Myrmecological News
Myrmecol. News 33: 35-75

ISSN 1997-3500
myrmecologicalnews.org

doi: 10.25849/myrmecol.news_033:035

26 January 2023

Original Article

Wonderfully weird: the head anatomy of the armadillo ant, Tatuidris tatusia(Hymeno­ptera: Formicidae: Agroecomyrmecinae), with evolutionary implications
Adrian Richter, Brendon E. Boudinot, Francisco Hita Garcia, Johan Billen, Evan P. Economo& Rolf G. Beutel

Abstract
Tatuidris tatusia Brown & Kempf, 1968, the armadillo ant, is a morphologically unique species found in low to highelevation forests in regions of Central and South America. It is one of only two extant representatives of the subfamily
Agroecomyrmecinae, and very little is known about the biology of these ants, which are almost exclusively collectedfrom leaf litter and have rarely been seen alive. Here, we illuminate the functional morphology and evolution of thisspecies via detailed anatomical documentation of their exceptionally modified head. We describe and illustrate theskeletomuscular system, digestive tract, and cephalic glands based on high-resolution micro-computed tomographyscan data. We hypothesize that the modifications which produce the unusual “shield-like” head shape are the resultof complex optimizations for mandibular power, physical protection, and balance. The most conspicuous cephalic features are the broadening of the frontal region and foreshortening of the postgenal region. The former characteristic islikely also associated with the lateral position of the antennal scrobe, the inverted antennal articulation, and the broad
attachment surface for the mandibular adductor muscles. This head geometry also comes with a degree of internal
restructuring of the tentorium and the antennal musculature, which have a unique configuration among ants studied
so far. The mandibular blades, and their articulations and muscles, are highly distinctive compared with previously
evaluated species. Using a 3D-printed model, we were able to hypothesize their entire range of motion as the mandibles
fit tightly into the oral foramen. Finally, we compare T. tatusia across other related subfamilies and discuss the evolutionof the Agroecomyrmecinae and other species-poor and phylogenetically isolated “relictual” lineages.
Key words: Micro-computed tomography, evolutionary morphology, relictual lineage, adaptation, Paraponera,
Ankylomyrma.
Received 14 February 2022; revision received 10 November 2022; accepted 10 November 2022
Subject Editors: Flávia Esteves, Florian M. Steiner
Adrian Richter (contact author), Brendon E. Boudinot & Rolf G. Beutel, Institute of Zoology and Evolutionary
Research, Friedrich Schiller University Jena, Erbertstr. 1, Jena 07743, Germany, E-mail: adrichter@gmx.de
Francisco Hita Garcia, Biodiversity and Biocomplexity Unit, Okinawa Institute of Science and Technology
Graduate University, Onna-son, Okinawa, 904-0495, Japan.
Johan Billen, Zoological Institute, KU Leuven, Naamsestraat 59, Box 2466, 3000 Leuven, Belgium.
Evan P. Economo, Biodiversity and Biocomplexity Unit, Okinawa Institute of Science and Technology Graduate
University, Onna-son, Okinawa, 904-0495, Japan; Radcliffe Institute for Advanced Study, Harvard University,
Cambridge, MA, 02138, USA.

Introduction
Tatuidris tatusia Brown & Kempf, 1968 is the “armadillo ant” in both form and name. The silhouette of thesestrongly armored ants is compact and profoundly curved,
while both elements of the binomen refer to armadillos:
The genus name is a combination of the Tupi word for theorder Cingulata (tatu) and the Greek word for ant (idris),and the specific epithet is derived from a synonymized
generic name for those armored mammals. This mor-

© 2023 The Author(s).

phologically singular species was described based on two
specimens from El Salvador (Quezaltepeque), which weresent to the original authors by Roy Snelling from the Los
Angeles County Museum (Brown & Kempf 1968). Since
that time, the number of collected specimens has considerably increased, and the known distribution range
extended north to Mexico and south to Peru and Brazil(Donoso 2012, 2017).

Open access, licensed under CC BY 4.0
https://creativecommons.org/licenses/by/4.0

Initially considered to be a lineage of Myrmicinaethat was transitional from Ponerinae sensu lato, Tatuidris tatusia was placed in the tribe Agroecomyrmecini
alongside the Eocene genera †Agroecomyrmex Wheeler,
1910 and †Eulithomyrmex Carpenter, 1935 (Brown &Kempf 1968). Because of its well-developed postpetioleand “dacetine”-like facies, T. tatusia featured prominently
as a point of contention among morphological systematists
and was subjected to a protracted series of back-and-forth
rearrangements (Baroni Urbani & al. 1992, Baroni Urbani & de Andrade 1994, Bolton 1998, 1999, 2000, 2003,
Baroni Urbani & de Andrade 2007). The disagreement
was ultimately settled via the generation and analysis ofmolecular datasets (e.g., Brady & al. 2006, Moreau &al. 2006, Rabeling & al. 2008, Ward & al. 2015), whichunambiguously supported the elevation of Agroecomyrmecini to subfamily level (Bolton 2003). These analyses,
moreover, revealed that the ornate African species Ankylomyrma coronacantha Bolton, 1973 is sister to T. tatusia(see Ward & al. 2015), for which a current diagnosis wasprovided by Fisher & Bolton (2016). That Agroecomyrmecinae is nested within the Poneria (“poneroids”) is
supported by the aforementioned studies as well as more
recent molecular analyses (e.g., Branstetter & al. 2017,
Borowiec & al. 2019), and a sistergroup relationshipbetween Agroecomyrmecinae and Proceratiinae, with
Paraponerinae sister to both, was even more recently supported by the phylogenomic analysis of Romiguier & al.
(2022).
Even half a century since its original description, thespecies-level diversity and natural history of Tatuidris
tatusia remain uncertain. The occurrence of apparently
distinct pilosity among forms of T. tatusia, differentiatedby setational curvature, stature, and density, led to the
hypothesis of multiple morphospecies (Longino 2010),
and eventually the recognition of one form as “Tatuidris
kapasi” Lacau & Groc, 2012 in Lacau & al. (2012). Soon
thereafter, Donoso (2012) systematically evaluated themorphological and cytochrome oxidase subunit I variationof T. tatusia across its range, finding that the primary
signal was of latitudinal variation without discrete morphological gaps and that the multiple hair forms occurred
within defined sequence clusters. Donoso (2012), therefore, tentatively concluded that T. tatusia represents asingle species in the process of allopatric differentiation,
stressing that the recognition of multiple species may
yet result from larger datasets and more sophisticatedanalyses. Recent advances in these regards have shown
Donoso’s foresight (2012), as complicated and morphologically subtle species boundary patterns can indeed beresolved in taxa of Formicidae using the approaches hesuggested (e.g., Prebus 2021, Branstetter & Longino
2022, Ward & Branstetter 2022, Williams & al. 2022).
To date, the most extensive contribution to the biology of
T. tatusia was provided by Jacquemin & al. (2014); theirfield- and lab-based videos showed that T. tatusia ants are
slow to the point of clumsiness, with an oversized headand body size exceeding that of other ant species of the36

Neotropical soil or leaf litter. The nitrogen stable isotope
ratio analysis of those authors indicated that T. tatusia islikely a top predator of the brown food web (as defined by
Kaspari & Yanoviak 2009), and their laboratory “cafeteria experiments” suggested that T. tatusia may be rather
specific in their prey choice. However, the prey preferenceof T. tatusia remains a mystery as the ants rejected all
offered food items.
The primary objective of the present study is to document the cephalic anatomy of this unusual ant species
in detail using modern techniques, while the secondary
objective is to understand the morphological evolution of
Tatuidris tatusia in the light of recent phylogenomic advances. In addition to providing a considerably expanded
and precise treatment of the external morphology of T.
tatusia, this work expands our knowledge of the variationof internal head structures, including muscles, the cephalicdigestive tract, and cephalic glands, complementing similar analyses for representatives of Formicinae, Ponerinae,
Myrmicinae, Leptanillinae, Dorylinae, and †Sphecomyrm­
inae (Richter & al. 2019, 2020, Boudinot & al. 2021,
Richter & al. 2021a, 2022). We employed micro-computed tomography (µ-CT) scanning for our anatomical
analysis and volume renderings to illustrate the anatomy.
Furthermore, we printed a 3D model of the head capsuleand mandible to investigate the unusual mandibular articulation of T. tatusia and the mandibular movement
pattern resulting from it. Our contribution provides new
information on a little-known group of ants and presents
thoughts on the morphological evolution and survival of ataxonomically depauperate and phylogenetically isolated
lineage, otherwise known as phylogenetic “relicts”.

Material and methods
Material examined
Tatuidris tatusia (Agroecomyrmecinae): The single
specimen used in this study for µ-CT scanning was collected in Costa Rica: Alajuela, 5 km east of Monteverde,
10° 17' 47.2" N, 84° 46' 16.8" W, 1230 m above sea level(a.s.l.), wet forest, ex sifted leaf litter, Winkler, collection
code JTL8695-s, 18.V.2014, leg. J. Longino. After µ-CT
scanning, the specimen was given the unique specimen
identifier (USI) CASENT0790526 and is now stored at the
research collection of the Biodiversity and Biocomplexity
Unit at the Okinawa Institute of Science and Technology.
The two workers used for histological examination werecollected by Thibaut Delsinne in the Copalinga Private
Reserve, Ecuador, 4° 5' 28.3" S, 78° 57' 38.5" W, 1050 ma.s.l., and were obtained via Winkler extraction of leaf
litter samples. They are stored in the embedding database of Johan Billen at Katholieke Universiteit Leuven
under numbers 2355-1 and 2355-2, with the corresponding section series kept in drawer 38C, numbers 32 to
46. Additional specimens of T. tatusia examined fromstacked photographs or scanning electron micrographs(SEMs) are indicated in Table 1. For the purpose of comparing T. tatusia with other Formicidae in a phylogenetic

Fig. 1: Macrophotographs of living Tatuidris tatusia. Upper row: ventral view on the left and lateral view on the right. Lower
row: dorsoventral view. Note the smooth, shining cuticle, inclination of the head, and the massive size of the head relative to themesosoma and gaster. Photographs by Michael Branstetter and used with permission. Scale bars represent an estimation basedon the dimensions of our 3D model.

context, we also examined specimens from Keller’s (2011)
SEM atlas, for which images are available on AntWeb(2022); the USIs for these specimens are listed in Table 1.
USIs were not recorded for other specimens evaluatedfrom AntWeb (2022) because of the varying taxonomic
granularity of our comparisons coupled with the default
display settings of AntWeb (2022) (see also "Comparative
morphology" below). One Ankylomyrma coronacantha
specimen (Ent:728292) was examined directly by AR at
the Harvard Museum of Comparative Zoology in Harvard,
Massachusetts.
µ-CT scanning and 3D-reconstruction
The head of the sample for µ-CT scanning was removed
with forceps and stained in an alcoholic iodine solution
for three days before it was mounted in a pipette tip filledwith 99.9% ethanol. The images were captured with aZeiss Xradia 510 Versa 3D X-ray microscope (Oberkochen,
Germany) operated with the Zeiss Scout-and-Scan Control
System software (version 11.1.6411.17883) at the Okinawa
Institute of Science and Technology Graduate University,
Japan. The scanning parameters chosen consisted of a 40
kV (75 μA) / 3 W beam strength with 20 s exposure time
under a 4 × magnification, which resulted in a pixel image
size of 1.41 μm. The distance between the sample andsource was set to 13 mm and the distance to the detector to
50 mm. 3D reconstructions of the resulting scan projection
data were done with the Zeiss Scout-and-Scan Control
System Reconstructor (version 11.1.6411.17883) and saved
in DICOM file format. Postprocessing of DICOM raw data
was performed with Amira (version 6.0) software (Visage
Imaging GmbH, Berlin, Germany). Individual structures37

were segmented into discrete materials. For segmenting
the head capsule, every 20th slice was pre-segmented inAmira, and the online platform Biomedisa (Lösel & al.
2020) was used to semiautomatically complete the segmentation. The segmented materials were then exportedwith the plugin script “multiExport” (Engelkes & al. 2018)
in Amira (version 6.1) as Tiff image stacks. VG-Studio Max
3.4 (Volume Graphics GmbH, Heidelberg, Germany) wasused to create volume renders out of the Tiff image series.
3D-printing for functional morphology
In addition to the volume renderings generated toillustrate our anatomical results, surface objects of thehead capsule and mandible were generated with Amira(version 6.0) and exported as .stl files. These surface files
were printed with an objet350 3D printer (Stratasys, Rehovot, Israel), using the solid, white digital acrylonitrile
butadiene styrene (ABS) material. The models were used
to investigate the potential mandibular movement of Tatuidris tatusia. A series of images of the opening movement
at different angles was taken with a Huawei mate 20 lite
(Huawei, Shenzhen, China) smartphone camera and ispresented in the discussion section.
Histological sections
The anterior part of the head of two workers was cut off
with a sharp razor blade and fixed in 2% glutaraldehyde ina sodium cacodylate buffer, followed by post-fixation in 2%
osmium tetroxide in the same buffer. After dehydration ina graded acetone series, tissues were embedded in araldite(Agar Scientific Ltd, Sansted, UK) and sectioned with a
Leica EM UC6 ultramicrotome (Wetzlar, Germany), one
head in longitudinal and one in a transverse orientation.
Semithin sections of 1 μm thickness were stained withmethylene blue and thionin and examined in an Olympus
BX-51 (Tokyo, Japan) microscope. With an Olympus Camedia C-3040 Zoom digital camera (Tokyo, Japan), imagesof histological sections were taken at 10 µm intervals using
the 20 × objective, and additional images of anatomical details were taken with a 40 × objective and a 100 ×objective with immersion oil with a focus on glandular
features.
Image processing
All images were edited with Adobe Photoshop® CS6(Adobe System Incorporated, San Jose, USA) and arranged
into figure plates (Figs. 1 - 17). On SEM images and images
from the section series, a tonal correction was performed.
The selective sharpener (30% strength) was used on all
images. Adobe Illustrator® CS6 (Adobe Systems Incorporated, San Jose, USA) was used to label the figure plates.
Data availability
The µ-CT scan data generated for this study were
uploaded to Zenodo.org and can be accessed under thefollowing DOI: https://doi.org/10.5281/zenodo.6046240.
The Zenodo repository also contains the surface renders of the head capsule and mandible. Using this,38

anyone with access to a 3D printer can reproduce themandibular movement experiments performed in thisstudy.
Terminology
The terminology of the head and its appendages isbased on Keller (2011) and Richter & al. (2020), withadditions made to hypostomal and mandibular terminology from Richter & al. (2022). The names and numbers
used for head muscles are based on Wipfler & al. (2011).
For previous applications of this system to Formicidae andcomparative muscle tables, see Richter & al. (2019: tab. 1)
and Richter & al. (2021a: tab. 2). Orientational descriptors (anterior, posterior, dorsal, ventral) assume prognathy
to facilitate comparison with other ant species, although
the head of Tatuidris tatusia is typically more strongly
inclined than in many other lineages (e.g., Richter & al.
2019, 2020, 2021a). Operationally, this means that theoral foramen is anterior and the vertexal area of the headis posterior while the occipital foramen is on the ventral
and the frontal area on the dorsal side of the head. Terminology for surface sculpturing follows Harris (1979). The
systematic terminology follows Boudinot & al. (2022a), in
which the “poneroid” and “formicoid” clades are referred
to as Poneria and Doryloformicia, respectively, to avoid
confusion with the superfamilial rank (suffixes: nominal-oidea, adjectival -oid).
Based on the functional, morphological investigation
presented here, it was necessary to derive new concepts,
hence terms, that are specific to the craniomandibular
articulations and associated structures of Tatuidris tatusia. Because of the uncertain homology of several ofthese structures to other groups, we prefer to use these
specific terms based on functional relevance to facilitate
our discussion of mandible movement. We also discuss
the concepts “trulleum” and “canthellus”. These new andrevised concepts are listed below.
Clypeal condyle rail: the long, transversely oriented part of the cranial condyle (ccr, Fig. 2D). This rail
may correspond to the triangular process of the clypeus
described in other ants (see Richter & al. 2019), but thatprocess does not interact with the mandible or function
as a condyle. Alternatively, it may be a modified part ofthe clypeal condyle.
Clypeal condyle lobe: the short lobe of the cranialcondyle perpendicularly oriented to the condyle’s long part(ccl, Fig. 2D). This corresponds in position to the cranialcondyle of other aculeatan species (e.g., Richter & al.
2020, 2022).
Distal contact surface: the dorsal groove on themandibular base in which the clypeal condyle rail rests
in a closed position (dcs, Fig. 4C ).
Medial mandibular groove: the dorsal groove
into which the clypeal condyle rail slides during mandible
opening (mg, Fig. 4C, E). The medial groove is distally
bordered by a short lobe, which we call the dorsal canthellar lobe (dcl, Fig. 4C, E). One or both of these grooves
may correspond to the “trulleum” of Ettershank (1966),

but the homology of this structure is unclear at the moment(see below).
Lateral and medial mandibular acetabular
grooves: the lateral (mdagl, Fig. 4C) and medial (mdagm,
Fig. 4C) portions of the mandibular acetabulum, which
receives the clypeal condyle and is generally in the formof a single groove but in Tatuidris tatusia is divided into
two by a triangular ridge.
Fimbrial line carina: the carina (flc, Fig. 4C, E) that
accompanies the line of setae along the medial surface ofthe masticatory margin (termed fimbrial line after Richter & al. 2022 and Michener & Fraser 1978).
Stipitopremental conjunctival thickening: themore or less sclerotized part of the conjunctivum thatconnects the inner side of the stipes to the prementum
and hypopharynx, in distinction to the surrounding thinand flexible regions of the conjunctivum. Previously, thiswas referred to as the “stipitopremental conjunctivum”
in Richter & al. (2020) and (2021a) and corresponds,
for example, to the “conjunctival thickening” of Porto &Almeida (2019) and possibly to the “anterior conjunctival thickening” of the Hymeno­ptera Anatomy Ontology(HAO:0002072, e.g., used in Prentice 1998), althoughthat definition includes a connection to the labrum.
Trulleum: The trulleum was originally defined in
species of Myrmicinae with a distinct “canthellus” (see corresponding section below). The canthellus, basal mandibular margin, and proximal mandibular base together form
the roughly triangular walls of the depressed trulleum(Ettershank 1966: fig. 2). However, as this specific condition is restricted to Myrmicinae (Richter & al. 2019), andmandibular grooves / depressions of various other shapes
occur in some, but by no means all, Formicidae in similar
positions (e.g., Richter & al. 2021b), Richter & al. (2019)suggested that use of the term should be restricted to themyrmicine condition. We follow this interpretation here,
and while “trulleum” could conceivably be established as apragmatic referral to various kinds of mandibular grooves,
we would prefer this to be based on a thorough evaluationof such grooves and their homology, which is outside thescope of the current contribution. As with other terms, wechoose to employ a homology-neutral term, emphasizing
functional role instead (here “distal contact surface” and“medial groove”, see corresponding sections above). We
also suggest here a modified use of Ettershank’s (1966)
term “canthellus”.
Canthellus: The term “canthellus”, as used by Etter­shank (1966), refers to the dorsal outgrowth / ridge of amedial thickening of the mandibular base. Instead, we
propose to apply this term to the medial thickening as
a whole as the latter appears to be widely present acrossFormicidae (Richter & al. 2019: fig. 4D, therein referred
to as “canthellar elevation”, see also Richter & al. 2020,2021a) while the dorsal ridge is restricted to some taxa
such as Myrmicinae. This usage makes the term more
widely applicable across Formicidae. Because the dorsal
outgrowth of the canthellus observed in Tatuidris tatusia
is restricted to part of the overall canthellus length, we callit “canthellar lobe” here (see above). “Canthellar ridge”
may be more appropriate for the condition observed in
Myrmicinae.
Comparative morphology
In order to comprehend the evolutionary derivation of
Tatuidris tatusia, we compared the external morphologyof the head, mesosoma, and metasoma with a sample ofother Poneria. We relied on Keller’s (2011) SEM atlasfor detailed comparisons of the head and on AntWeb(2022) for comparisons across the rest of the body. Additional sources of information for individual characters
are indicated where necessary. Our comparative process
was structured by the phylogenies of Branstetter &al. (2017), Borowiec & al. (2019), and Romiguier & al.
(2022), that is, using the character concepts of Bolton(2003), Keller (2011), Boudinot (2015), and Boudinot& al. (2022a) as a basis, we iteratively compared sistergroups where reasonably supported from the species tothe supra-subfamilial level in a node-spanning manner.
To evaluate image arrays, we used the subfamily, genus,
and species classification tool of AntWeb (2022) to view
specimens of given castes and taxa side by side, withmultiple cycles of higher-to-lower and lower-to-higher
taxon comparison. We recorded morphological variables
in Table 2 at the species level for Tatuidris, Ankylomyrma,
Paraponera, and Apomyrma (given that they are currently monotypic), while our scoring for the comparatively
diverse subfamilies Amblyoponinae, Ponerinae, and Proceratiinae represents a generalization across the species
and genera contained therein. As evaluating patterns ofmorphological transformation in these three larger subfamilies is not the focus of the present study, we usually
did not record variables within these clades.
The characters listed in Table 2 were coded as statearguments that are either TRUE (1) or FALSE (0), ina manner similar to Boudinot & al. (2022a) but with
“0” representing the inferred plesiomorphic condition.
Hypotheses of state polarities, presented in Figure 12,
are based on an informal, parsimony-based evaluation
informed by the ancestral state estimates of Boudinot &al. (2022a) and Richter & al. (2022). If a given structure
could not be evaluated based on available images, it wasrecorded as “?”. Overall, the coding accounts for uniqueas well as shared features of all scored taxa and mostly
includes features that are externally visible; internal observations were restricted to Tatuidris tatusia and areconsidered comparatively ambiguous.
We note that the resultant table of variables is not
specifically intended for statistical phylogenetic analysis.
Rather, the table is intended as a summary of possible morphological transformations. Given their distribution acrossthe focal taxa, these transformations may be reasonably
hypothesized to have evolved along particular branches
and to have been inherited by ancestors at deep nodes
within the Poneria (Fig. 12). Future studies that aim fora more formal assessment may require refined character
definitions, possibly based on precise quantifications,39

Tab. 1: Taxa and specimens from Keller’s Scanning Electron Microscopy atlas (Keller 2011) that were examined plus specimensfrom AntWeb (2022) that were included in the figures of this contribution.
Subfamily

Genus

Species

Authority

Identifier

Agroecomyrmecinae

Agroecomyrmex

duisburgi(Mayr, 1868)

BMNHP18831

Ankylomyrma

coronacantha

Bolton, 1973

CASENT0005904
CASENT0902015
MCZ:Ent:728292

Tatuidris

tatusia

Brown & Kempf, 1968

ANTWEB1008593
ANTWEB1008605
CASENT0423526
CASENT0178755

Amblyoponinae

Apomyrminae

Adetomyrma

caputleae

Yoshimura & Fisher, 2012

ANTWEB1008494

Amblyopone

australis

Erichson, 1842

ANTWEB1008497

Fulakorachilensis(Mayr, 1887)

ANTWEB1008496

bath5

-

QMT152681

Prionopelta

amabilis

Forel, 1909

ANTWEB1008581

Prionopelta

concenta(Brown, 1974)

ANTWEB1008513

Stigmatomma

pallipes(Haldeman, 1844)

ANTWEB1008501

Apomyrma

stygiaBrown & al., 1971

ANTWEB1008505
CASENT0000077

Paraponerinae

Paraponera

clavata(Fabricius, 1775)

ANTWEB1008572
CASENT0006789

Ponerinae

40(F. Smith, 1858)

ANTWEB1008574

Platythyreapunctata
turneri

Forel, 1895

ANTWEB1008575

Anochetus

emarginatus(Fabricius, 1804)

ANTWEB1008504

BelonopeltadeletrixMann, 1922

ANTWEB1008507

Bothroponera

pachyderma(Emery, 1901)

ANTWEB1008567

Brachyponera

croceicornis(Emery, 1900)

ANTWEB1008564

Centromyrmex

brachycola(Roger, 1861)

ANTWEB1008505

Cryptopone

gilva(Roger, 1863)

ANTWEB1008514

Dinoponera

lucidaEmery, 1901

ANTWEB1008517

Dolioponera

fustigera

Brown, 1974

ANTWEB1008521

Emeryopone

buttelreepeni

Forel, 1912

ANTWEB1008525

Hagensia

h. marleyi

Arnold, 1926

ANTWEB1008566

Harpegnathos

saltator

Jerdon, 1851

ANTWEB1008532

Hypoponera

mx01

-

ANTWEB1008535

Leptogenys

mx01

-

ANTWEB1008541

np02

-

ANTWEB1008542

podenzanai(Emery, 1895)

ANTWEB1008543

Loboponera

vigilans

Bolton & Brown, 2002

ANTWEB1008546

Myopiasmaligna(F. Smith, 1861)

ANTWEB1008550

Neoponera

apicalis(Latreille, 1802)

ANTWEB1008561

villosa(Fabricius, 1804)

ANTWEB1008571

Odontomachus

bauri

Emery, 1892

ANTWEB1008557

Odontoponeratransversa(F. Smith, 1857)

ANTWEB1008558

Ophthalmopone

berthoudi

Forel, 1890

ANTWEB1008562

Pachycondyla

crassinoda(Latreille, 1802)

ANTWEB1008563

Subfamily

Proceratiinae

Genus

Species

Authority

Identifier

Paltothyreus

tarsatus(Fabricius, 1798)

ANTWEB1008570

Phrynoponera

pulchella(André, 1892)

ANTWEB1008573

Platythyrea

lamellosa(Roger, 1860)

CASENT0252018

Plectroctena

strigosa

Emery, 1899

ANTWEB1008576

Poneraalpha

Taylor, 1967

ANTWEB1008578

Psalidomyrmex

procerus

Emery, 1901

ANTWEB1008585

Pseudoneoponera

porcata(Emery, 1897)

ANTWEB1008568

Pseudoponera

stigma(Fabricius, 1804)

ANTWEB1008569

Simopelta

oculata

Gotwald & Brown, 1967

ANTWEB1008588

transversa

Mackay & Mackay, 2008

ANTWEB1008589

Streblognathus

peetersi

Robertson, 2002

ANTWEB1008591

Thaumatomyrmex

fraxini

D’Esquivel & Jahyny, 2017

ANTWEB1008597

Wadeuraguianensis

Weber, 1939

ANTWEB1008565

Discothyrea

oculata

Emery, 1901

ANTWEB1008518

testacea

Roger, 1863

ANTWEB1008519

creek

De Andrade, 2003

CASENT0104439

croceum(Roger, 1860)

ANTWEB1008583

pergandei(Emery, 1895)

ANTWEB1008584

guineensis

Taylor, 1965

ANTWEB1008582

Proceratium

Probolomyrmex

CASENT0249253

considerations of character interdependencies, and expanded datasets including µ-CT-scans and additional SEM
images. For heuristic purposes, we group the characters in
Table 2 into possible apomorphies of certain clades and,for the head of Tatuidris tatusia, informal sets based on
hypothesized functions of the included character states.
Table 2 is not exhaustive given the limitation of image
availability, yet we hope that it will be a useful foundation
for future hypothesis testing and for future studies as asummary of similarities and differences across the taxa
under consideration.

Results
Head capsule
A digital 3D model of the head to view from all angles isavailable on Sketchfab (https://skfb.ly/o8UAW). The headis roughly as large as the mesosoma in apparent total volume (Fig. 1). It appears orthognathous in dead specimens,that is, the mouthparts are oriented downwards, and thelongitudinal cephalic axis is perpendicular to the longitudinal axis of the body. However, in living specimens, thehead often has an oblique, subprognathous orientation,with an angle of the longitudinal axes of about 30° relative to the mesosoma (Fig. 1, see also live observationsof a colony of Tatuidris tatusia; [https://www.youtube.
com/watch?v=AkeNF9PW92s]; Jacquemin & al. 2014,2018). The postocciput and the occipital foramen are set
in a deep concavity on the ventral side of the head; they
are almost perpendicular to the longitudinal axis of thehead (Fig. 2C). The postocciput is hourglass-shaped inposterior view, with the upper portion much wider thanthe lower one and both separated by the postoccipital
condyles (pocn, Fig. 2C). Proximad the ventromedianmargin of the foramen, a secondary median evagination
is present; this subforaminal process is hook-like andcurved ventrad (sfp, Figs. 2C; 3B). The concave occipitalregion above the postocciput (occ, Fig. 2C) fits with theparabolic anterodorsal part of the pronotum, and theposterior declivity of the vertexal area (vt, Fig. 2C) fits
against the complementary convex contact surface of the
pronotum.
The strongly sclerotized head capsule is distinctly
broader than long, rounded posterolaterally, and almost
straight along the posterior margin, depending on theangle of view (Fig. 2A). The dorsal (frontal) surface forms
an even and broad convexity (Brown & Kempf 1968:
shield-shaped); its anterolateral margins consist of large,slightly raised frontal lobes (frl, Fig. 2A, B, D); they are
weakly curved in lateral view and obliquely oriented in
dorsal view, diverging posteriorly.
The shield-like frontal surface is almost indistinguishable from the clypeus (border roughly indicated as dottedline in Fig. 2A); only a slight swelling of the cuticle and thesite of origin of the clypeobuccal and clypeopalatal muscles(0ci1, 0bu1) mark its location. The clypeus' anterolateralcorners project anterad above the base of the mandible
together with the frontal lobes (lcl, Fig. 2A). The anterior
clypeal lamina (cll, Fig. 2D) forms the apparent anterior
margin of the clypeus; it is broad and slightly sinuate,

41

Fig. 2: Volume renderings of the head of Tatuidris tatusia, based on micro-computed tomography scan. A: Dorsal view. B:
Lateral view. C: Ventral view. D: Frontal view of oral foramen showing articular surfaces of the mouthparts. Abbreviations:al – atala; alf – atalar fossa of the pleurostoma; asc –antennal scrobe; ascf – stabilizing ridges in the antennal scrobe; ata –anterior tentorial arm; atp – anterior tentorial pit; ccl – clypeal condylar lobe; ccr – clypeal condylar rail; ce – compound eye;cl – clypeus; clg – clypeal groove; cll – clypeal lamina; fr – frontal area; frl – frontal lobe; hc – hypostomal cardinal condyle;hyc – hypostomal corner; hyg – hypostomal groove; hyl – anterior lobe of the hypostomal process; hysci – inner hypostomal
carina; hyscou – outer hypostomal carina; hysp – triangular hypostomal process; lb – labrum; lcl – lateral clypeal lobe (fused
to frontal lobe); md – mandible; mdc – mandibular condyle; oca – occipital carina; occ – occipital area; pgb – postgenal bridge;
pm – prementum; pocn – postoccipital condyle; pta – posterior tentorial arm; ptp – posterior tentorial pit; sfp – subforaminal
process; st – stipes; tb – tentorial bridge; to – torulus; vcb – posterolateral ventral cephalic bulge; vma – ventral mandibular
articulation fossa; vt – area of the vertex. Symbols: cyan dotted line – outline of the hypostomal process lobe in D and roughindication of the clypeal margin in A. Color marking: cyan – clypeal condylar rail; purple – clypeal condylar lobe.

Fig. 3: Volume renderings of the head, antenna, antennal articulation, and antennal musculature of Tatuidris tatusia, basedon micro-computed tomography scan. A: Antennal musculature and tentorium, dorsal view on the left and ventral view on theright. B: Antennal musculature and inner skeleton in sagittal view. C: Lateral view of head with antenna in antennal scrobe.

42D: Medial view of antenna. E: Antennal articulation in ventrolateral view. F: As in C but with antenna removed. G: Antennal
articulation in internal posterior view. Abbreviations: 0an1 – M. tentorioscapalis anterior; 0an2 – M. tentorioscapalis posterior; 0an3 – M. tentorioscapalis lateralis; 0an4 – M. tentorioscapalis medialis; 0an6 – M. scapopedicellaris lateralis; 0an7
– M. scapopedicellaris medialis; aa – antennal ampulla; anVII – antennomere seven / apical antennomere; ant – antennifer;asc – antennal scrobe; ata – anterior tentorial arm; bb – bulbus; bbn – bulbus neck; cl – clypeus; clg – clypeal groove; cli– clypeal inflection; ce – compound eye; dta – dorsal tentorial arm; esr – vestige of the epistomal ridge; fr – frontal area; frl– frontal lobe; hyg m – median hypostomal groove; hyl – anterior lobe of the hypostomal process; hyscou – outer hypostomal
carina; hysp – triangular hypostomal process; lcl – lateral clypeal lobe (fused to frontal lobe); md – mandible; ml – medial
lamella; pd – pedicel; pgr – postgenal ridge; poc – postocciput; pta – posterior tentorial arm; sc – scapus; sfp – subforaminal
process; tb – tentorial bridge; tba – anteriomedial process of the tentorial bridge; to – torulus; toa –torular apodeme; tol –lateral torular arch / torular lobe; tom – median torular arch. Symbols: cyan dotted line – outline of the hypostomal process.

43

protruding medially; its marginal area presents a fine,transverse imbricate surface pattern; on each lateral sideof the lamina groups of conspicuous mesally directed,
curved setae are present. The inflected portion of theclypeus forms a deeply concave and dorsoventrally broad
groove / sulcus (clg, Fig. 2D); it is continuous across thedorsal margin of the oral foramen and curves lateradaround the mandibular base; the groove is dorsally andlaterally walled off by the anterior clypeal lamella; it bears
two pairs of thin setae. Laterally, the clypeus is delimitedby the anterior tentorial pit, which is situated inside theantennal scrobe close to the antennal socket (atp, Fig. 2B),
and by a short internal ridge between the antennal socket
and the dorsal mandibular articulation. The clypeal surfaces articulating with the mandibles are L-shaped andwide along the lateromedial axis; the short clypeal condylar lobes (ccl, Fig. 2D) are longitudinal to the long axisof the head, while the long clypeal condylar rails (ccr,Fig. 2D) are transversely oriented. A supraclypeal area ora frontal line are at most very indistinctly indicated by aflat cuticular depression (Fig. 2A).
The widely separated antennal foramina are not visible
in dorsal (frontal) view; their position is only indicatedby low, dome-like bulges of the frontal lobes. The torularsclerite itself is situated within a deep antennal scrobe
and oriented ventrad (to, Fig. 2B). The antennal scrobe islarge, triangular, and deep, covering a major portion of thelateral head surface (asc, Fig. 2B); it reaches the posterior
third of the head capsule, where the small, roughly circular and convex compound eyes are inserted (diameter ca.
80 µm). The scrobe is delimited dorsally by the expanded
frontal lobe (frl, Fig. 2A, B, D); a narrower but distinct
longitudinal ridge forms its ventral border. The scrobe
is medially divided into a dorsal and a ventral portion by
the lateral edge of the anteriorly produced clypeus; somelongitudinal costae / ridges stretch posteriorly into theantennal scrobe from this point (ascf, Fig. 2B). Internally,
the scrobal surface does not bear muscular attachments.
On the ventral side of the head, the posterolateral regions form pronounced bulges (vcb, Fig. 2C); these bulges
extend well below the ventromedian cephalic surface andnarrow anteriorly towards the ventral (primary) mandibular articulations; curved, longitudinal ridges / costae arepresent on their surface. Internally, the bulges are filled
out with fibers of the mandible adductor / closer muscle(0md1). Between the bulges, the ventral surface of thehead is roughly triangular (Fig. 2C), with a boxy postgenal
region delimited laterally by wide grooves that narrow as
they enter the occipital concavity. The postgenal region(pgb, Fig. 2C) is divided into anterior and posterior portions by an arcuate, discontinuous carina that posteriorly
ends laterad the posterior tentorial pits; this carina may
be the ventral rim of the occipital carina. The posterior
portion of the postgenal region bears a few short setae.
The postgenal bridge and the internal postgenal ridge areshort relative to the head length (pgb length / cephaliclength = 0.415, n = 1), resulting in the subprognathous /oblique head orientation.

44

Seen from below, the hypostomal margin appears
weakly concave (hyscou, Fig. 2C), eclipsing the anterior
clypeal margin in a low, oblique posteroventral view; thedepth of this margin from the anterolateral hypostomal
corners (hyc, Fig. 2C) is about 1 / 5 of its lateromedial
width (vs. ~ 1 / 2 in other ants). Due to the shallownessof the hypostoma, the maxillolabial complex is held in analmost vertical orientation in its retracted position. It isnoteworthy that the curvature of the hypostomal margin
is similar to that of the posterior clypeal margin (compare
Fig. 2A, C). The outer hypostomal carina (hyscou, Fig. 2C,D) bears transverse cuticular ridges externally (Fig. 2C).
Ventrally, the outer carina delimits the deep hypostomal
groove (hyg, Fig. 2D); laterally, it reaches the flat hypostomal corners (hyc, Fig. 2C, D). The inner hypostomal carina(hysci, Fig. 2D) ends laterally in the tips of the triangularhypostomal processes (hysp, Fig. 2D) and reaches the outerhypostomal carina medially. The anterolateral margins of
the triangular processes bear broad, anteriorly directed,
rounded lobes (hyl, Fig. 2D).
The cuticle of the head is dark brown and shiny. A regular vestiture of setae of about 70 µm length is present; theaverage distance between setae is about 50 µm.
Endoskeleton
The anterior tentorial arms are long and curved in
a complicated manner (ata, Fig. 3A, B); they are slightly
sinuous, in both sagittal as well as dorsal or ventral view;in dorsal or ventral view their posterior halves are roughly
parallel while their anterior halves are strongly diverging.
The posterior tentorial arms are dorsally directed andsharply curved anteriad (ca. 90°) at the level of the tentorial bridge (pta, Fig. 3A, B). This configuration results inan almost horizontal orientation of the anterior tentorialarms relative to the longitudinal axis of the head capsule(Fig. 3B). The medial tentorial lamella (ml, Fig. 3A, B)
forms a short lobe that is twisted upwards, resulting inalmost horizontal orientation of the extrinsic antennal
muscles (0an1 - 4, Fig. 4A, B). The lateral lamella is absent.
The dorsal tentorial arm is long and consists of a relatively
thick base and a thin, thread-like extension; the latterstructure is directed backward on one side and forward
on the other, indicating that it is relatively flexible (dta,Fig. 3A, B). The tentorial bridge is very short and thinnerthan the posterior arms (tb, Fig. 3A, B); a rather long andthick anteromedian process is continuous with the tendonof 0bu5 / 6 (tba, Fig. 3A, B). The epistomal ridge is vestigial,
and only a slight thickening of the cuticle can be observedin its place (esr, Fig. 3B). The postgenal ridge extends overthe entire width of the short, box-like postgenal bridge(pgr, Fig. 3B). The torular apodemes are formed by dorso­
ventrally flattened, rod-like processes, extending mesadfrom the antennal socket and slightly curved anteriad(toa, Fig. 3B, G).
The antennae and their articulations
The antennal foramen and antennal base are coveredby the large frontal lobe; the ventrally facing antennal

socket apparatus is placed on the ventral surface of thisprojection (Fig. 3C). Due to this rotation of the antennal
socket, the medial arch of the torulus (tom, Fig. 3E) is positioned laterad the lateral arch (tol, Fig. 3E). The torular lobeof the medial arch is well-developed, roughly triangular,
but apically rounded; it is located on the posterior half ofthe torulus (tom, Fig. 3E, F). The antennal acetabulum isa large concave area on the dorsal, anterior, and ventral
torular surfaces; the massive antennifer is located at theanterior margin of the foramen and is almost transversely
oriented (ant, Fig. 3F, G). It articulates with the bulbus (bb,Fig. 3C - E), which is hemispherical and deeply countersunk into the acetabulum. The short bulbus neck is almost
cylindrical but slightly widens distally (bbn, Fig. 3E); it is
attached to the posterior side of the bulbus and separated
from the main shaft of the scapus by a constriction. The
seven-merous (= “segmented”) geniculate antenna fits
almost completely into the antennal scrobe except for thelarge apical flagellomere, with the scape and flagellum
fitting into the dorsal and ventral portions of the scrobe,
respectively (Fig. 3C). When the antenna is pulled back, themovement is restricted by the large torular lobe (Fig. 3E),
confining it to a resting position in the antennal scrobe.
The scapus (sc, Fig. 3C - E) is slightly longer than the pedicellus (pd, Fig. 3C, D) and flagellomeres 1 - 4 combined;
it is bent at about 45° close to the bulbus neck, curving
anteriad in its proximal half; its anterior margin forms abroad distal lobe, resulting in a clavate appearance of theentire antennomere; this distal lobe is broadly concave
and receives the pedicellus when the flagellum is flexed
towards the scapus body; the anterior distal margin of
the scapus is deeply notched to create space for pedicellus
movements (Fig. 3D). The pedicellus is about 1 / 5 as longas the scapus; its base is narrowed and slightly curved
anteriad. Flagellomeres 1 - 4 widen distally, appearing
cup-shaped; their size increases towards the antennal
apex. The spindle-shaped apical antennomere is enlarged,
about 2 / 3 as long as the scapus (anVII, Fig. 3C, D); thedensity of setae is slightly increased apically; a field of sensilla placodea, each with a distinct perforation, is present
apically (Keller 2011).
Musculature (Fig. 3A, B): Due to the switch in
the orientation of the antennal socket, the bulbus is also
shifted. Its medial margin is dorsal and more lateral thanthe ventrally shifted lateral margin, and the bulbus isrotated so that the anterior muscle is inserted medially,
the posterior one laterally, the lateral one anteriorly, andthe medial one anteriorly. Nevertheless, relative positionsof origin on the tentorium are retained, so that homologization with other ants is possible. Musculus tentorioscapalis anterior (0an1): origin (= O): anterior/ dorsal surface of the medial tentorial lamella, on its
anterior corner; insertion (= I): on a tendon originating
medially on the bulbus. M. tentorioscapalis posterior(0an2): O: posteriorly on the anterior / dorsal surface ofthe medial tentorial lamella, posteriad 0an4; I: on a tendon
originating laterally on the bulbus. M. tentorioscapalis
lateralis (0an3): largest of the four muscles; O: laterally

on anterior tentorial arm; laterad / ventrad the otherthree muscles, partly on the dorsal tentorial arm; I: on atendon originating posteriorly on the bulbus. M. tentorioscapalis medialis (0an4): O: anterior / dorsal on
medial tentorial lamella, in between 0an1 and 0an2; I: ona tendon originating anteriorly on the bulbus. M. scapopedicellaris lateralis (0an6): O: dorsally on the distal
half of the scapus; I: on a short tendon originating fromthe dorsal base of the pedicellus. M. scapopedicellaris
medialis (0an7): O: ventrally on the distal half of thescapus; I: on a short tendon originating from the ventral
base of the pedicellus.
The mandibles and their articulations
The massive, shovel-shaped mandibles are triangularin dorsal view, with the proximal stem and basal marginof the blade set at about a 90° angle (Fig. 4A, C). In ventral
view, the thick mandibular stem appears swollen, with alateral bulge towards the level of the mandibular blade(Fig. 4D); the bulge continues onto the mandibular blade
dorsally and is visible in medial view near the basal margin(mdd, Fig. 4A, B, D). Posteriad the bulge, the blade tapers
towards the slightly concave basal margin (bm, Fig. 4C,D), creating a broad, sloping surface (sms, Fig. 4C, E). The
basal margin fits into the clypeal groove (bmc, Fig. 4B) but
only in the medial clypeal region at the level of the labral
articulation; it is subtended by a row of setae on the innerside of the mandible (Fig. 4D). A narrow ridge from themandibular condyle to the apical tooth, the condylar carina (cdc, Fig. 4D), forms the ventral mandibular margin.
At full closure, the medial / masticatory margins of themandibles are aligned along their length; only the shortand blunt triangular apical (ai, Fig. 4D, E) and subapicalincisors (sai, Fig. 4C, D) overlap. The blade is roughly
triangular in cross-section, with massive ventral andlateral portions and a thin medial portion towards themasticatory margin. In medial view, the blade is distally
bent downwards and appears cup- or bowl-shaped, witha slightly concave internal surface (Fig. 4B, E).
The tool edge of the mandible comprises a complex
of structures, including the masticatory margin (mm,Fig. 4C), the fimbrial line carina (flc, Fig. 4C, E), two specified sets of chaetae (“traction setae”) (mdb, Fig. 4B, D, E),
and thin sensory hairs (setae): The masticatory margin
consists of a crenulate bead forming a row of denticles (cb,Fig. 4C, E); this bead is closely subtended ventrally by the
fimbrial line carina (flc, Fig. 4C, E), which is a straight,
sharp rim that continues proximally along the basal margin. Closely subtending the fimbrial line carina ventrallyis the first specified line of chaetae, which is a single-filerow of very short and bristle-like chaetae that are even inlength and, with the masticatory margin, sandwich thefimbrial carina between sharp points. This chaetal line isventrally subtended by the second chaetal set; this dense
patch of robust and acuminate chaetae (mdb, Fig. 4A, B, D,
E) is curved toward the masticatory margin and graduallylengthens toward the lateral mandibular edge; the longest
chaetae are very slightly sinuate; all chaetae are set in deep45

Fig. 4: Volume renderings of the mandible, mandibular musculature, and mandibular gland of Tatuidris tatusia, based on microcomputed tomography scan. A, B: Mandibular muscles and gland. A: Dorsal view left, ventral view right. B: Sagittal view.
C - E: Details of the mandible. C: Dorsal view. D: Ventral view. E: Medial view. Abbreviations: 0md1 – M. craniomandibularis internus; 0md1ab – M. craniomandibularis internus apodeme; 0md3 – M. craniomandibularis externus; 0md8 – M.
tentoriomandibularis; ai – apical incisor; adc – adductor carina; al – atala; bm – basal margin; bmc – basal margin resting inclypeal groove; ca – canthellus; cac – canthellar carina; cag – canthellar groove; cb – crenulate bead; cdc – condylar carina;
dcl – dorsal canthellar lobe; dcs – distal mandibular contact surface; flc – fimbrial line carina; hyl – anterior lobe of the hypostomal process; hysp – triangular hypostomal process; ma – mandalus; md – mandible; mdag l – lateral mandibular acetabular
groove; mdag m – medial mandibular acetabular groove; mdb – mandibular chaetal brush; mdc – mandibular condyle; mdd– mandibular dorsal bulge; mdg – mandibular gland; fimbrial carina; mg – medial mandibular groove; mm – masticatory
margin; sai – subapical incisor; sms – sloping surface of proximal mandibular blade. Color marking: cyan – contact surfacesof clypeal condylar rail; purple – contact surfaces of clypeal condylar lobe.

pores (Fig. 5B) and have a surface pattern of oblique riffles.
Sensory setae are loosely arranged along the smooth region of the concave inner mandibular surface. The acetabular carina (adc, Fig. 4E) is a well-developed, oblique ridge
accompanied by an oblique row of setae, separating thefimbrial / brush area from the convex ventral mandibular
surface; distally, it reaches the subapical tooth.

46

The ventral (primary) mandibular condyle (mdc,
Fig. 4A, D, E), a short and narrow ventral projection on thecentral area of the mandibular stem, is slightly curved laterad; the corresponding articulatory fossa (vma, Fig. 2D)is placed at the very anterior edge of the ventral headcapsule on the hypostomal corner (hyc, Fig. 2C, D); thefossa is slightly wider than long; it is stabilized by theFig. 5: Longitudinal histological sections through the head of Tatuidris tatusia. A: Base of the mandible, showing part of themandalus and the inner mandibular gland. B: Tip of the mandible, showing the chaetal brush in its deep pores. C: Lateral headregion, section through the center of the 0md1 muscle and its apodeme, showing different fiber types. D: Roughly sagittal section showing labium, distal hypopharynx, and infrabuccal pouch. E: Section slightly laterad of B, additionally showing galea
and stipes. F: Roughly sagittal sections showing buccal tube. Abbreviations: 0hy7 – M. praementosalivarialis; 0hy12 – M.
hypopharyngosalivarialis; 0la12 – M. praementoglossalis; 0md1ad – M. craniomandibularis internus apodeme; 0md1ls – M.
craniomandibularis internus, long sarcomere fiber; 0md1ss – M. craniomandibularis internus, short sarcomere fiber; bpb –basiparaglossal brush; bt – buccal tube; cl – clypeus; dhy – distal hypopharynx; dw – dorsal (epipharyngeal) prepharyngeal
wall; ep – epipharynx; epm – epipharyngeal microtrichia; eplm – epipharyngeal lip microtrichia; fmo – functional mouth
opening; ga – galea; gl – glossa; glds – glossa dorsal sclerite; glvs – glossa ventral sclerite; hyb – hypopharyngeal rod; hym –hypopharyngeal microtrichia; hysp – triangular hypostomal process; ibp – infrabuccal pouch; lb – labrum; ma – mandalus;
md – mandible; mdb – mandibular chaetae brush; mdig – inner mandibular gland; mxc – maxillary comb; pgb – postgenal
bridge; pm – prementum; pma – premental arm; psm – postmentum; st – stipes; sv – salivarium; vw – ventral (hypopharyngeal) prepharyngeal wall.

47

longitudinal costae on the ventrolateral surface of thehead capsule, which converge in this point. The evenly
rounded atala (al, Fig. 4A, D) is a large lateral projectionof the mandibular stem; it reaches about half the verticalextension of the mandibular base and is inserted in a large
acetabulum laterad the oral foramen (alf, Fig. 2D). The
dorsal (secondary) mandibular articulation consists of an
L-shaped clypeal condyle, described in the head capsulesection above (ccr, cdl Fig. 2D), and the corresponding,complicated acetabular surfaces of the mandibular stem.
In the resting position, the condylar lobe is received by
the small lateral mandibular acetabular groove (mdagl,
Fig. 4C); the lateral acetabular groove is separated from a
medial acetabular groove by a short ridge (mdagm, Fig. 4C).
When the mandible opens, the condylar lobe traverses theridge and locks into the medial groove, as shown by our
3D-printed model manipulations (see section “Possible
mandibular motions” below). Anteromediad of the acetabular grooves, a smooth, broad area forms the distal
contact surface of the mandibular stem (dcs, Fig. 4C);
this smooth region serves as a secondary acetabular region receiving the condylar rail. Mesally, the surface isbordered by a lobe of the canthellus (dcl, Fig. 4C) and aslanted, slightly concave area on the medial side of themandibular stem, which forms the medial mandibular
groove (mg, Fig. 4C). According to our mechanical tests,
the condylar rail slides along the canthellar lobe and the
medial groove as the mandible opens (see section “Possible
mandibular motions”). The canthellus itself is an oblique
medial thickening on the dorsal half of the mandibularbase (ca, Fig. 4E); it is capped ventrally by a carina (cac,Fig. 4E) that overhangs a groove (cag, Fig. 4E) into which
the hypostomal process inserts when the mandible isflexed (closing) (Fig. 4B). The mandalus (ma, Figs. 4C, 5A)
is represented by a small, paddle-shaped area dorsally on
the mandibular stem, located laterad the medial groove(dorsal acetabular surface).
Musculature (Figs. 4A, B; 5A): Musculus craniomandibularis internus (0md1): by far the largest muscle in the head, filling most of the lateral headcapsule; O: all of the posterior, most of the ventral, partof the dorsal, and some of the lateral head capsule except
for the area of the antennal scrobe; I: Most fibers insert
directly on the massive, sheet-like adductor apodeme. Only
the outermost fibers of the muscle insert on the apodeme
through rather short, thin cuticular fibrillae. Additionally,
the fibers of a bundle originating dorsad of the occipitalforamen insert with thin cuticular fibers on an accessory
branch of the main apodeme. While sarcomeres are notvisible in the µ-CT data of the central, directly attachingfibers, they are clearly recognizable in the thread attachedfibers and also some of the directly attaching fibers at theedges of the apodeme, indicating that these may have
longer sarcomeres. A few muscle fibers are visible on our
histological sections, confirming that the fibers without
visible sarcomeres in µ-CT data have shorter sarcomeres(Fig. 5A). M. craniomandibularis externus (0md3):
O: mostly on the postgenal ridge and partly on the ventral

48

head capsule; I: with a short tendon on the ventral side ofthe large atala. M. tentoriomandibularis medialis
inferior (0md8): minute muscle of only two thin fibers;
O: ventrally on the anterior tentorial arm; I: dorsomesal
inner margin of the mandible.
Maxillae
The maxillae are closely connected to the labium by
a conjunctiva, thus forming the maxillolabial complex.
The cardines articulate with thin hypostomal cardinal
condyles (hc, Fig. 2D). The external stipital sclerite (st,Fig. 6C) is elongate, more than twice as long as wide; its
external surface bears a longitudinal furrow that is deepest
proximally; a deep oblique groove runs across the sclerite
distomedially (msg, Fig. 6C) and receives the distal labralmargin in resting position. Apicolaterally, the stipitalsclerite is produced as an acutely triangular process witha narrow, rounded tip, a concave medial side, and a convex
lateral side (stp, Fig. 6C); in resting position, this processpresses against the proximolateral labral process, locking
the labrum in place (lbrp, Fig. 7B). The lateral margin of
the stipes inserts into the concave anterior surface of thetriangular hypostomal process (Fig. 7A). Most of the stipital surface bears a scale-like pattern of cuticular ridges;
these ridges are arranged transversely on the proximal
region and longitudinally on the lateral region; the concavity in the stipital center is mostly smooth except forsome oblique rugae. The base of the inner stipital wall
forms a short inner stipital sclerite; it continues onto thebasal lacinial sclerite. The stipitopremental conjunctival
thickening (spc, Fig. 7B) is large, curved, and connects theinternal stipital sclerite with the premental arms and thehypopharynx at the lateral edge of the infrabuccal pouch(ibp, Fig. 7B). The large galeolacinial complexes are bent
across the labium and distal hypopharynx. The apex of the
galea (ga, Fig. 6C - E) is roughly triangular with roundededges, the remaining galea roughly rectangular. It bears
the maxillary comb of densely set, thick setae on its inner surface (mxc, Figs. 5C; 6E). The lacinia (lc, Fig. 6D,E) is roughly triangular and its margin is set with thick,spine-like hairs forming the lacinial comb (lcc, Fig. 6D).
The maxillary palp is reduced and lacks musculature.
The presence of a potential remnant of the palp could not
be verified here; Bolton (2003) recorded one maxillary
palpomere for Tatuidris tatusia.
Musculature (Fig. 6A, B): Musculus craniocardinalis externus (0mx1): O: posterior postgenal bridge
below occipital foramen; I: lateral proximal base of the
cardo. M. tentoriostipitalis anterior (0mx4): O: on
the anterior tentorial arm close to the anterior tentorial
pit; I: on a short process of the stipes in the region of the
cardinostipital hinge. M. tentoriostipitalis posterior(0mx5): two bundles; O: anterior bundle on the anteriortentorial arm on the level of the medial lamella and partly
on this lamella, posterior bundle posteriad the dorsal
tentorial arm on the anterior arm; I: uniting tendons of
the two bundles insert on the inner stipital sclerite. M.
stipitolacinialis (0mx6): O: in the distolateral region

Fig. 6: Volume renderings of the maxilla and maxillary musculature of Tatuidris tatusia, based on micro-computed tomographyscan. A, B: Maxillary muscles. A: Dorsal view left, ventral view right. B: Sagittal view. C - E: Details of the maxilla. C: Frontal
view. D: Posterior view. E: Medial view. Abbreviations: 0mx1 – M. craniocardinalis externus; 0mx3 – M. tentoriocardinalis; 0mx4 – M. tentoriostipitalis anterior; 0mx6 – M. stipitolacinialis; 0mx7 – M. stipitogalealis; cd – cardo; ga – galea;lc – lacinia; lcc – lacinial comb; msg – medial stipital groove; mxc – maxillary comb; st – outer stipital sclerite; sti – innerstipital wall; stp – stipital process.

of the external stipital sclerite; I: base of the lacinia. M.
stipitogalealis (0mx7): O: centrally on the external
stipital sclerite; I: with a short, thin tendon at the base of
the galea. M. stipitopalpalis externus (0mx8): absent.
Intrinsic muscles of the maxillary palp: absent.
Labium and distal hypopharynx
The short and narrow postmentum (psm, Figs. 5B;
8C, D) is located between the cardines and connected tothem by the articulatory conjunctiva; together with the
cardines, it is inserted in the deep hypostomal groove.
In resting position, the base of the maxillolabial complex
formed by the cardines and postmentum is covered by

the medial edges of the external stipital sclerites and thus
not visible externally (Fig. 7A). The ventral premental faceconsists of two roughly equally-sized parts, delimited by a
deep transverse furrow (pmf, Fig. 8B - D); this furrow receives the distal margin of the labrum at full labral closure(Figs. 7A; 8B). The proximal portion (pmvp, Fig. 8C, D) has
the shape of a circular sector, is irregularly wrinkled, andis the only part of the prementum visible externally in resting position; the distal premental portion (pmvd, Fig. 8C,
D) is semi-elliptic and completely smooth. The thin andclavate two-segmented labial palps (plb, Fig. 8C - E) are
inserted apicolaterally at the edge of the distal premental
portion; both palpomeres are flattened basally and widen

49

Fig. 7: Volume renderings of the head, labrum, and maxillolabial complex of Tatuidris tatusia, based on micro-computed
tomography scan. A: Frontoventral view of maxillolabial-labral locking. B: Lateral view of maxillolabial-labral locking. C - E:
Labrum. C: Outer surface. D: Lateral view. E: Inner surface. Abbreviations: asc – antennal scrobe; ccl – clypeal condylar
lobe; ccr – clypeal condylar rail; cd – cardo; clg – clypeal groove; cli – clypeal inflection; ga – galea; ibp – infrabuccal pouch;lb – labrum; lbrp – proximolateral labral process; lcl – lateral clypeal lobe (fused to frontal lobe); plb – labial palp; pmvp –ventral premental surface; spc – stipito-premental conjunctival thickening; st – stipes; stp – stipital process.

distally; the proximal palpomere is especially narrowbasally as it is squeezed between prementum and labrum
when the mouthparts are retracted, leaving only the apical
palpomere visible (Fig. 7A); the base of the spindle-shaped
apical palpomere is slightly curved mediad; it bears a
medium length seta at mid-level, a long seta apically, anda short one sub-apically. The glossa, placed far behindthe distal premental margin (gl, Fig. 8C - E), consists ofa rather narrow base and a broad lobe; the latter is set
with rows of curved microtrichia (Fig. 5B), resulting in amushroom-like appearance in lateral view; the extended
surface between the glossal base and the distal premental
margin is stabilized by the massive ventral glossal sclerite(glvs, Fig. 5B); ligamentous projections extend laterally
into the ventral region of the glossa from the ventral
sclerite; the similarly massive dorsal glossal sclerites(glds, Fig. 5B) stabilize especially the narrow base of theglossa. The basiparaglossal brushes (bpb, Figs. 5E; 8D, E)
are well-developed, but paraglossae are not recognizable.
The salivarium (sv, Figs. 5B; 8B) between glossa and distal hypopharynx is stabilized by a long U-shaped sclerite
ventrally and on the dorsal side by two longitudinallyoriented sclerotized bars, which posteriorly fuse with thehypopharyngeal buttons. The salivary duct (svd, Fig. 8A,B) is relatively thick, especially its posterior portion; it

50

forms a large, dorsally directed loop directly anteriad
the suboesophageal ganglion. The thin premental arms
(pma, Figs. 5C; 8D) are connected to the well-developed
hypopharyngeal rods (hyb, Fig. 5C) which stabilize thedistal hypopharynx laterally. The distal hypopharynx (dhy,Figs. 5B, C; 8D, E) is continuous with the large infrabuccal
pouch (ibp, Figs. 5B, C; 9D), which was deflated and almost
empty in the µ-CT scanned specimen; the small amountof material it contained partly appeared like compacted
small pieces of cuticle, judging from shape and histologicalstaining; a pattern of dorsal folds indicates that the pouch
can be inflated depending on feeding status.
Musculature (Fig. 8A, B): Musculus tentoriopraementalis (0la5): O: partly on posterior tentorialarm and partly on ventral head capsule close to occipitalforamen; I: tendons of the two sides fusing into one that
inserts on the proximal premental margin. M. praementoparaglossalis (0la11): absent. M. praementoglossalis (0la12): O: distal part of the ventral premental
face; I: base of the dorsal glossal wall / sclerite. M. praementopalpalis externus (0la14): very thin muscle;
O: premental arm; I: base of the proximal palpomere. M.
palpopalpalis labii primus / secundus (0la16 / 17):absent / not recognizable. M. tentoriohypopharyngalis (0hy3): O: posterior tentorial arm and small part of

Fig. 8: Volume renderings of the labium and labial musculature of Tatuidris tatusia, based on micro-computed tomography scan.
A, B: Labial muscles and salivary duct. A: Dorsal view left, ventral view right. B: Sagittal view. C - E: Details of the labium. C:
Frontal view. D: Lateral view. E: Dorsal view. Abbreviations: 0hy3 – M. tentoriohypopharyngalis; 0hy7 – M. praementosalivarialis; 0hy12 – M. hypopharyngosalivarialis; 0la5 – M. tentoriopraementalis; 0la12 – M. praementoglossalis; 0la14 – M.
praementopalpalis externus; bpb – basiparaglossal brush; dhy – distal hypopharynx; gl – glossa; lb – labrum; plb – labial palp;
pmd – premental ditch; pmf – premental furrow; pml – lateral premental surface; pmvd – distal ventral premental surface;pmvp – proximal ventral premental surface; psm – postmentum; sv – salivarium; svd – salivary duct.

the head capsule laterad 0la5; I: hypopharyngeal button.
M. praementosalivarialis (0hy7): O: distal premental part proximad 0la12; I: ventrally on the sclerotizedsalivarium. M. hypopharyngosalivarialis (0hy12):
O: dorsolaterally from the stabilizing sclerites of the distal hypopharynx (hypopharyngeal rods); I: dorsally andlaterally on the salivarium.
Labrum and distal epipharynx
The labrum is a trapezoidal plate with almost straight,
distally converging lateral edges (Fig. 7C); at full closure,
it is nearly vertically oriented (assuming a prognathous
head, Fig. 7B); its distal margin is concave and thus appears bilobed (Fig. 7A, C, E). The proximolateral labralprocesses (lbrp, Fig. 7B, D, E) are large, lobe-like, andabout half as long as the entire labrum; when the mouthparts are retracted, the distal processes of the external
stipital sclerites push against the proximolateral labralprocesses (Fig. 7B); this interlocking mechanism and thetight fit of the labral margin in the premental and stipitalgrooves lead to tight closure of the buccal cavity (Fig. 7A).
The external surface of the labrum is densely covered withthick, long, and apicomedially directed setae (Fig. 7C);
they are distributed across most of the surface apart fromthe distal margin and a triangular distal medial area; thesetae decrease in length from the base to the apex of thelabrum and are most densely set apicolaterally. The proximal internal margin of the labrum is continuous with theflexible distal epipharynx (ep, Fig. 9A, D), which is slightly
longer than the labrum; this anteriormost portion of the51

Fig. 9: Volume renderings of the digestive tracts, its muscles and glands, and the central nervous system of Tatuidris tatusia,
based on micro-computed tomography scan. A: Dorsal view of digestive tract, select muscles, glands, and the nervous system.
B: Ventral view of digestive tract, glands, and nervous system. C: Ventral view of cut open head capsule showing dorsal headmuscles. D: Sagittal view of digestive tract, muscles, glands, and nervous system. E - G: Distal digestive tract with focus onprepharynx and oral arms. E: Dorsal view. F: Lateral view. G: Posteroventral view. Abbreviations: 0bu1 – M. clypeobuccalis; 0bu2 – M. frontobuccalis anterior; 0bu3 – M. frontobuccalis posterior; 0bu5 – M. tentoriobuccalis posterior; 0ci1a – M.
clypeopalatalis, unpaired portion; 0ci1b – M. clypeopalatalis, paired portion; 0hy1 – M. frontooralis; 0hy2 – M. tentoriooralis;
0hy9 – M. oralis transversalis; 0lb2 – M. frontoepipharyngalis; 0pe1d – M. pharyngoepipharyngalis, dorsal portion; 0pe1dl–
M. pharyngoepipharyngalis, lateral dorsal portion; 0pe1l – M. pharyngoepipharyngalis, lateral portion; 0ph2 – M. tentoriopharyngalis; br – brain; bt – buccal tube; dhy – distal hypopharynx; ep – epipharynx; fg – frontal ganglion; fmo – functional
mouth opening; ibp – infrabuccal pouch; lb – labrum; nan – antennal nerve; no – optical nerve; oa – oral arm; oal – oral arm
lamella; oalw – oral arm lateral wall; oapr – oral arm process; ph – pharynx; phg – pharyngeal gland; pph – prepharynx;
pphg – prepharyngeal gland; sog – suboesophageal ganglion; toa – torular apodeme.

52cibarium is followed by the anterior / upper wall of thelong buccal tube.
Musculature (Fig. 9A, C): Musculus fronto­
epipharyngalis (M. 9 / 0bl2): O: frontal area shortly
posteriad the antennal bases, laterad 0bu1 and 2; I: upona thin, long tendon attached at the lateral inner cornersof the labrum.
Cephalic digestive tract
The anterior prepharynx (pph, Fig. 9A, B, D - G), especially the region of the buccal tube (bt, Figs. 5D; 9B, D,
F, G), is broad, about 4 / 7 as wide as the minimum widthof the head capsule internally (e.g., Fig. 9A, B); it narrows
gradually to slightly less than half this width towards thepharynx (Fig. 9E). The dorsal (epipharyngeal) prepharyngeal wall (dw, Fig. 5F) is partially robust, with a maximum
thickness of about 11 µm in the region around the insertion site of muscle 0ci1b and a minimum of about 2 µm
in the buccal tube close to the functional mouth opening(fmo, Figs. 5F; 9G); with about 7 µm thickness, the ventral(hypopharyngeal) wall of the prepharynx (vw, Fig. 5F) isslightly thinner throughout its length. The hypopharyngeal wall of the buccal tube is covered with scale-like, flatridges set with microtrichia (ca. 4 µm), with increasing
length (up to ca. 10 µm) towards the functional mouth
opening (hym, Fig. 5F); the dorsal (epipharyngeal) wallis set with thin, short microtrichia (ca. 2 - 3 µm); longer
microtrichia (up to ca. 8 µm) are only present on thewrinkled epipharyngeal wall, forming a lip-like structure
close to the functional mouth opening (epm, Fig. 5F); thetip of the lip bears a brush of longer microtrichia (ca. 15 20 µm, eplm, Fig. 5F) on its central area. To stabilize thebroad distal part of the prepharynx, the oral arms diverge
distally, appearing as mirrored angle brackets (><) on theventral (hypopharyngeal) wall (oa, Fig. 9G); the proximal
part of the oral arms, close to the anatomical mouth opening / frontal ganglion, curves sharply upward (Fig. 9F);
this curved part forms a broad, concave lateral wall (oalw,
Fig. 9E, F) and is continuous with broad dorsal plates, theoral arm lamellae (oal, Fig. 9E, F); posteriorly, the plates
are extended as short, laterally diverging processes (oapr,
Fig. 9E, F).
Musculature (Fig. 9A, C, D): Musculus frontooralis (0hy1): O: centrally in the frontal region(Fig. 9C); I: posteriorly on the oral arm process (Fig. 9A).
M. tentoriooralis (0hy2): almost twice as long as 0hy1;
O: on the base of the torular apodeme (Fig. 9C); I: anteriorly on the oral arm process (Fig. 9A). M. oralis transversalis (0hy9): transverse muscle of the prepharynx,
connecting the oral arm lamellae of both sides (Fig. 9A, D);a second, smaller transverse muscle bundle is also present on the ventral side of the pharynx at the level of 0bu3
(Fig. 9D). M. clypeopalatalis (0ci1): two distinct parts:
0ci1a: very small, unpaired muscle; O: anteriorly on theclypeus (Fig. 9C); I: buccal tube close to the functional
mouth (only partly visible in 3D reconstruction due to minute muscle size). 0ci1b: distinctly larger than 0ci1a andpaired; O: lateral region of the clypeus (Fig. 9C); I: along

a long area of the thickened dorsal prepharyngeal wall(Fig. 9D). M. clypeobuccalis (0bu1): flattened but broadmuscle; O: on cuticular swelling likely corresponding toepistomal sulcus (Fig. 9C, D); I: on posterior wall of dorsal
prepharyngeal depression (Fig. 9D). M. frontobuccalis
anterior (0bu2): O: frontal area at the level of the frontal
ganglion (Fig. 9C, D); I: dorsal prepharyngeal wall at thelevel of the frontal ganglion, curving around it (Fig. 9D).
M. frontobuccalis posterior (0bu3): O: frontal area
directly behind 0bu2 (Fig. 9C, D); I: directly behind 0bu2
but straight, not curved (Fig. 9D). M. tentoriobuccalis anterior (0bu5) (& possibly posterior, 0bu6):
two closely set bundles that either represent the anterior
and posterior tentoriobuccal muscle or just the anterior
one that is slightly separated in two parts; O: anteromedian process of the tentorial bridge (Fig. 9D); I: ventral
prepharyngeal wall anteriad of 0bu2 / the anatomicalmouth (Fig. 9D). M. pharyngoepipharyngalis (0pe1):
well-developed longitudinal musculature of the prepharynx. The main lateral bundles (0pe1l, Fig. 9A) connect the
concave lateral walls of the oral arms and the anteriorprepharynx close to the buccal tube, the much smaller
median bundles (0pe1dl, Fig. 9A) connect the dorsal oralarm lamellae and the central prepharynx. The unpaired
dorsal bundle (0pe1d, Fig. 9D) is short and extends fromthe pharynx around the level of 0bu3 to the prepharynx
around the level of 0bu1.
Cephalic glands
The mandibular gland consists of a moderately sized,
drop-shaped reservoir (Fig. 4A, B) that is cupped by roundto oval glandular cells with a diameter of around 20 - 25
µm (mdgc, Fig. 10A, B). The short gland duct enters themandibular base and opens on the mandalus (ma, Fig. 4C,
5A). The inner mandibular gland, first described by Billen& Delsinne (2014), is formed by spherical secretory cellswith a diameter of 25 - 30 µm that occupy most of the mandibular lumen (mdig, Fig. 10C, D). Each cell is connected
to a duct cell, and all ducts form a bundle that opens to theexterior through a sieve plate at the ventroproximal innerside of the mandible (mdigo, Fig. 10D). The pharyngeal
gland is the largest gland of the head (phg, Fig. 9A, B, D);
it opens into the pharynx close to the anatomical mouth
opening (phgo, Fig. 10E) and consists of a large sack ofglandular epithelium with a thickness of about 10 - 20
µm (Fig. 10F); its anterior lobes extend ventrally into thespace between the brain, the infrabuccal pouch, and theprepharyngeal gland (Fig. 9D); the posterior lobes almost
completely cover the brain dorsally (Figs. 9A; 10F). The
prepharyngeal gland (pphg, Figs. 9A, B; 10G, H) forms twopaired clusters of large cells laterad and below the anteriorprepharynx. The round cells have a diameter of 50 - 60
µm and are filled with numerous secretory vesicles thatstain darkly in histological preparation and most likely
contain digestive enzymes. Their accompanying ducts(pphgc, Fig. 10G, H) open through a small sieve plate in
the buccal tube close to the functional mouth opening(pphgo, Fig. 10H).

53

Fig. 10: Histological sections through the head of Tatuidris tatusia. A - F, H: Cross sections. G: Longitudinal section. A: Overview section showing position of the mandibular gland. B: Enlarged box from A. C: Overview of the mandible showing innermandibular gland. D: Detail of the inner mandibular gland opening. E: Opening region of the pharyngeal gland. F: Posterior
region of the pharyngeal gland and brain. G: Anterior digestive tract with prepharyngeal gland. H: Detail of the prepharyngeal
gland opening. Abbreviations: 0lb2 – M. frontoepipharyngalis; 0md1ad – M. craniomandibularis internus apodeme; asc– antennal scrobe; ata – anterior tentorial arm; br – brain; bt – buccal tube; cl – clypeus; ep – epipharynx; fr – frontal area;
frl – frontal lobe; hyg – hypostomal groove; lb – labrum; lbm – labium; md – mandible; mdb – mandibular chaetae brush;54

Fig. 11: Photographs of the 3D-printed model of the head capsule and mandible of Tatuidris tatusia, showing our hypothesis forthe mandible opening motion based on physical experiments with the model. We show the motion in medial (upper row), ventral(second row), dorsal (third row), and lateral (last row) view. Images were taken in Closed position (first column); Opening step 1(second column), in which the mandible performs an upward / outward rotation based on sliding of the dorsal mandibular stem
along the clypeal condylar ridge, the canthellar groove along the hypostomal process, the lateral mandibular acetabular groove
along the clypeal condylar lobe and the mandibular condyle mesially along its articulatory fossa; the Transition point (third column)
is reached when the clypeal rail sits in the medial mandibular groove, the canthellar groove loses contact with the hypostomal
process, the mandibular condyle rests in its mesal position (black arrow in ventral view), and the condylar lobe sits on top of theridge separating medial and lateral articulatory grooves; in Opening step 3 (fourth column), the mandible rotates outward andreaches its Open lock state (fifth column) as the clypeal condylar lobe completely locks into the medial mandibular acetabular
groove. Abbreviations: ccl – clypeal condylar lobe; ccr – clypeal condylar rail; adc – adductor carina; cac – canthellar carina;
hysp – triangular hypostomal process; mdag l – lateral mandibular acetabular groove; mdc – mandibular condyle; mg – medialmandibular groove. Color marking: cyan – clypeal condylar rail; green – medial mandibular; orange – triangular hypostomal
process; pink – clypeal condylar lobe; purple – lateral mandibular acetabular groove.

Possible mandibular motions
Mandibular opening has never been directly observedor documented in living Tatuidris tatusia. However, via
manipulation of a 3D-printed model of the head capsuleand mandible, we found that the mandible fits tightly
into the cranial contact surfaces, from the closed to thefully open posture. Because of this tight fit, we hypo­thesize here a mode of motion for the mandible based on
our handling of the model (Fig. 11). It should be notedthat this is a preliminary interpretation that does notaccount for soft structures and does not represent
a precise kinematic assessment of mandibular movement. Moreover, a mechanical model has not been
proposed for more “generalized” ants, thus the extendibility of our observations to other genera and subfamilies must be approached with caution, particularly
given the derived nature of the mandibular articulations. To replicate our manipulations, we have provided
our model as supplementary material for 3D printing(Zenodo DOI: 10.5281/zenodo.6046240).

mdgc – mandibular gland cells; mdig – inner mandibular gland; mx – maxilla; oa – oral arm; pgb – postgenal bridge; phg– pharyngeal gland; pph – prepharynx; phgo – prepharyngeal gland opening; pphg – prepharyngeal gland; pphgc – prepharyngeal gland canals; pphgo – prepharyngeal gland opening.

55

With the caveats mentioned above, we observe thatthe tight fit highly constrains the range of motion of themandible, revealing that two axes of rotation are required
to obtain maximal gape (~160°) and that the opening motion has three phases. The first phase of opening, startingwith the mandible at closure and comprising the first axisof motion, is an outward and upward (dorsal) rotation
where the masticatory margin becomes dorsomediallydirected from its medially directed position at full closure.
The second phase is a distinct and stable transition point.
The third phase, comprising the second axis of motion, isa posterolateral swing, which continues to the point where
the mandible physically cannot be opened further. At thismaximal gape, the masticatory margin is directed almost
completely anteriad. The first and third phases are eachstructured by a separate mechanism, which must be physically engaged in order to guide the mandibular motion.
Neither mechanism is engaged during the second phase,
which is transitional, and mandibular motion cannot bemade from the transition point without engaging one or
the other mechanism.
Mechanically, Phase I is defined by a tricondylic pivot-slide articulation with three main components: a pivot
and paired lateromedial rail systems. The pivot is formedby the primary (ventral) mandibular condyle, which fits
in the deep and narrow cranial fossa and which has alimited degree of lateromedial motion therein (see blackarrow, Fig. 11 mesal view, transition point). The slide isdicondylic, comprising the medial and lateral rail systems:
The medial rail is formed by the canthellar carina of themandible (cac, Fig. 11 medial view), which slides along
the inner hypostomal carina to the point of transition; thelateral rail is formed by the condylar lobe of the cranium(ccl, Fig. 11 lateral view), which slides from the lateral ace­tabular groove (mdagl, Fig. 11 lateral) onto the pyramidalridge of the mandible that separates the medial and lateral
acetabular grooves. When the lateromedially paired rails
are at the limit of their contact from Phase I and before
the mechanism of Phase III engages, the mandible is at astable point of transition, with very little play in its socket,
which we consider to be Phase II.
Phase III is a complex tongue-and-groove slidinghinge articulation with a lateral stop mechanism at maximum gape. The dicondylic sliding hinge is formed ventrally
by the primary mandibular condyle and dorsally by the
tongue-and-groove mandibulocranial articulation. While
the primary condyle simply pivots in its fossa, the medial
groove of the mandible (mg, Fig. 11 medial view) engages
with the condylar rail of the clypeus (ccr, Fig. 11 medialview) and guides the mandible to its maximal gape. Uponreaching the maximal gape, the mandible is stopped by
the lateral condylar lobe (ccl, Fig. 11 lateral view), whichitself fits tightly into the median acetabular groove of themandible, preventing lateral or posterior overextension.
The closure of the mandible is a complete reverse of thissequence. At total closure, the basal margin of the mandible contacts the inflected surface of the clypeal groove, another potential stop mechanism. Because Phases I and III

56

represent two axes of movement that are part of one continuous range of motion, punctuated by Phase II, the motion of the mandible may be conceived of as a “wanderingaxis” that is constrained by four condyles, of which three
are transitional while only the primary condyle is fixed.
Morphology of Tatuidris tatusia comparedwith other Poneria
Our comparative morphological observations of theworker caste across Poneria are summarized in Table 2 andFigures 12 - 17. The polarities of the mapped characters
were reasoned based on the principle of parsimony acrossthe preexisting topologies of Poneria (Romiguier & al.
2022), Ponerinae (Branstetter & al. 2017, Borowiec &al. 2019, Branstetter & Longino 2022), Amblyoponinae(Ward & Fisher 2016), and Proceratiinae (Borowiec & al.
2019), with groundplan conditions inferred from Richter& al. (2022) and Boudinot & al. (2022a). Inferences aboutmouthpart polarities and internal structures are limitedby the availability of fine anatomical data such as theSEM images from Keller’s (2011) atlas and completedµ-CT studies of head anatomy (Richter & al. 2019, 2020,2021a, 2022, also Hita Garcia & al. 2019 for externalfeatures of Discothyrea). In this respect, the observations are biased toward Tatuidris tatusia and those taxa
for which we have 3D renders. We have not considered†Eulithomyrmex in the present comparisons as this compression fossil has inadequate preservation of structuraldetail for meaningful interpretation, at least given thecurrently available documentation (Carpenter 1930 andspecimen UCM17019 on AntWeb 2022). Future work on
ponerine anatomy will absolutely advance the knowledgeof this clade, particularly through focused sampling of
Apomyrma, Ankylomyrma, and Paraponera, as well as
the morphologically highly modified ponerines, such as
Harpegnathos, Thaumatomyrmex, and the genera of the
Plectroctena clade.
Across Poneria, we tabulated 92 apomorphies (Tab. 2).
Of these, we counted 47 unambiguous and 13 ambiguous
autapomorphies, with the latter accounting for uncertainty
due to limited sampling (Fig. 12). The 13 ambiguous autapomorphies are restricted to Tatuidris tatusia. Considering only unambiguous conditions, T. tatusia has both themost total apomorphies and the most autapomorphies,
with 29 and 15, respectively (Figs. 13 and 14). The taxa
with the next greatest number of apomorphies based on
our present tabulation are Apomyrma stygia, Ankylomyrma coronacantha, and Paraponera (Paraponera
clavata + †Paraponera dieteri), which have 17, 16, and
12 apomorphies and 9, 5, and 4 autapomorphies, respectively (Figs. 15 - 17). Of the clades with > 100 species,
Amblyoponinae is the clade with the most derived features(7 apo-, 4 autapomorphies), followed by Proceratiinae(2 apomorphies, 1 autapomorphy).
We observe that Ponerinae is poorly defined morphologically, having no known unambiguous autapomorphies
based on our assessment. For the two synapomorphiesof this subfamily as recognized by Bolton (2003) –Tab. 2: Summary matrix of numbered apomorphies, based on the female castes, which are presented as a hypothetical transformation series in Figure 12 in order to sense the relative number of derivations in ponerine lineages. The presented charactersemphasize the cranium. Ground plan conditions drawn from the estimates of Boudinot & al. (2022a) and Richter & al. (2022),
following up on the intuition-based hypotheses of Bolton (2003). Characters and synapomorphies for Ponerinae, Amblyoponinae, and several for the Apomyrminae are drawn from Bolton (2003), Keller (2011), Boudinot (2015), and Boudinot & al.
(2022a), which are abbreviated as “B03”, “K11”, “B15”, and “B22” below; other characters were composed and scored based on
the present study. Due to the limited availability of completed micro-computed tomography (µ-CT) studies and scanning electron
microscopy (SEM)-imaged specimens, this list is largely restricted to external features. Also note that this is not an exhaustive
list of apomorphic conditions in the Poneria, and that not all conditions are illustrated. Note further that our sampling may misssubfamilial variation for some characters due to these limitations. While this level of variation would be unlikely to change ourinferences for deeper nodes, a more complete and formal reconstruction – outside the scope of this contribution – may refine
evolutionary interpretations. We reiterate that our sampling is limited, especially for the mouthparts, thus future studies would
benefit from systematic evaluation using SEM and µ-CT. # = number, Po = Ponerinae, Ap = Apomyrminae, Am = Amblyoponinae,
Pa = Paraponera, Pr = Proceratiinae, An = Ankylomyrma, Ag = †Agroecomyrmex, Ta = Tatuidris, 1 = state TRUE, 0 = state
FALSE, - = state inapplicable, ? = state uncertain, (#) = state known to be reversed within taxon, 0 / 1 = variation between statesobserved, 0 / ? = state 0 observed but state 1 not ruled out for all taxa, 1 / ? = state 1 observed but state 0 not ruled out for all
taxa. State 1 is used here for the reasoned apomorphic condition in the context of the Poneria.
#

Definition

Po

Am

Ap

Pa

Pr

An

Ag

Ta

1–2: Possible synapomorphies of Ponerinae
1

Frontal carinae and medial torular arches atleast partly “fused” (B03)

1

1

0

0

0

0

0

0

2

Frontal carinae / torular arches forming lobes
which distinctly curve posteromedially, thus
with “pinched” appearance (B03)

1(0)

0/1

0

0

0(1?)

0

0

0

3: Possible synapomorphy of Agroecomyrmecinae, also for Amblyoponinae plus Apomyrminae
3

Eyes situated in extreme posterior portion ofhead (B15, B22)

0(1)

0/1

1

0

0

1

1

1

4–6: Possible synapomorphies of Amblyoponinae plus Apomyrminae
4

Mandible not shovel-shaped (= “triangular”)(sensu Richter & al. 2022) (B22)

0(1)

1

1

0

0

0

0

0

5

At least one tooth occurring on mandibular
basal margin (B22)

0(1)

0/1

1

1

0

0

0

0

6

Labrum with chaetae (“dentiform” or “traction
setae”) (B22)

0

0/1

1

0

0

0

0

0

7–13: Apomorphies or possible synapomorphies of Amblyoponinae
7

Mandibles elongate thus relatively bar-shaped,
e.g., Stigmatomma (B22)

0(1)

1/0

0

0

0

0

0

0

8

Mandibular teeth large and triangular, fang-likein the form of, e.g., Stigmatomma rather thanBelonopelta (B22)

0

1/0

0

0

0

0

0

0

9

Anterior clypeal margin with chaetae (B03,
K11, B15, B22)

0

1(0)

0

0

0

0

0

0

10

Genal region with anterolateral spines (note 1)
(B22)

0

1/(0)

0

0

0

0

0

0

11

Petiolar tergum without distinct posterior face
(B03, B22)

0

1

0

0

0/1

1

0

0

12

Helcium weakly constricted posteriorly (B03,
B22)

0

1

0

0

0/1

0

0

0

13

Helcium supraaxial relative to a line drawn
through the midheight of the segment (K11,
B22)

0

1

0

0

0/1

0

?

1

. Probolomyrmex has antennal toruli that are directed dorsally rather than laterally, but these structures are close-set, being
only separated by the median longitudinal carina / lamina, whereas those of Apomyrma are wideset.
1

57

#

Definition

Po

Am

Ap

Pa

Pr

An

Ag

Ta

14–27: Possible synapomorphies of Apomyrminae
14

Median portion of clypeus raised and pyramidal(B15, B22)

0

0

1

0

0

0

0

0

15

Frontal carinae absent (B15, B22)

0

0

1

0

0

0

0

0

16

Antennal toruli wideset and directed dorsallyrather than laterally (B03, K11, B22) (see note 1)

0

0

1

0

0

0

0

0

17

Propleurae bulgingly muscular, i.e., strongly
convex and clearly visible in lateral view (B15)

0/1

0/1

1

0

0/1?

0

0

0

18

Promesonotal articulation modified,
hyperflexible (B03, B15, B22)

0

0

1

0

0

0

0

0

19

Propodeal lobes absent (B03, B15, B22) (seenote 2)

0

0

1

0

0

0

0

0

20

Petiolar tergum forming a pedunculate collar
around segment anteriorly, restricting sternite
to small posterior plate surrounded by two flat
lobes that are sutured anteroventromedially

0

0

1

0

0

0

0

0

21

Petiolar node subrectangular, resembling arounded inverted trapezoid (see Fig. 17)

0

0

1

0

0

0

0

0

22

Helcium infraaxial relative to a line drawn atsegment midheight (B22)

0/1

0

1

0

0

1

?

0

23

Abdominal segment III spiracles directedventrad due to bulging of the segment over thehelcium (B03) (see also Bolton 1990)

0

0

1

0

0

0

0

0

24

Abdominal tergum III fused posteroventrad
helcium, thus transverse sulcus present onventral surface (B03, K11, here)

0

0

1

0

0

0

?

0

25

Prora of abdominal sternum III absent (B22)

0/1

0/1

1

0

0

?

1

0

26

Abdominal segment IV not tubulated (B22)

0

0/1

1

0

0

?

?

0

27

Presclerites of abdominal segment IV
unconstricted (B03, B22)

0/1

0/1

1

0

0

0

0

0

1

1

1

28, 29: Possible synapomorphies of Paraponerinae, Proceratiinae, Agroecomyrmecinae
28

Promesonotal articulation fused, prothorax
immobile (see notes 3, 4)

0

0

0

1

1

. While Apomyrma appears to have propodeal lobes, these may be seen to be the posterolateral corners of the metapleural region,
rather than ventrolateral lobate processes of the carina bordering the propodeal foramen. That is, the apparent lobes of Apomyrma
are the “posterolateral process of the metapleural gland orifice” (mpgop) rather than the “lateral lobe of the propodeal foramen”(prffl) of Lieberman & al. (2022: fig. 4).
2. Two conditions previously used for subfamilial and tribal diagnosis by Bolton (2003) can now be reasonably polarized, thatof promesonotal and petiolar fusion (for the latter, see note 5 below). Fusion of the promesonotal articulation is an apomorphy
shared by Paraponerinae, Proceratiinae, and Agroecomyrmecinae, while loss of the promesonotal suture is an apomorphy sharedby Proceratiinae and Agroecomyrmecinae; this articulation is unsutured and capable of flexion in Ponerinae, Amblyoponinae,
and Apomyrminae. In contrast to Paraponerinae, the metanotal spiracle is lost in Proceratiinae and Agroecomyrmecinae, whichhave complete mesosomal fusion. Loss of the promesonotal suture gives the mesosoma of these two subfamilies a dome-shaped
appearance, with elongation of the mesosoma resulting in a tubular shape in Probolomyrmex. We expect that the compactionof the mesosoma conceals additional, internal synapomorphies of Paraponerinae, Proceratiinae, and Agroecomyrmecinae. Seenote 4 below for more structural detail about the promesonotal suture.
3. When the pro- and mesonota are fused, the promesonotal suture forms. This suture passes from the posterolateral corner of the
pronotum through the mesothoracic (anterior) spiracle of one side over the mesosomal dorsum and down through the spiracleof the other side to the corner. The spiracle, therefore, divides the suture into dorsal and ventral portions. The dorsal portion is
obliterated in both Proceratiinae and Agroecomyrmecinae, while the ventral portions are retained in the latter and obliterated
in the former.
4

58

#

Definition

Po

Am

Ap

Pa

Pr

An

Ag

Ta

29

Petiole with complete tergosternal fusion (seenote 5)

0

0

0

1

1?(0)

1

?

1

30–36: Possible synapomorphies of Paraponerinae
30

Scrobe developed, capable of receiving scapeand flagellum

0

0

0

1

0

1

1

1

31

Body size large, i.e., head width > 3.5 mm (seeBreed & Harrison 1985)

0/1

0

0

1

0

0

0

0

32

Scrobe above/ mediad compound eye

0

0

0

1

0

0

0

0

33

Paramedian clypeal processes and lateralnotches developed

0/1

0

0

1

0

0

1

0

34

Paired pronotal tubercles developed

0(1)

0

0

1

0

1

1

0

35

Tibiae rectangular in cross-section, tarsomeres
widened, tarsomeres II–V covered ventrallywith dense setal brushes

0

0

0

1

0

0

0

0

36

Petiole hatchet-shaped, i.e., pedunculate andwith angular anterodorsal corner of the node(B03, B15)

0

0

0

1

0

0

0

0

37, 38: Possible synapomorphies of Proceratiinae plus Agroecomyrmecinae
37

Mesosomal segments completely fused,promesonotal suture absent or vestigial andmetanotal spiracle vestigial or absent (see notes
3, 4)

0

0

0

0

1

1

1

1

38

Abdominal tergum IV strongly vaulted (see note
6 and Figs. 13, 16, 17)

0/1

0

0

0

1(0)

1

1

1

39, 40: Possible synapomorphies of Proceratiinae
39

Toruli situated near to (touching) or exceeding
anterior clypeal margin

0/1

0/1

0

0

1

0

0

0

40

Ventral promesonotal suture, completely fused(externally obliterated or sulcate) ventrad theanterior/ mesothoracic spiracle (see note 4)

0

0

0

0

1/0

0

?

0

41–44: Possible synapomorphies of Agroecomyrmecinae
41

Scrobe below/ laterad compound eye

0

0

0

0

0

1

1

1

42

Scapes thickened distally, medial (apparent
dorsal) articulatory ridge enlarged andelongated

0

0

0

0

0/1

1

1

1

43

Postgenal bridge less than half of head length(corresponds to length of mandibular abductor/
opener muscle 0md3 origin in Tatuidris)

0

0

0

0

0/1

1

?

1

44

Abdominal segment III strongly petiolated (seeFigs. 13, 16, 17)

0

0

0

0

0

1

1

1

. This new polarity hypothesis with respect to petiolar tergosternal fusion is dependent on the topologies of the Poneria andclades therein. The following observations may be made (see also Bolton 2003): The petiole is unfused in Ponerinae, where thelaterotergites are usually present; the petiole is unfused and lacks laterotergites in many Amblyoponinae, although almost no
amblyoponine has complete fusion (F. Esteves, pers. comm., draws attention to the complete fusion of Onychomyrmex doddi);
it is unfused and possibly with laterotergites in Apomyrma; in contrast, it is without laterotergites in Paraponerinae, Proceratiinae, and Agroecomyrmecinae, and is completely fused in Paraponerinae and Agroecomyrmecinae, and partially fused in
at least some Proceratium (Proceratiinae). The variability in Proceratiinae, including complete absence (or loss) of fusion in
Probolomyrmex, raises the uncertainty of the proposed hypothesis. Notably, there is known variability in tergosternal fusion,
as loss of this condition has also been observed for the abdominal segment IV in Amblyoponinae via Adetomyrma Ward, 1994
(e.g., Ward & Fisher 2016).
5. Species of the ponerine genus Loboponera have vaulted abdominal terga IV, for example, representing an instance of convergence with the Proceratiinae + Agroecomyrmecinae clade.

6

59

#

Definition

Po

Am

Ap

Pa

Pr

An

Ag

Ta

45–47: Apomorphies within Agroecomyrmecinae that depend on fossil placement
45

Antennomere count reduced, less than 12 (seenote 11 in †Agroecomyrmex)

0

0/1

0

0

0/1

0

1

1

46

Petiole extremely supraaxial and nearlyvertically aligned, with posterior foramen raised
high above anterior foramen (e.g., Boudinot
2015; it may be reversed in Ankylomyrma,
whose condition is similar to that in someAmblyoponinae, but without a supraaxial
helcium)

0

0

0

0

0

0

1

1

47

Postpetiole cylindrical and cheese-wheel-like,with sternum large and broadly convex

-

-

-

-

-

0

1

1

48–54: Possible synapomorphies of Ankylomyrma
48“Crown of thorns” flange developed alongposterior head margin, anteriad occipital carina

0

0

0

0

0

1

0

0

49

Clypeus produced anteriorly as a lateromediallybroad lobe

0/1

0

0

0

0

1

0

0

50

Postoccipital region encircled by dorsal andventral occipital carinae

0/?

0

0

0

1?/?

1

?

0

51

Paired spines developed on mesonotum,
propodeum, and petiole

0

0

0

0

0

1

0

0

52

Sternum of abdominal segment IV, whenforming postpetiole, greatly reduced in size,
being much shorter dorsoventrally than tergum

-

-

-

-

-

1

0

0

53

Abdominal tergum IV completely ovoid andwith strong, laminar rim encircling terminal
segments

0

0

0

0

0

1

0

0

54

Abdominal spiracle IV situated very distantfrom postpetiole (see Figs. 13, 16, 17)

0

0

0

0

0

1

?

0

55–92: Synapomorphies of Tatuidris
55–60: Bite force optimized
55

Head capsule broader than long (not accounting
for lateral spines of Ankylomyrma)

0

0

0

0

0

0

0

1

56

Head posteriorly enlarged forming massive
posterolateral lobes and posterior declivity
dorsad postocciput

0

0

0

0

0

0

0

1

57

Mandibular adductor apodeme enlarged andsheet-shaped (see note 7)

0/?

?

?

?

?

?

?

1

58

Adductor muscle massively enlarged and only
short tendon-fibrillae present (see note 7)

0/?

?

?

?

?

?

?

1

59

Anterior arms at right angle to long posteriorarm, resulting in elevated position in headcapsule (see note 7)

0/?

?

?

?

?

?

?

1

60

Resting position of head subprognathous (close
to orthognathous) (note: due to postoccipitalangle and position)

0

0

0

0

0/?

0

?

1

0

?

?

0

0

?

?

1

61–66: Mandibular articulation modified
61

Mandibular acetabulum of cranium produced toapex of hypostoma (see note 8). Internal anatomy and fine-scale morphology based on comparisons with Brachyponera (Richter & al. 2020).

7. Examination of the 3D models provided in Hita Garcia & al. 2019 (https://sketchfab.com/arilab/collections/discothyrea)
suggests that the fimbrial carina may form part of the gripping edge of the mandible in Discothyrea; this will be of high value
to formally evaluate. Based on images from Keller’s (2011) atlas, the fimbrial carina is neither elongate nor forming part of thegripping edge in Proceratium or Probolomyrmex.
8

60

#

Definition

Po

Am

Ap

Pa

Pr

An

Ag

Ta

62

Medial portion of clypeal condyle elongated,
forming clypeal condylar rail (first part of pivotslide mechanism)

0

?

?

0

0

0

?

1

63

Lateral portion of clypeal condyle shortened,clypeal condylar lobe incurved (first part ofstop-lock mechanism)

0

?

?

0

0

0

?

1

64

Atala massively enlarged compared to
mandibular stem size with consequent
broadening and deepening of atalar acetabulum

0

0

0

0

0

0

0

1

65

Proximomedial lobate process (canthellar lobe)
developed on mandibular stem (second part ofpivot-slide mechanism)

0

?

?

0

0

?

?

1

66

Mandibular acetabulum medially divided
by pyramidal process with medial portion
forming deep fossa (second part of stop-lockmechanism)

0

?

?

0

0

0

?

1

67–72: Mandibular grip enhanced
67

Blade of mandible robustly thickened andstrongly arched, thus bowl-shaped

0

0

0

0

0

0

?

1

68

Masticatory margins aligned for their whole
length, thus mandibles not overlapping

0

0

0

0

0

0

0

1

69

Masticatory teeth in form of numerous, evencrenulations

0

0

0

0

0

0

0

1

70

Fimbrial carina elongate and forming primarycontact margin for gripping (see note 9)

0

0

0

0

0/1

0

?

1

71

Chetae of first fimbrial field comb-like,sandwiching fimbrial carina ventrally with themasticatory margin

0

0

0

0

0/1

0

?

1

72

Robustly thickened, curving chaetae developedin a second fimbrial field

0

0

0

0

0

0

?

1

73, 74: Antennal mechanical protection enhanced
73

Frontal region expanded such that antennal
scrobe encompasses entire lateral head surface,and antennal toruli wideset from each other

0

0

0

0

0

0

0

1

74

Scrobe deepened and widened such that entire
antenna capable of concealment, consequently
antennal toruli upside-down and scapeproximally bent

0

0

0

0

0

0

0

1

75–78: Maxillolabial complex mechanical protection enhanced
75

Proximolateral labral processes enlarged (firstpart of lateral labromaxillary lock mechanism)(see note 9)

0/1

0/1

?

0

1

?

?

1

76

Distal stipital processes elongated (second partof lateral labromaxillary lock mechanism) (seenote 10)

0/1

0/1

?

0

0

?

?

1

77

Labral distal notch both shallow and fitting into
the premental furrow (see note 11)

0/1

0/1

0

0

0

?

?

1

. The specimen of Proceratium pergandei imaged in Keller’s (2011) SEM atlas (ANTWEB1008584) has a disarticulated labrum. The lateral processes are as large as those of Tatuidris tatusia. We were unable to evaluate the labra of other proceratiines.
9. The specimens of Discothyrea oculata (ANTWEB1008518), Proceratium pergandei (ANTWEB1008584), and Probolomyrmex
guineensis (ANTWEB1008582) in Keller’s (2011) SEM atlas appear to have elongate distal maxillary processes; they are of a
distinct shape relative to Tatuidris, and thus are scored 0 here.
10. The distal labral notch of the proceratiines imaged in Keller’s (2011) SEM atlas may be shallow, but all of them have labra
that overlap the prementum, rather than fitting into the premental groove, thus this lock mechanism is not formed.
11

61

#

Definition

Po

Am

Ap

Pa

Pr

An

Ag

Ta

78

Transverse premental furrow/groove clearly
developed (part of the labrolabial lockmechanism) (see notes 12, 13)

0/1

0/1

0

1

0

1

?

1

79–92: Other apomorphies
79

Cuticle highly polished in appearance (also,
thick)

0/1

0/1

0

0

0

0

0

1

80

Clypeal furrow developed as deep groove

0

0

0

0

0

0

?

1

81

Anterior margin of clypeal lobe laminar

0

0

0

0

0

0

0

1

82

Antennomere count further reduced (seenote 14)

0

0/1

0

0

0/1

0

0

1

83

Scape clavate (presumably flagellar muscles
strengthened)

0

0

1

0

0/1

0

0

1

84

Antennomere proportions and sensillaarmament of a complex form with enlarged
apical antennomere with a group of sensillaplacodea ventrally

0/1

?

0/?

0/?

0/1

0/?

0/?

1

85

Palpomere count reduced (1,2 or 0,2?) (seenote 15)

0/1

0

0

0

0/1

0

?

1

86

Dorsal tentorial arm elongated, longer than
2/5 of anterior arm

0/?

?

?

?

?

?

?

1

87

Ventral thoracic contact surface of headwidened (i.e., ventral occipital carina shifted
orad)

0/?

0/?

0

0

0

0

?

1

88

Secondary ventromedial postoccipital process(subforaminal process) developed (see note 16)

0/?

?

?

0

1?

?

?

1

89

Posterior delimitation of clypeus weakly
developed; supraclypeal area indistinct

0

0

1

0

0

0

0

1

90

Compound eyes reduced

0/1

1

1

0

0/1

0

0

1

91

Protibia with conspicuous apical seta brush on
anterior/dorsal surface

0

0

0

0

0

0

0

1

92

Mesometapleural suture obliterated (= notsulcate) ventrad metathoracic (posterior)spiracle

0/1

0/1

1

0

0/1

0

?

1

. The transverse premental furrow is absent in a narrow majority of the ponerine specimens imaged in Keller’s (2011) atlas. The
furrow is absent, indistinctly developed, or otherwise modified beyond direct recognition in the following: Platythyrea, Ponera,
Cryptopone, Dinoponera, Neoponera apicalis (but present in Neoponera villosa), Belonopelta, Harpegnathos, Brachyponera,
Centromyrmex, Dolioponera, Hagensia, Ophthalmopone, Paltothyreus, Phrynoponera, Streblognathus, Leptogenys (variable),Anochetus, and Odontomachus. All taxa from the atlas that are not listed here have a distinct furrow, with the exception of Emeryopone, for which the imaged specimen has had the maxillolabial complex dissected out.
12

. From Keller’s (2011) atlas, the proximal prementum of Discothyrea testacea (ANTWEB1008519), Proceratium pergandei,
and Probolomyrmex guineensis can be seen to be longer than that of Tatuidris.

13

14. See also Hita Garcia & al. (2019) for anatomical details about antennomere count reduction in Discothyrea.

15. Scoring for Amblyoponinae based on Bolton (2003).

. The specimen UFV-LABECOL-000032 of Discothyrea bobi, dissected and imaged by Júlio Chaul (Univ. Federal do Viçosa),
appears to have this proximomedial process of the postocciput.
16

62

Fig. 12: Summary chronogram adapted from Borowiec & al. (2019) and Romiguier & al. (2022) displaying relationships, ageestimates, and potential morphological transformation series within Poneria. Numbers: red = reasonable apomorphies; ochre= traits that are polymorphic across the given clade; blue = reasonable reversals; black = count of autapomorphies. Ambiguous
apomorphies in terms of total autapomorphy count indicated by square brackets []. Tatuidris tatusia has 15 unambiguous and
13 ambiguous autapomorphies (i.e., which could not be evaluated across multiple terminals due to sampling limitations, chars
57 - 59, 61 - 63, 65, 66, 75, 76, 84, 86, 88 of Tab. 2), while three reasonable apomorphies are shared between †Agroecomyrmex
and Tatuidris. The blue bar on the Agroecomyrmecinae node is the 95% highest probability densities for the age of the node from
Borowiec & al. (2019). Colored boxes in the background correspond to major changes in global temperature based on the dataof Zachos & al. (2001): light yellow = hot Earth; dark yellow = Eocene optimum; first blue = Oligocene cool Earth; second blue
= early Miocene warm period; third blue = late Miocene cold period; fourth blue = Pliocene to Pleistocene colder period. Major
or otherwise notable extinction events are indicated by vertical bars: K / Pg = end-Cretaceous crisis; PETM = Paleocene-Eocene
thermal maximum; TEE = terminal Eocene event; MMD = mid Miocene disruption. See Table 2 for numbered morphologicaltransformations. Note that the locations of the tick marks on the chronogram are not intended to indicate the exact geologicaltime when the transformations occurred.

“pinching” of the frontal carinae and “fusion” of the medial
torular arch and frontal carina – the former is reversedin Platythyrea, and the latter is homoplastic with respect
to some Amblyoponinae. There are definitely complex
patterns of variation within Ponerini (e.g., Schmidt &Shattuck 2014), but these derivations are not tabulated
here as Ponerinae are not a focus of our study. Extensive
study of Ponerinae, particularly the highly derived lineages, is desirable for future study.
For the newly recovered “PPA” clade (Paraponerinae+ Proceratiinae + Agroecomyrmecinae) and “PA” clade(Proceratiinae + Agroecomyrmecinae) (Romiguier &al. 2022), we recognize two synapomorphies each plus a
new synapomorphy for Proceratiinae. In contrast to other
Poneria, the promesonotal articulation is fused in the PPA
clade, with the dorsal suture retained in Paraponera andobliteration of this suture in the PA clade as a synapomorphic condition. The PPA clade may also be definedby complete tergosternal fusion of the petiole, although
there is variability as the fusion is absent or incomplete inseveral Proceratiinae. The PA clade has a strongly vaulted
abdominal segment IV. Vaulting and the fusions of thepromesonotum and petiole have been recorded in priorstudies (e.g., Bolton 2003, Keller 2011), but their polarities were not possible to reason out previously due totopological uncertainty. Finally, whereas Agroecomyrmecinae and Discothyrea retain a fine, external markingof the ventral (lateral) promesonotal suture, in other
Proceratiinae, this marking is reduced (Proceratium) tocompletely obliterated (Probolomyrmex), completing a
trend of mesosomal fusion.

Discussion
Overview
Among all ants, Tatuidris tatusia is one of the mostalien in appearance, being lumbering, tank-like predators,
with a highly compact mesosoma, strongly arched gaster,long sting, and a powerful, massive head that bears deep
antennal scrobes. The head is “oversized” in comparison
with other ants, approaching or exceeding the individual volumes of the mesosoma and metasoma. Like themesosoma and the “waist”, the cuticle of the cranium
and mandibles is thick and strongly sclerotized, likely
providing substantial mechanical protection as well as
structural integrity. We hypothesize that the specific headarchitecture of T. tatusia results in high biting strength,mechanical protection, and balance via significant expansion of the frontal area in all directions, which also
results in the highly unusual “shield-like” cranial form.
We group these and other modifications that we record forthe head of T. tatusia into six sets of structural charactersof the head skeleton and the head appendages based on
their presumed mechanical consequences (mechanical63

Fig. 13: Summary of morphological characters of Agroecomyrmecinae, based on photographs of point-mounted specimens
and an amber fossil from AntWeb (2022). A, B: Tatuidris tatusia. C, D: Ankylomyrma coronacantha. E, F: †Agroecomyrmex
duisburgi. A, C, E: Full face view of the head. B, D, F: Lateral view of whole body, oblique in F. B’: Detail of the petiole andpostpetiole of T. tatusia. Characters are marked by number, corresponding to Table 2. Image credits: T. tatusia CASENT0423526(A, B), CASENT0178755 (B’), and Ankylomyrma coronacantha CASENT0005904 by April Nobile; † Agroecomyrmex duisburgi
BMNHP18831 by Vincent Perrichot.

“faculty” but not “biological role”; Bock & von Wahlert1965). The modifications of these structural sets appear to
be broadly related to: (1) mandibular closing kinematics(the mandible adductor / closer muscle 0md1, the forehead, and posterolateral bulges); (2) mandibular opening
kinematics (the mandibular abductor / opener muscle

64

0md3, the postgenal box); (3) head orientation / balance(the cervical articulation, that is, the postocciput and its
surrounding cuticular surfaces); (4) mouthpart interlocking (the oral foramen, that is, the clypeus and mouthpart
articulation surfaces); (5) antennal kinematics / protection(the antenna, the antennal scrobe, and torular apparatus);

Fig. 14: Summary of morphological characters of Agroecomyrmecinae, based on scanning electron micrographs of a dissected
specimen of Tatuidris tatusia, based on the morphological atlas of Keller (2011), accessed from AntWeb (2022). A: Head infrontal view. B: Ventral view of apical antennomere, showing sensilla. Abbreviations: cl – clypeus; ess – epistomal sulcus; fr– frontal area; sc – sensillum coeloconicum; sss – sickle shaped sensillum. Characters are marked by character number detailedin Table 2. Image credits: ANTWEB1008593 by Roberto Keller.

Fig. 15: Summary of morphological characters of Agroecomyrmecinae, based on photographs of a point-mounted, dissected
specimen of Ankylomyrma coronacantha, from AntWeb (2022). A: Head in ventral view. B: Mandible in outer view. Characters
are marked by number, corresponding to Table 2. Image credits: CASENT0005904 (A) by Estella Ortega, CASENT0902015 (B)by Will Ericson.

and (6) spatial organization in the head (the tentorium
bearing muscular origins for the antenna and maxillolabial complex). We propose these sets for biomechanical /physiological studies in concert with ecological studies tobetter understand in how far the structural modificationsof T. tatusia are “adaptations” to their ecology and lifestyle.
For this contribution, we hypothesize potential biologicalroles of the modifications found in T. tatusia from ananatomical-functional perspective based on the limited
knowledge available. Additionally, we assess the functional
morphology of the mandible, we compare the pharyngeal
apparatus with that of previously studied species, andwe provide special considerations on the evolution of T.
tatusia in the context of the Poneria and relative to otherhighly specialized and depauperate ant lineages. Notethat the sets named above are only partly congruent withthose used to organize characters in Table 2. The latter are
convenient groupings to organize our entire character list,
partly based on potential functions, the former are more
specific structural modifications with possible mechanical consequences that we want to discuss here in moredetail.
Hypothesized adaptations relating to aggression, defense, and balance
The visually most striking modifications of the Tatuidris tatusia head, compared with species retaining more
plesiomorphies of the ant ground plan (e.g., Richter & al.
2022), are the shortening and broadening of the cranium,
the lateral and posterior expansion of the frontal cephalicsurface, and arguably thickening of the cuticle. We proposethat all of these simultaneously enhance power (through65

Fig. 16: Summary of morphological characters of Paraponerinae and Proceratiinae, based on photographs of point-mounted
specimens from AntWeb (2022). A, B: Paraponera clavata. C, D: Proceratium creek. E, F: Probolomyrmex guineensis. A, C,
E: View of the head, full face. B, D, F: Lateral view of whole body. Characters are marked by number, corresponding to Table 2.
Image credits: Paraponera clavata CASENT0006789 and Proceratium creek CASENT0104439 by April Nobile, Probolomyrmex
guineensis CASENT0249253 by Shannon Hartman.

mandibular muscles, see, e.g., Gronenberg & al. 1997),
protection, and balance.
The shortening of the head is evinced by the relative
length of the postgenal bridge (set 2), which is ~42% of thetotal head length in Tatuidris tatusia. This relative lengthis similar to that observed in stem ants (such as †Gerontoformica Nel & Perrault, 2004) and prognathous66

Aculeata (such as Methocha Latreille, 1804 and Ampulex Jurine, 1807) but is distinctly shorter than occurs inmany other crown Formicidae (see Richter & al. 2022).
A potential limit to this shortening is the length of thearea of origin of the mandibular abductor / opener muscle(0md3), which extends across the entire postgenal ridge
surface in T. tatusia (Fig. 4A, B); a case not yet observed

Fig. 17: Summary of morphological characters of Ponerinae, Amblyoponinae, and Apomyrminae, based on photographs ofpoint-mounted specimens from AntWeb (2022). A, B: Platythyrea lamellosa. C, D: Fulakora bath5. E, F: Apomyrma stygia.
A, C, E: View of the head, full face. B, D, F: Lateral view of the whole body. Characters are marked by number, corresponding
to Table 2. Image credits: Platythyrea lamellosa CASENT0252018 by Bradley Reynolds, F. bath5 QMT152681 by Erin Prado,and A. stygia CASENT0000077 by April Nobile.

in other ants (Richter & al. 2019, 2020, Boudinot & al.
2021, Richter & al. 2021a, 2022). The expansion of thefrontal surface correlates with two other modifications:the shift of the antennal scrobes to the lateral sides of thehead (set 5) and the presence of the massive, muscle-filled
posterolateral bulges that extend onto the ventral side of
the head (Fig. 1C, set 1), resulting in the posterodorsal cranial declivity around and above the postoccipital complex(set 3). The posterolateral bulges also minimize the expo-

sure of the cervical region, another potential defensive
benefit.
In addition to these modifications, the cuticle is relatively thick, especially in the dorsal and lateral regions of
the head – likely improving mechanical protection andintegrity – and the mandibular adductor muscles areenormously enlarged. A relatively thick cuticle was also
recorded for other Poneria, especially members of theclosely related Proceratiinea (Peeters et al. 2017), but

67

ecological and phylogenetic patterns are not understood in
detail at the moment. Although we do not have quantitative(i.e., volumetric) data for the entire body, it is apparent
that the head of Tatuidris tatusia is massive comparedwith the mesosoma (Fig. 1). Thus, it is worth considering
how the head balances. Similar to other large-headed
ants (such as Acanthomyrmex Emery, 1893 and Pheidole
Westwood, 1839 soldiers; see Moffett 1986, Wilson2003), we observe that the postocciput is situated close
to the mouth due to the short postgenal bridge, thus lying
well beneath the mass of the posterior and posterolateralcranial elongations.
Modifications of the antennal apparatus andtentorium
Due to the lateral expansion of the frontal area of thehead, the frontal lobes broadly overhang the wide and deep
antennal scrobes dorsally, and the antennal articulations
are oriented nearly upside-down, that is, facing the underside of the head rather than the dorsal / facial direction.
The completely inverted antennal socket is likely uniqueamong ants (Keller 2011). However, other ant species
with wide frontal lobes may also approach this condition(e.g., Phalacromyrmex Kempf, 1960; see Bolton 2003
and Donoso 2012). A nearly complete inversion was also
recently observed in a pselaphine beetle (Beutel & al.
2021). Together with the deep antennal scrobe that provides coverage for almost the entire antenna and the clavate scapus, this configuration results in a high degree ofprotection for the antennae of Tatuidris tatusia, although
it likely also restricts antennal movement. This restriction
is especially emphasized by the large torular lobe, whichmechanically “forces” the antenna to rest inside the scrobe
when it is retracted (Fig. 3C).
Corresponding to the far anterolateral position andinverted orientation of the antennal socket, a suite of tentorial modifications occurs (set 6). The attachment area
for the extrinsic antennal muscles faces almost directly
toward the antennal socket due to the shape of the anteriorarm and, notably, the twist of its medial lamella (Fig. 3A,
B). This condition presumably results in optimized useof the available space for muscle attachment and is likely
necessary to reach the upward-facing rim of the bulbus. If
the antennal muscles originated further ventrally relative
to the antennal socket, as in other ants (e.g., Kubota &al. 2019, Richter & al. 2019, 2020), their tendons would
have to curve downward to reach the bulbus, which might
be mechanically unfavorable. A considerable portion of
Musculus tentorioscapalis lateralis (0an3) originates on
the dorsal tentorial arm. As this shifts its origin further
dorsad compared with other ants studied so far (Richter& al. 2019, 2020, 2021a), this can likely be seen in the samefunctional context. In Tatuidris tatusia, this antennal
muscle configuration is also supported by elongation anda 90° bend of the posterior tentorial arms, which result inan overall elevated position of the tentorium in the lumenof the head. This may also allow for maximal use of spaceby the mandibular adductor muscles.

68

Some of these modifications are an interesting parallel to the condition in Protanilla lini Terayama, 2009,
where the zigzag-shaped anterior arms and the twist of
the medial lamellae presumably also optimize the attachment area for the extrinsic antennal muscles to thealmost anteriorly directed antennal socket. Richter &al. (2021a) hypothesized that the origin of 0an3 on thedorsal tentorial arm could be a plesiomorphic condition
retained in Leptanillomorpha. Even though this may
be the case, the observations made in the present study
suggest that such a condition could have evolved (or reevolved) several times in Formicidae and other groupsof Aculeata (Richter & al. 2022), indicating that this isa highly variable condition possibly linked with specificfunctional or spatial constraints.
Mandibular form and function
The mandibles, the mandibular articulations, and the
adductor muscles of Tatuidris tatusia are modified in a
way that may enhance the physical capacities for powerexertion, energy transfer, and grip (see section “Possible mandibular motions” above, and Fig. 12, Tab. 2). The
mandibles are massive, with a bowl-shaped blade thatbears a dense brush of chaetae subtending two rims; oneis crenulate and corresponds to the “masticatory margin”
of other ants, the other is sharp, straight, and corresponds
to the fimbrial line carina (Fig. 4). The shape of the bladeis nearly unique among ants, with parallels only observedin the myrmicines Ishakidris Bolton, 1984 and Phalacromyrmex (see Kempf 1960, Bolton 1984, 1998, Donoso2012) and stem ants of the †Camelomecia Barden &Grimaldi, 2016 genus group (Boudinot & al. 2020). Similar chaetal brushes, however, have also been observed in†Zigrasimecia Barden & Grimaldi, 2013 (Cao & al. 2020)
and the extant genera Protanilla Taylor, 1990 (Richter& al. 2021a) and to a lesser extent Discothyrea Roger,
1863 (Hita-Garcia & al. 2019). The fact that reduction of
the mandibular teeth in these lineages correlates with thedevelopment of chaetal brushes suggests a possible functional tradeoff between friction- and incisor-based grip.
Although the taxonomic sampling of published depictions (e.g., based on µ-CT-scans) of ant mandibular
bases is still limited, it is apparent that the mandibular articulations of Tatuidris tatusia are highly derived
within the family (set 4). Both the ventral (primary) anddorsal (secondary) mandibular articulations differ strikingly from the corresponding joints in other ant species(Richter & al. 2019, 2020, Boudinot & al. 2022a). These
unique conditions coincide and possibly correlate withthe general modification of the clypeus and its anterior
and lateral projections above the mandible. Due to thisconfiguration, the cephalic condyle is not on the anterolateral corner of the clypeus, as in most other ants, but on
the ventral surface of the clypeal projection. Additionally,
the condyle is L-shaped and divided into two distinctive
parts (Fig. 2D): the condylar lobe, a short dorsoventral
projection corresponding more or less with the condyleof other ants, and the condylar rail, a transverse ridge

(long part of the “L”) that is either a modification of a partof the condyle or a modification of the triangular clypeal
projection, observed, for example, in Wasmannia Forel,1893 (Richter & al. 2019). Consequently, the articulatory
surface of the mandible is unusually complex (Fig. 4C).
The mandibular acetabulum is divided into a medial and alateral groove by a short ridge. The condylar lobe interacts
with these two grooves, while the condylar rail interacts
with the distal contact surface and the medial groove,
areas of the mandibular stem unmodified in many otherants (Richter & al. 2019, 2020, Boudinot & al. 2022a).
Additional notable modifications of the mandible are theshift of the primary mandibular condyle medially in its
broad articulatory fossa of the cranium and the thickening and rotation of the canthellus, forming the canthellar
groove interacting with the triangular hypostomal process.
Based on these structural modifications, we havepresented our hypothesized movement pattern for themandible in the results section. This adds to the known
mechanical diversity of mandible opening / closing in ants,
so far described for trap-jaw ants (e.g., Gronenberg 1995,
Larabee & al. 2017, Booher & al. 2021), Protanilla lini(see Richter & al. 2021a), and Harpegnathos Jerdon,
1851 (Zhang & al. 2020). If 3D-printed models are more
precisely constructed in the future and potentially include
soft parts of the articulation, they may be used to conduct
actual kinematic analyses to better understand the rangeof mandibular motion. One exciting application would bemodeling mandibular motions of extinct species such as†Camelomecia janovitzi Barden & Grimaldi, 2016 (seealso Boudinot & al. 2020), which has mandibles that are
exceptionally similar in shape to those of Tatuidris tatusia.
In the context of mandibular functional morphology, it
is also important to address the anatomy of the adductor
muscle (0md1). This muscle is massive, filling out almost
the entire lateral space within the head capsule in Tatuidris tatusia (Fig. 4A). Additional space for this muscle isprovided not only by the broad and posteriorly expanded
head capsule but also by the ventral lateral bulges andthe bent tentoria. Further, most of the muscle fibers in
the head likely are fast-contracting fibers (Fig. 5A). As
revealed by Gronenberg & al. (1997), the fibers in the
adductor muscle of ants can have either long or shortsarcomeres, specialized for either powerful or fast contractions, respectively. Long-sarcomere fibers come in two
further configurations: They are either directly attachedto the main apodeme of the muscle or connect to it via
thin cuticular threads. Our µ-CT data were insufficient toassess sarcomere length accurately, and our histologicalsections were not optimized (e.g., in orientation) for sarcomere-length measurements. However, we observed thatthose fibers attached to the central region of the massive
apodeme lack threads. Our histological data corroborate ashort sarcomere condition for these fibers. Only the fibers
attached to the edges of the apodeme have both threads
and long sarcomeres.
The apodeme is expanded as a large, flat sheet of cuticle
and provides sufficient attachment space for the many di-

rectly attaching fibers. The high proportion of short sarcomere fibers and the muscle size likely produce a relatively
fast yet powerful bite of the mandible (see also Püffel
& al. 2021 for a discussion of functional consequencesof mandibular muscle architecture). Tatuidris tatusia,
thus, have morphological characteristics that enable themto employ the “static” method of prey control (Bolton1998), that is, remaining clamped on the prey until thesting is used. Another remarkable aspect concerning theunusual mandible of T. tatusia is the presence of the innermandibular gland, opening with a sieve plate at its lateralbase. A gland of this specific configuration has not beenfound in any other ant species so far (Billen & Delsinne2014). The function of the gland is currently unknown, but
it may be functionally related to the peculiar mandibularform and movement.
While the prey of Tatuidris tatusia remains unknown(Jacquemin & al. 2014), it is conceivable that T. tatusia
rely on their chaetal comb to grip small, round arthropods,such as Oribatida or other Acari. However, the proportionof fast-contracting fibers combined with the friction-supporting mandibular brush could conceivably also allow
T. tatusia to catch fast prey such as Collembola. We agreewith Jacquemin & al. (2014) that the overall slow movements, highly defensive external morphology, and reducedeyes suggest that T. tatusia may be slow ambush predatorsof the leaf litter or upper soil layers rather than active
hunters. Previously, it was proposed that some species in
the genus Strumigenys Smith, F., 1860 use chemical lures
to attract prey (Dejean 1985). If T. tatusia indeed werean ambush predator, its intramandibular gland may serve
such a function. More observational and experimental
work is needed.
Comparative anatomy of the pharyngeal apparatus
As the pharyngeal apparatus is a source of phylogenetic
information that has only recently been tapped into, we
dedicate a brief comparative discussion to this subject.
The anterior prepharynx of Tatuidris tatusia is similar
to that of Protanilla lini (see Richter & al. 2021a) andBrachyponera Emery, 1900 (Richter & al. 2020) in its
overall shape and pattern of sclerotization, suggestingthat this may represent a plesiomorphic configurationin crown Formicidae. As in P. lini, the upward curving
posterior parts of the oral arms form a broad concavewall. As in Brachyponera, the dorsal plates are high andelongated. Both features are also similar in male Dorylus
helvolus (Linnaeus, 1764) (see Boudinot & al. 2021) but
not Wasmannia affinis Santschi, 1929 (Richter & al.
2019) or Formica rufa Linnaeus, 1761 (Richter & al.
2020). The ventral prepharyngeal dilatator has anterior
and posterior insertion areas that are distinctly separated;
this supports the previous interpretation that this muscle
might consist of both 0bu5 and 0bu6 (Richter & al. 2019,2020). The long tentorio-oral muscle 0hy2 resembles thecondition in Protanilla (see Richter & al. 2021a). However, this is most likely a result of convergent / parallel69

evolution caused by the anterior shift of the antennal
socket relative to the anatomical mouth opening in both
species. The shape of the pharyngeal gland is similar tothat described in Brachyponera (see Richter & al. 2020)and several other representatives of Poneria (Schoeters& Billen 1996), supporting the notion that this condition
may be a synapomorphy of the whole clade.
Evolutionary implications: derived conditionsof the Agroecomyrmecinae
At the given scale of comparison (Fig. 12, Tab. 2), Tatuidris tatusia is the taxon with the most derived conditions
in our present assessment of morphological evolution in
Poneria, followed by the exceptional and monotypic lineage Apomyrminae (Apomyrma stygia). Various aspectsof the exceptional morphology of T. tatusia are paralleledin the fossil record in lineages such as †Zigrasimecia
and †Camelomecia janovitzi, which raises a number ofquestions. What is the timescale of extreme derivation,
and what are the selection forces that drive the evolutionof these forms? What do these ants do in nature, and can
T. tatusia shed light on the paleoecology of Mesozoic
Formicoidea? In overall cephalic and mesosomal form,
the big-headed and tank-like T. tatusia is superficiallybut still remarkably similar to †Zigrasimecia (Cao & al.
2020; see also Boudinot & al. 2022b). In the unusual andspecific form of the mandibular tool edge, T. tatusia and†C. janovitzi are nearly identical. While there is far too
little known about the behavioral ecology of T. tatusia
and these Mesozoic ants to address potential behavioral
similarities, the phylogenetic relationships of the Agroecomyrmecinae may provide some insight into the timescaleand selective context driving their phenotypic evolution.
Table 2 and Figures 12 - 17 provide an overview of ourfavored phylogenetic topology and a series of potential
morphological transformations in Agroecomyrmecinae
and related subfamilies based on our present study, forwhich the implicit ground plan hypothesis was informedby the ancestral state analyses of Boudinot & al. (2022a)
and Richter & al. (2022).
As outlined in the Introduction, the extant sister groupof Tatuidris tatusia is known to be Ankylomyrma Bolton,
1973 (Ward & al. 2015), the two comprising the extant
members of the subfamily Agroecomyrmecinae. This clade
has been variably supported as sister to the “bullet ant”,
Paraponera Smith, F., 1858 (Brady & al. 2006, Moreau& al. 2006, Branstetter & al. 2017, Borowiec & al.
2019), with a recent phylogenomic study more strongly
supporting a sister-group relationship to Proceratiinae(Romiguier & al. 2022). The relationships of the two fossil
genera †Agroecomyrmex (Baltic amber) and †Eulithomyrmex (Florissant shale) are far less certain, although†Agroecomyrmex and T. tatusia do clearly share a series ofderived conditions (Figs. 12 and 13, Tab. 2; Wheeler 1915).
Previous molecular phylogenetic analyses supporting
Paraponerinae and Agroecomyrmecinae were criticized bybeing unexpected based on morphology alone (e.g., Baroni
Urbani & de Andrade 2007); we observe that, with the70

topology of Romiguier & al. (2022), it is possible to reason
out character polarity hypotheses for promesonotal fusionof the mesosoma and, to some degree, the tergosternalfusion of the petiole. The remarkable development ofantennal scrobes that can contain the entire scape would
be a potential synapomorphy of Agroecomyrmecinae +Paraponerinae. However, considering the different position of the scrobe relative to the eye, the Proceratiinae +Agroecomyrmecinae relationship indicates that this may
be parallelism. The vaulting of abdominal tergum IV in
Proceratiinae and Agroecomyrmecinae, however, becomesa reasonable synapomorphy as well as the obliteration of
the dorsal portion of the promesonotal suture, which isretained in Paraponera. Based on the divergence dating
estimates of Borowiec & al. (2019) and Romiguier & al.
(2022), the crown ages of the Poneria, Ponerinae, Amblyoponinae, and Proceratiinae are Cretaceous. While thecrown ages of Agroecomyrmecinae and Paraponerinae
are almost certainly Cenozoic, the ancestor of Agro­ecomyrmecinae + Proceratiinae likely displayed a compact mesosoma with promesonotal fusion prior to theend-Cretaceous turnover event. By or perhaps after the
Paleocene-Eocene thermal maximum, the ancestor of theAgroecomyrmecinae may have derived posteriorly shiftedcompound eyes, well-developed frontal lobes, and robust
scapes, among other features (Figs. 12, 13). Comparisonof †Agroecomyrmex duisburgi and Ankylomyrma with
Tatuidris tatusia using µ-CT data will certainly enrich
our knowledge of the history of this group.
Despite the derivation of the “crown of thorns” cranial lamella, Ankylomyrma coronacantha has retained
a head morphology characterized by more plesiomorphic
features than Tatuidris tatusia. Some of these relative
plesiomorphies include the flat mandible with five teeth(Fig. 15B; resembling the weakly torqued planar form as
defined by Keller 2011), the absence of the chaetal field,
the less expanded frontal region / frontal lobes, absenceof the posteroventral cephalic lobes, the 12-merous antennae, the relatively large compound eyes (although eye
size is generally variable and polarity uncertain), and amore open configuration of the mouthparts (Fig. 15A).
Specifically, the hypostoma of Ankylomyrma is much
more open in ventral view. Moreover, the maxillary andlabial palps are significantly longer with more palpomeresthat are also individually longer; however, the elongation
may be a secondary derivation of Ankylomyrma comparedto the short palpomeres of relatives such as Paraponera
and Proceratiinae.
While Ankylomyrma shows more plesiomorphic features relative to Tatuidris tatusia, many morphologicalcharacteristics of this genus are likely specialized in theirown right, considering the arboreal lifestyle of this species(Bolton 1973). Clear indications of this are the comparatively larger compound eyes and the protective spines.
The ecological correlates of ant maxillolabial complex
architecture are uncertain, and while the long palps ofAnkylomyrma may intuitively be related to foraging in
more open, for example, arboreal, habitats, there is noclear evidence for such an association. In contrast, due totheir shorter palpomeres and hypostomal configuration,
T. tatusia may seal the oral foramen more tightly, which
intuitively would lead to improved protection in cryptic
habitats or from potentially defensive prey. However, thefunctional context of tight mouthpart closure is similarly
poorly known as that of palpomere length.
Interestingly, the mandibular shape of Ankylomyrma
could be associated with canopy living to a certain degree.
It is similar to the “planar” mandibular form, which also
occurs in the arboreal genus Pseudomyrmex Lund, 1831
(see Keller 2011), although the specific mandibular shapeis different. A detailed study of the internal anatomy ofAnkylomyrma may reveal additional cephalic synapomorphies with T. tatusia and will likely contribute to thefurther polarization of the character states of the armadillo
ant (Fig. 12, Tab. 2).
Evolutionary implications: “relictual” lineages
We reiterate that, among examined representatives of
Poneria, Tatuidris tatusia, Ankylomyrma, Paraponera,
and Apomyrma represent morphologically disparate andphylogenetically isolated lineages for which relatively little
morphological variation is known in the extant populations. Pie & Feitosa (2016) consider lineages characterizedby phylogenetic isolation and a species-poor extant fauna
as “relictual”; they indicate T. tatusia and Paraponera as
typical representatives of such lineages. Based on theirmodeling, the survival of such lineages along evolutionary
time is unlikely, so they propose two specific scenarios fortheir persistence: adaptation to a stable but specialized
niche (hypothesis 1) or survival on insular landmasses
with depauperate competition (hypothesis 2). Due to thewidespread occurrence of T. tatusia, they recognized thisgenus as an example for the first hypothesis.
Considering the incomplete investigation of species
diversity in Tatuidris tatusia and with the potential forsome species to be delimited in the future (Donoso 2012;
J. T. Longino, pers. comm.), the “relictuality” of this lineage from a diversity perspective will require additional
scrutiny. Nevertheless, a > 40 million year old clade(Fig. 12), with a few species, may still fulfill the criterionof “disproportionately low species richness” given by Pie& Feitosa (2016). At this point it is important to note a
critical distinction as the concept of “relictuality” has also
been used to refer to paleogeographical relicts that retain
an ancient geographical range relative to related lineages
with a novel distribution, exemplified by the highly morphologically distinct Leptomyrmex relictus Boudinot &al. 2016 (see also Barden & al. 2017). In the following,
we will focus our thoughts on the phylogenetic isolation
concept of “relictual” lineages.
While not being a criterion for the definition of phylogenetic relictuality, we consider morphological stabilityor stasis to be an interesting aspect of understanding theevolution of “relictual” lineages: Should a phylogenetically isolated species (or species complex of a few similar
species) be considered a paleomorphological “relict” if its

derivations are extreme and relatively recent or at least
not evinced in the fossil record? Among the four genera
indicated at the beginning of this section, we observe thatAnkylomyrma displays several symplesiomorphies of thehead with †Agroecomyrmex duisburgi that have beenstable since at least the Eocene (~40 million years (Ma))
and which are not shared with Tatuidris tatusia (Fig. 12,
Tab. 2). Many of these same plesiomorphies are preserved
in Paraponera; however, little can be said about the age ofits defining synapomorphies except that this lineage has
been stable since the mid Miocene (~15 Ma), as evincedby the high degree of similarity between †P. dieteri (seeBaroni Urbani 1994) and P. clavata. Regardless, there
is a spectrum of morphological derivation across these
isolated lineages, with Apomyrma and T. tatusia at one
extreme of the range (based on number of apomorphies).
While the history of such character state transformations
in geological time is elusive, fossils can be a way to tracethem. Due to its position (Fig. 12), †A. duisburgi indicatesthat many of the autapomorphies of T. tatusia date to the
Eocene. This further suggests a short timespan of morphological transformation and a possibly increased rate ofmodification relative to Paraponera and Ankylomyrma,
despite the unusual morphology of the latter.
While we have made an effort to avoid bias in our character assessment, our coding scheme necessarily remains
subjective; thus, the count of apomorphies is expected tochange with a different approach to character coding andanalysis (see section “Comparative morphology”) andespecially with additional data. Nevertheless, it seems
obvious that Tatuidris tatusia is a species with an extreme
accumulation of derived features; Figure 12 and Table 2
show that T. tatusia has, in our estimation, several times
more apomorphies than Ankylomyrma and Paraponera
and almost twice as many as Apomyrma. Moreover, thehead of T. tatusia is characterized by a complex suiteof interconnected modifications, based on our present
assessment. The structural transformations of its head
involve many characters that are not closely correlated in
an obvious way according to our interpretation but instead
belong to at least five different mechanical syndromes(Tab. 2). As a counterexample, the functionally and structurally modified head of species of the genus Strumigenys,
defined by a trap jaw mechanism, evolved several times
independently from non-trap-jaw relatives. Small but
significant realignments of mouthpart structures lead to aswitch-like reorganization of many head structures, which
ultimately resulted in similar morphology in all lineages
that have this mechanism, including specific modificationsof musculature and the nervous system (Booher & al.
2021). In T. tatusia, the extreme derivation and total lackof morphologically intermediate lineages pose a conundrum: How does an extremely modified, phylogeneticallyisolated lineage come to be so, and what would this tell us
about its “relictuality”?
We can only speculate about the exact evolutionary
processes resulting in the wonderfully weird head morphology of Tatuidris tatusia. Did the many structural71

derivations of the head assemble gradually over time, oris there a morphological switch, that is, a key change thattriggered the large-scale reorganization of the head, similar to what occurs in Strumigenys? A candidate for sucha switch may be the broadening of the frontal region andhead in general, which directly leads to the rotated antennal socket and additional space for musculature and influences the mandibular articulation through the projectionof the clypeus above the clypeal condylar area. However,
the exact conformation of the mandibular articulation as
well as the mandibular shape and chaetal brush are notnecessarily logical outcomes of this change, indicatingthat there may have been intermediate states leading up
to the extremely derived condition currently seen. Thatsuch morphological intermediates have apparently notsurvived to the present indicates that climatic events andvegetational turnovers may have played an important rolein the evolution of the genus, with subsequent “runaway”
derivation of the head and associated mechanisms, potentially as optimization of the functional traits discussed
previously (predation, protection, and balance).
Certainly, Ankylomyrma and Paraponera also experienced similar turnover, but they accumulated much fewer
morphological apomorphies (particularly of the head).
Whether the derivation of the Tatuidris tatusia morph
was a rapid burst (switch-like), long fuse (gradual), or
jittery sprint (a combination of both), the armadillo ant
represents a special case among its unusual relatives. Is
their unique head a recent evolutionary event or are they
paleomorphological relicts that retained this configuration for tens of millions of years? A detailed molecularinvestigation of the living population may help in at least
establishing a minimum age for this question. Ultimately,
future findings of Tatuidris-line fossils would provide
a window into their evolutionary history and potential
solution to the conundrum of “runaway relicts”.
Conclusions
Our study of the head anatomy of Tatuidris tatusia
builds on the recently established conceptual framework
for the head morphology of Formicidae (Richter & al.
2019, 2020, 2021a, 2022, Boudinot & al. 2021). Throughdetailed documentation of the cephalic anatomy of T.
tatusia and comparison with other ants, we reconstruct
the complexity of modifications that have evolved in thislineage. These have resulted in a head morphology thatappears to be optimized for prey capture (mandibularmorphology and muscle architecture), protection (thickcuticle, antennal protection, tight maxillolabial complex
closure), and balance (position of the occipital foramen,
distribution of mass in the head). We find that the mandibular motion of T. tatusia is structurally constrained bytheir complex articulations, allowing us to precisely reason
the mechanical function of opening and closing, which is
unique among ants studied to date.
Benefitting from the phylogenetic resolution provided
by DNA-sequence data – thus removing the confounding
factor of relationship uncertainty to some degree – we72

propose a transformation series leading to the modern
form of the Tatuidris tatusia head. Further, we recognizethat T. tatusia is sui generis among Poneria, being an exceptionally modified “runaway relict”. While more formal
testing the evolutionary sequence leading to modern T.
tatusia is outside of the scope of the present work, ourconsiderations set up three major goals for the understanding of T. tatusia and other phylogenetically isolated
and species-poor lineages, whether their morphology isdominated by relatively plesiomorphic or derived characters: (1) population-level phylogenomic analysis and reassessment of morphological variation (how old is the crowngroup; are there multiple sympatric species that have notbeen detected?), (2) analysis and exploration of the fossil
record (evaluate the anatomy of †Agroecomyrmex, at theleast), and (3) focused study on the natural history of thesesingular ants. Resolving these questions will contributeto our understanding of the patterns and processes ofmorphological macroevolution and may shed light on
the evolutionary dynamics of the Formicoidea from theirMesozoic origin to their Cenozoic modernity. Interrogationof fossil and extant anatomy is a necessity.

Acknowledgments
We gratefully acknowledge the donation of several specimens of Tatuidris tatusia for µ-CT scanning by Jack
Longino and by Thibaut Delsinne for histological examination. AR thanks Crystal Maier and Charles Whittemore
Farnum for facilitating his stay at the Harvard Museumof Comparative Zoology and for databasing the specimenof T. tatusia examined there. We thank the Okinawa
Institute of Science and Technology Graduate University(OIST) Imaging Section for providing access to the Zeiss
Xradia µ-CT scanner and Shinya Komoto for general
support. An Vandoren prepared the histological sectionsof T. tatusia, which we gratefully acknowledge. We arealso very thankful to Michael Branstetter for providing
us with pictures of living T. tatusia, which are valuable
considering the rare collection of living specimens of thisspecies. We thank Toni Wöhrl for taking the time to discuss the mandibular opening mechanism with us from abiomechanical perspective. We also thank the anonymous
reviewers and Subject Editor Flávia Esteves for commentsthat improved the quality of the present study. AR gratefully acknowledges support by a scholarship and travel
grant of the Evangelisches Studienwerk Villigst eV, whichallowed him to visit the MCZ. BEB is supported by an
Alexander von Humboldt Research Fellowship. FHG was
supported by Japan Society for the Promotion of Science(JSPS) grants-in-aid KAKENHI grants (No. 18K14768 &21K06326).

Conflict of interest
None.

Contributions of authors
Conceptualization: AR, BEB. Methodology: AR, BEB,
FHG, JB. Investigation: AR, BEB. Resources: FHG, RGB,JB, EPE. Data curation: AR, BEB, FHG. Writing, originaldraft: AR, BEB. Writing, review & editing: all authors.
Visualization: AR, BEB. Supervision: RGB, EPE.

References
AntWeb 2022: AntWeb Version 8.68.7. California Academyof Science. – <https://www.antweb.org.>, retrieved on 25
January 2022.
Barden, P., Boudinot, B.E. & Lucky, A. 2017: Where fossils dare
and males matter: combined morphological and molecularanalysis untangles the evolutionary history of the spider ant
genus Leptomyrmex Mayr (Hymeno­ptera: Dolichoderinae).
– Invertebrate Systematics 31: 765-780.
Baroni Urbani, C. 1994: The identity of the Dominican Paraponera (Amber Collection Stuttgart: Hymeno­ptera, Formicidae. V: Ponerinae, partim). – Stuttgarter Beiträge zur
Naturkunde. Serie B (Geologie und Paläontologie) 197: 1-9.
Baroni Urbani, C. & de Andrade, M. 1994: First description offossil Dacetini ants with a critical analysis of the current classification of the tribe (Amber Collection Stuttgart: Hymeno­
ptera, Formicidae. VI. Dacetini). – Stuttgarter Beiträge zur
Naturkunde. Serie B (Geologie und Paläontologie) 198: 1-65.
Baroni Urbani, C. & de Andrade, M. 2007: The ant tribe
Decetini: limits and constituent genera, with descriptionsof new species. – Annali del Museo Civico di Storia Naturale“Giacomo Doria” 99: 1-191.
Baroni Urbani, C., Bolton, B. & Ward, P.S. 1992: The internal
phylogeny of ants (Hymeno­ptera: Formicidae). – Systematic
Entomology 17: 301-329.
Beutel, R.G., Luo, X.-Z., Yavorskaya, M.I. & Jałoszyński, P.
2021: Structural megadiversity in leaf litter predators – thehead anatomy of Pselaphus heisei (Pselaphinae, Staphylinidae, Coleo­ptera). – Arthropod Systematics & Phylogeny
79: 443-463.
Billen, J. & Delsinne, T. 2014: A novel intramandibular glandin the ant Tatuidris tatusia (Hymeno­ptera: Formicidae). –Myrmecological News 19: 61-64.
Bock, W.J. & Von Wahlert, G. 1965: Adaptation and theform-function complex. – Evolution 19: 269-299.
Bolton, B. 1973: A remarkable new arboreal ant genus (Hym.
Formicidae) from West Africa. – Entomologist’s Monthly
Magazine 108: 234-237.
Bolton, B. 1984: Diagnosis and relationships of the myrmicine
ant genus Ishakidris gen. n. (Hymeno­ptera: Formicidae). –Systematic Entomology 9: 373-382.
Bolton, B. 1998: Monophyly of the dacetonine tribe-group andits component tribes (Hymeno­ptera: Formicidae). – Bulletinof the Natural History Museum Entomology Series 67: 65-78.
Bolton, B. 1999: Ant genera of the tribe Dacetonini (Hymeno­
ptera: Formicidae). – Journal of Natural History 33: 16391698.
Bolton, B. 2000: The ant tribe Dacetini. – Memoirs of theAmerican Entomological Institute 65: 1-1028.
Bolton, B. 2003: Synopsis and classification of Formicidae. –Memoirs of the American Entomological Institute 71: 1-370.
Booher, D.B., Gibson, J.C., Liu, C., Longino, J.T., Fisher,
B.L., Janda, M., Narula, N., Toulkeridou, E., Mikheyev,
A.S., Suarez, A.V. & Economo, E.P. 2021: Functional innovation promotes diversification of form in the evolution of
an ultrafast trap-jaw mechanism in ants. – Public Libraryof Science Biology 19: art. e3001031.
Borowiec, M.L., Rabeling, C., Brady, S.G., Fisher, B.L.,
Schultz, T.R. & Ward, P.S. 2019: Compositional heterogeneityand outgroup choice influence the internal phylogeny of theants. – Molecular Phylogenetics and Evolution 134: 111-121.
Boudinot, B.E. 2015: Contributions to the knowledge of Formicidae (Hymeno­ptera, Aculeata): a new diagnosis of thefamily, the first global male-based key to subfamilies, and atreatment of early branching lineages. – European Journalof Taxonomy: art. 120.
Boudinot, B.E., Khouri, Z., Richter, A., Griebenow, Z.H., vande Kamp, T., Perrichot, V. & Barden, P. 2022a: Evolution
and systematics of the Aculeata and kin (Hymeno­ptera), withemphasis on the ants (Formicoidea: †@@@idae fam. nov.,
Formicidae). – bioRxiv; doi: 10.1101/2022.02.20.480183.
Boudinot, B.E., Moosdorf, O.T.D., Beutel, R.G. & Richter, A.
2021: Anatomy and evolution of the head of Dorylus helvolus(Formicidae: Dorylinae): patterns of sex- and caste-limited
traits in the sausagefly and the driver ant. – Journal of Morphology 282: 1616-1658.
Boudinot, B.E., Perrichot, V. & Chaul, J.C.M. 2020: † Camelosphecia gen. nov., lost ant-wasp intermediates from the
mid-Cretaceous (Hymeno­ptera, Formicoidea). – ZooKeys
1005: 21-55.
Boudinot, B.E., Probst, R.S., Brandão, C.R.F., Feitosa, R.M.
& Ward, P.S. 2016: Out of the Neotropics: newly discovered
relictual species sheds light on the biogeographical historyof spider ants (Leptomyrmex, Dolichoderinae, Formicidae).
– Systematic Entomology 41: 658-671.
Boudinot, B.E., Richter, A., Katzke, J., Chaul, J.C.M., Keller,
R.A., Economo, E.P., Beutel, R.G. & Yamamoto, S. 2022b:
Evidence for the evolution of eusociality in stem ants anda systematic revision of †Gerontoformica (Hymeno­ptera:
Formicidae). – Zoological Journal of the Linnean Society
195: 1355-1389.
Brady, S.G., Schultz, T.R., Fisher, B.L. & Ward, P.S. 2006:
Evaluating alternative hypotheses for the early evolution anddiversification of ants. – Proceedings of the National Academyof Sciences of the United States of America 103: 18172-18177.
Branstetter, M.G. & Longino, J.T. 2022: UCE phylogenomics of New World Cryptopone (Hymeno­ptera: Formicidae)
elucidates genus boundaries, species boundaries, and thevicariant history of a temperate-tropical disjunction. – Insect
Systematics & Diversity 6: art. 6.
Branstetter, M.G., Longino, J.T., Ward, P.S. & Faircloth,
B.C. 2017: Enriching the ant tree of life: enhanced UCE bait
set for genome-scale phylogenetics of ants and other Hymeno­
ptera. – Methods in Ecology and Evolution 8: 768-776.
Breed, M.D. & Harrison, J.M. 1985: Worker size, ovary
development and division of labor in the giant tropical ant,
Paraponera clavata (Hymeno­ptera: Formicidae). – Journalof the Kansas Entomological Society 61: 285-291.
Brown, W.L., Jr. & Kempf, W.W. 1968 [1967]: Tatuidris, a remarkable new genus of Formicidae (Hymeno­ptera). – Psyche
74: 183-190.
Cao, H., Boudinot, B.E., Wang, Z., Miao, X., Shih, C., Ren,
D. & Gao, T. 2020: Two new iron maiden ants from Burmese
amber (Hymeno­ptera: Formicidae: †Zigrasimeciini). – Myrmecological News 30: 161-173.
Carpenter, F.M. 1930: The fossil ants of North America. –Bulletin of the Museum of Comparative Zoology 70: 1-66.
Dejean, A. 1985: Étude éco-éthologique de la predation chez lesfourmis du genre Smithistruma (Formicidae – Myrmicinae –Dacetini). II. Attraction des proies principals (Collemboles).
– Insectes Socieux 32: 158-172.
Donoso, D.A. 2012: Additions to the taxonomy of the armadillo ants (Hymeno­ptera, Formicidae, Tatuidris). – Zootaxa
3503: 61-81.

73

Donoso, D.A. 2017: Capitulo 20. Subfamilia Agroecomyrmecinae. In: Fernández, F., Guerrero, R.J. & Delsinne, T.
(Eds.): Hormigas de Colombia. – Universidad Nacional de
Colombia, Bogotá, pp. 631-636.
Engelkes, K., Friedrich, F., Hammel, J.U. & Haas, A. 2018:
A simple setup for episcopic microtomy and a digital image
processing workflow to acquire high-quality volume data and3D surface models of small vertebrates. – Zoomorphology
137: 213-228.
Ettershank, G. 1966: A generic revision of the world Myrmicinae related to Solenopsis and Pheidologeton (Hymeno­ptera:
Formicidae). – Australian Journal of Zoology 14: 73-171.
Fisher, B.L. & Bolton, B. 2016: Ants of Africa and Madagascar: A Guide to the Genera. – University of California Press,
Berkeley, CA, I-IX + 503 pp.
Gronenberg, W. 1995: The fast mandible strike in the trap-jaw
ant Odontomachus I. Temporal properties and morphological characteristics. – Journal of Comparative Physiology A:
Neuroethology, Sensory, Neural, and Behavioral Physiology
176: 391-398.
Gronenberg, W., Paul, J., Just, S. & Hölldobler, B. 1997:
Mandible muscle fibers in ants: fast or powerful? – Cell andTissue Research 289: 347-361.
Harris, R.A. 1979: A glossary of surface sculpturing. – Occasional Papers in Entomology 28: 1-31.
Hita-Garcia, F., Lieberman, Z., Audisio, Z.L., Liu, C. &Economo, E.P. 2019: Revision of the highly specialized ant
genus Discothyrea (Hymeno­ptera: Formicidae) in the Afrotropics with X-ray microtomography and 3D cybertaxonomy.
– Insect Systematics and Diversity 3: art. 5.
Jacquemin, J., Delsinne, T., Maraun, M. & Leponce, M.
2014: Trophic ecology of the armadillo ant, Tatuidris tatusia,
assessed by stable isotopes and behavioral observations. –Journal of Insect Science 14: art. 108.
Jacquemin, J., Delsinne, T., Maraun, M. & Leponce, M. 2018:
Erratum to “Trophic ecology of the armadillo ant, Tatuidris
tatusia, assessed by stable isotopes and behavioral observations”. – Journal of Insect Science 18: art. 9.
Kaspari, M. & Yanoviak, S.P. 2009: Biogeochemistry andthe structure of tropical brown food webs. – Ecology 90:
3342-3351.
Keller, R.A. 2011: A phylogenetic analysis of ant morpho­
logy (Hymeno­ptera: Formicidae) with special reference tothe poneromorph subfamilies. – Bulletin of the American
Museum of Natural History 355: 1-90.
Kempf, W.W. 1960: “Phalacromyrmex”, a new ant genus fromSouthern Brazil (Hymeno­ptera, Formicidae). – Revista
Brasileira de Biologia 20: 89-92.
Kubota, H., Yoshimura, J., Niitsu, S. & Shimizu, A. 2019:
Morphology of the tentorium in the ant genus Lasius Fabricius(Hymeno­ptera: Formicidae). – Scientific Reports 9: art. 6722.
Lacau, S., Groc, S., Dejean, A., de Oliveira, M.L. & Delabie,
J.H.C. 2012: Tatuidris kapasi sp. nov.: a new armadillo ant
from French Guiana (Formicidae: Agroecomyrmecinae). –Psyche 2012: art. 926089.
Larabee, F.J., Gronenberg, W. & Suarez, A.V. 2017: Performance, morphology and control of power-amplified mandibles
in the trap-jaw ant Myrmoteras (Hymeno­ptera: Formicidae).
– Journal of Experimental Biology 220: 3062-3071.
Lieberman, Z.E., Billen, J., van de Kamp, T. & Boudinot, B.E.
2022: The ant abdomen: the skeletomuscular and soft tissue
anatomy of Amblyopone australis workers (Hymeno­ptera:
Formicidae). – Journal of Morphology 283: 693-770.

74

Longino, J.T. 2010: Tatuidris of Costa Rica. – <https://ants.
biology.utah.edu/genera/tatuidris/home.html>, retrieved
24 October 2022.
Lösel, P.D., Van De Kamp, T., Jayme, A., Ershov, A., Faragó,T., Pichler, O., Tan Jerome, N., Aadepu, N., Bremer, S.,
Chilingaryan, S.A., Heethoff, M., Kopmann, A., Odar,J., Schmelzle, S., Zuber, M., Wittbrodt, J., Baumbach, T.
& Heuveline, V. 2020: Introducing Biomedisa as an opensource online platform for biomedical image segmentation.
– Nature Communications 11: art. 5577.
Michener, C.D. & Fraser, A. 1978: A comparative anatomical
study of mandibular structure in bees. – The University of
Kansas Science Bulletin 51: 463-482.
Moffett, M.W. 1986: Revision of the myrmicine genus Acanthomyrmex (Hymeno­ptera: Formicidae). – Bulletin of theMuseum of Comparative Zoology 151: 55-89.
Moreau, C.S., Bell, C.D., Vila, R., Archibald, S.B. & Pierce,
N.E. 2006: Phylogeny of the ants: diversification in the ageof angiosperms. – Science 312: 101-104.
Peeters, C., Molet, M., Lin, C.C. & Billen, J. 2017: Evolutionof cheaper workers in ants: a comparative study of exoskeleton thickness. – Biological Journal of the Linnean Society
121: 556-563.
Pie, M.R. & Feitosa, R.M. 2016: Relictual ant lineages (Hymeno­
ptera: Formicidae) and their evolutionary implications. –Myrmecological News 22: 55-58.
Porto, D.S. & Almeida, E.A. 2019: A comparative study of thepharyngeal plate of Apoidea (Hymeno­ptera: Aculeata), with
implications for the understanding of phylogenetic relationships of bees. – Arthropod Structure & Development 50: 64-77.
Prebus, M.M. 2021: Phylogenetic species delimitation in theants of the Temnothorax salvini group (Hymeno­ptera: Formicidae): an integrative approach. – Systematic Entomology
46: 307-326.
Prentice, M.A. 1998: The comparative morphology and phylogeny of apoid wasps (Hymeno­ptera: Apoidea). – Unpublished
PhD dissertation, University of California, Berkeley, 1439 pp.
Püffel, F., Pouget, A., Liu, X., Zuber, M., van de Kamp, T.,
Roces, F. & Labonte, D. 2021: Morphological determinantsof bite force capacity in insects: a biomechanical analysis ofpolymorphic leaf-cutter ants. – Journal of the Royal Society,
Interface 18: art. 20210424.
Rabeling, C., Brown, J.M. & Verhaagh, M. 2008: Newly
discovered sister lineage sheds light on early ant evolution.
– Proceedings of the National Academy of Sciences of theUnited States of America 105: 14913-14917.
Richter, A., Boudinot, B.E., Yamamoto, S., Katzke, J. &Beutel, R.G. 2022: The first reconstruction of the head
anatomy of a Cretaceous insect, †Gerontoformica gracilis(Hymeno­ptera: Formicidae), and the early evolution of ants.
– Insect Systematics & Diversity 6: art. 4.
Richter, A., Hita Garcia, F., Keller, R.A., Billen, J.,
Economo, E.P. & Beutel, R.G. 2020: Comparative analysis
of worker head anatomy of Formica and Brachyponera(Hymeno­ptera: Formicidae). – Arthropod Systematics &Phylogeny 78: 133-170.
Richter, A., Hita Garcia, F., Keller, R.A., Billen, J., Katzke,J., Boudinot, B.E., Economo, E.O. & Beutel, R.G. 2021a:
The head anatomy of Protanilla lini (Hymeno­ptera: Form­
icidae: Leptanillinae), with a hypothesis of their mandibular
movement. – Myrmecological News 31: 85-114.
Richter, A., Keller, R.A., Rosumek, F.B., Economo, E.P.,
Hita Garcia, F. & Beutel, R.G. 2019: The cephalic anatomy
of workers of the ant species Wasmannia affinis (Formicidae,
Hymeno­ptera, Insecta) and its evolutionary implications. –Arthropod Structure & Development 49: 26-49.

Richter, A., Schoeters, E. & Billen, J. 2021b: Morphology
and closing mechanism of the mandibular gland orifice inants (Hymeno­ptera: Formicidae). – Journal of Morphology
282: 1127-1140.

Ward, P.S. & Fisher, B.L. 2016: Tales of dracula ants: theevolutionary history of the ant subfamily Amblyoponinae(Hymeno­ptera: Formicidae). – Systematic Entomology 41:
683-693.

Romiguier, J., Borowiec, M.L., Weyna, A., Helleu, Q., Loire,
E., La Mendola, C., Rabeling, C., Fisher, B.L., Ward, P.S.
& Keller, L. 2022: Ant phylogenomics reveals a natural selection hotspot preceding the origin of complex eusociality.
– Current Biology 32: 2942-2947.

Wheeler, W.M. 1915 [1914]: The ants of the Baltic Amber. –Schriften der Physikalisch-Ökonomischen Gesellschaft zu
Königsberg 55: 1-142.

Schmidt, C.A. & Shattuck, S.O. 2014: The higher classificationof the ant subfamily Ponerinae (Hymeno­ptera: Formicidae),with a review of ponerine ecology and behavior. – Zootaxa
3817: 1-242.
Schoeters, E. & Billen, J. 1996: The post-pharyngeal glandin Dinoponera ants (Hymeno­ptera: Formicidae): unusualmorphology and changes during the secretory process. –International Journal of Insect Morphology and Embryology
25: 443-447.
Ward, P.S., Brady, S.G., Fisher, B.L. & Schultz, T.R. 2015:
The evolution of myrmicine ants: phylogeny and biogeographyof a hyperdiverse ant clade (Hymeno­ptera: Formicidae). –Systematic Entomology 40: 61-81.
Ward, P.S. & Branstetter, M.G. 2022: Species paraphyly andsocial parasitism: Phylogenomics, morphology, and geography clarify the evolution of the Pseudomyrmex elongatulus
group (Hymeno­ptera: Formicidae), a Mesoamerican ant
clade. – Insect Systematics & Diversity 6: art. 4.

Williams, J.L., Zhang, Y.M., LaPolla, J.S., Schultz, T.R. &Lucky, A.R. 2022: Phylogenomic delimitation of morphologically cryptic species in globetrotting Nylanderia (Hymeno­
ptera: Formicidae) species complexes. – Insect Systematics& Diversity 6: art. 10.
Wilson, E.O. 2003: Pheidole in the New World. A dominant,hyperdiverse ant genus. – Harvard University Press, Cambridge, MA, ix + 794 pp.
Wipfler, B., Machida, R., Müller, B. & Beutel, R.G. 2011:
On the head morphology of Grylloblattodea (Insecta) and thesystematic position of the order, with a new nomenclature forthe head muscles of Dicondylia. – Systematic Entomology
36: 241-266.
Zachos, J., Pagani, M., Sloan, L., Thomas, E. & Billups, K.
2001: Trends, rhythms, and aberrations in global climate 65
Ma to present. – Science 292: 686-693.
Zhang, W., Li, M., Zheng, G., Guan, Z., Wu, J. & Wu, Z. 2020:
Multifunctional mandibles of ants: variation in gripping behavior facilitated by specific microstructures and kinematics.
– Journal of Insect Physiology 120: art. 103993.

75