Ymboirana acrux Santos & Wilkinson & Ribeiro & Carvalho & Zaher 2024, sp.nov.
Creators: name: Santos, Rodolfo Otávio
name: Wilkinson, Mark
name: Ribeiro, do Couto Graziela
name: Carvalho, Alberto B.
name: Zaher, Hussam
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Amphibia
Gymnophiona
Typhlonectidae
Ymboirana
Ymboirana acrux

description: Ymboirana acrux sp.nov.
ZooBank registration: urn:lsid:zoobank.org:act:D6487148- 6E74-436A -83C9-C89AC455B4BB
Diagnosis: As for genus.
Holotype: DGM 1462-R (Figs 2–9, 12A, 13A, 14A, 15A 1, A 2; Supporting Information, Figs S1, S 2), housed at Museu de Ciências da Terra (formerly Divisão de Geologia e Mineralogia of the Departamento Nacional de Produção Mineral), Rio de Janeiro, Brazil. The specimen consists of part and counterpart preserving the anterior portion of a cae - cilian skeleton (with an estimated length of around 20 cm), including skull, mandibles, and the anteriormost portion of the vertebral column, including the atlas, 66 trunk vertebrae, and some ribs.
Type locality and horizon: Outcrop located on the right bank of the river Paraíba do Sul, 1.6 km north -east of the municipality of Tremembé, São Paulo, Brazil (22º94 ʹ 50 ʹʹ S; 45º53 ʹ 55 ʹʹ W). These deposits are assigned to the Oligocene age Tremembé Formation, and lie within Taubaté Basin.
Etymology: The epithet is a noun in apposition and refers to Alpha Crucis, also known as Star of Magellan. This star (actually a multiple star system) represents, in the Brazilian national flag, the state of São Paulo, where the fossil was found.
Description
Skull
The skull is severely damaged and strongly dorsoventrally compressed (Fig. 3). Most of the bones are broken into short or indistinct fragments; however, both parietals and the pos - terior portion of the left pseudangular are well preserved.
Some fragments are tentatively assigned to the nasopremaxilla, maxillopalatine, frontals, squamosals, quadrates, stapes, os basale, and pseudodentaries. Many tooth crowns are also pre - sent, most of them disarticulated and a few still attached to their pedicels and to their corresponding jaw bones. Given that the skull bones may be difficult to distinguish due to the damaged condition of the fossil, we also provide a figure of the skull in the Supporting Information (Fig. S2) with an outline line drawing and using lighter shades of colours.
Nasopremaxillae: Fragments of the presumed nasopremaxillae are present, identified primarily on the basis of their position. Due to their poor preservation, few diagnostic features can be ob - served. However, based on the relative position of the fragments in relation to the lower jaw, we interpret the nasopremaxillae as forming an anteriorly projected snout and, thus, the mouth to be subterminal. Several other fragments were also preserved but lack diagnostic features.
Maxillopalatines: Both maxillopalatines are partially pre - served though severely damaged. The outer tooth rows of both maxillopalatines are present but incompletely preserved, so we could recognize only 12 pedicels. A fragment tentatively inter - preted as a portion of the left vomeropalatine tooth row is lo - cated medially and bears seven pedicels. Several other fragments, possibly belonging to the maxillopalatines, are present but do not exhibit any diagnostic features.
Parietals: These are the best -preserved skull bones in the spe - cimen and are almost complete, except for a short dorsolateral portion missing from the left. In dorsal view, except for their anterolateral pointed corners, both parietals exhibit a roughly trapezoidal shape (Figs 4A, 12A). The dorsal surface is smooth, lacking ornamentations in the form of small neurovascular for - amina that are common in some caecilians. The parietals are also flat medially but have a pronounced ventrally oriented slope at their posterolateral margins. Collapsed fragments of the os basale cover the ventral surfaces of the parietals.
Frontals: Located immediately anterior to both parietals, lie two poorly preserved bones, tentatively identified as parts of the frontals. They are broken and show an irregular shape, and similar to the parietals, the preserved portions of the dorsal sur - face of both frontals lack small neurovascular foramina.
Squamosals: Some fragments are tentatively assigned to the left squamosal (Fig. 4F). They are located close to the left quadrate and above the left parietal. The squamosal is roughly rectangular and bears a postorbital process with a slightly concave margin.
The only evidence of ornamentation is a single neurovascular foramen.
Quadrates: Both quadrates are preserved but broken (Figs 3, 4B). Portions of the left quadrate are located just above the articular facet of the pseudangular and not so widely separated from the maxillopalatines. The right quadrate is preserved far forward, next to fragments interpreted as parts of the pseudodentary. Due to the damaged condition of the squamosals, the degree of dorsal exposure of the quadrates cannot be determined. The pterygoid process of the left quadrate is present and elongate. A distinctive otic process is absent from both quadrates.
Stapes: A small fragment located on the posterodorsal portion of the skull, close to the left quadrate, is tentatively interpreted as the left stapes (Figs 4G, 13A). This fragment is composed of two different portions (i.e. a short and slender stapedial style and a small footplate) and is imperforate.
Os basale: Several small broken parts, scattered about the skull but mainly at its posterior portion appear to represent different regions of the os basale. One of these fragments, located just behind the left pseudangular and still articulated with the atlas, corresponds to a posterior portion of the os basale, including the left occipital condyle (Fig. 4C–E; Supporting Information, Fig. S1). A jugular foramen is present just anterior to the condyle. The region of the otic capsule is located anterior to the jugular foramen, but its external wall is badly damaged, whereas parts of the internal walls of the otic capsule are better preserved and bear the perilymphatic foramen and other foramina for branches of the auditory nerve. A large fragment preserved collapsed on the ventral surface of the right parietal bears a carotid for - amen (Fig. 4A), and lateral to this foramen, a lip of bone with a distinctive articular surface is interpreted as the basipterygoid process.
Lower jaw
Pseudangulars: Only the well -preserved posteriormost portion of the left pseudangular is discernable (Figs 4B, 5C–E, 14A). In lateral view, a recurved retroarticular process is present, bearing a well -developed lateral crest. The sutural surface that contacts the pseudodentary is excavated in the anterolateral margin. The processus condyloides is limited to a weakly developed process. In dorsal view, the articular surface is anteriorly positioned, concave, and surrounded posteriorly by the short processus condyloides. There are no signs of a preserved fossa or a canalis primordialis immediately anterior to the articular surface. The distinct processus internus is present, anteromedially oriented, and shows a roughly quadrangular outline. The retroarticular process curves medially mainly along its posterior portion. In medial view, two foramina of the ramus intermandibularis are present, one located at the posterior base of the processus internus and the other excavated anteriorly to it.
Pseudodentaries: Both pseudodentaries are partially preserved, including the region of the mandibular symphysis (Fig. 5A, B). Whereas the left one is represented by its anteriormost portion, in the right one the posterior portion is also preserved. Pedicels are present in the dorsal surface of the right pseudodentary, al - though the exact number is difficult to estimate. The pedicels are arranged in two rows, with definitive inner mandibular (i.e. splenial) pedicels being limited to the symphyseal region (based on the preserved pedicels, at least four are present on the right pseudodentary). The outer tooth row is also represented only by damaged pedicels, and we are able to recognize just 16 of them.
Dentition
We recognized at least 48 scattered tooth crowns near the skull (Fig. 6). Some teeth were also preserved still attached to the pedicels. Most of them are similar in size (around 0.5 mm in length) and shape: slender, conical, recurved, and monocusped. Based on the size of the preserved pedicels, the teeth of the inner rows are a little smaller than those of the outer rows. The apex is pointed but lateral flanges are seemingly absent. Due to the dam - aged condition of the fossil, it was not possible to verify the exact total number of teeth; however, the pedicel arrangement allows confirmation of the ancestral caecilian double tooth rows in the upper and lower jaws.
Postcranium
Postcranial elements assigned to Ymboirana acrux comprise the fragmented atlas followed by 66 subsequent vertebrae (most of them still articulated, although in the median portion of the vertebral column they are scattered) and some ribs (Fig. 7). An unknown number of posteriormost vertebrae are not preserved.
Atlas: Broken elements interpreted as the atlantal centrum and the neural arch were preserved separately in the two slabs (Figs 8A–E, 15A 1, A 2). Both atlantal cotyles are present and fused anteroventrally but with a pronounced median notch between them. The spinal nerve foramen is preserved only in the pos - terior surface of the left cotyle and is oriented posterolaterally. The centrum has a length considerably shorter than the fol - lowing vertebrae, and its ventral surface is smooth, bearing a medially positioned fossa and no hypapophyseal keel. The pos - terior cotyle has a roughly oval outline and the notochordal fossa is deep. Only a few portions of the neural arch were preserved. Two separated and anteriorly directed projections are present and are interpreted as the anterior bases of the lateral pedicles. The dorsal surface of the atlas is smooth, with no evidence of a neural spine.
Second vertebra: The second vertebra is preserved still ar - ticulated to the atlas, but severely damaged (Fig. 8F, G). The neural arch is completely collapsed over the centrum. The two prezygapophyses are preserved and exhibit a flat articular surface. On the ventral surface of the centrum, there is a well -developed hypapophyseal keel, but basapophyseal processes seem to be ab - sent. No rib -like elements can be confidently identified near the second vertebra.
Trunk vertebrae: The exact number of preserved trunk verte - brae (Figs 7, 9A–C) cannot be determined because of exten - sive fragmentation and disarticulation. All of these vertebrae show diagenetic dorsoventral compression. They are elongate anteroposteriorly and have medially constricted amphicoelous centra, well -developed basapophyseal processes projecting anteroventrally, a pronounced hypapophyseal keel, and low neural arches. Poorly developed neural spines, limited to faint ridges, are present in some, but not all, of the posteriormost pre - served vertebrae. Basapophyseal processes bear well -developed and convex ventral ridges, and this feature is particularly pro - nounced in some vertebrae preserved at the posterior region of the holotype (Figs 7B, C, 9B, C).
Ribs: Preserved ribs are hard to distinguish. Some are still articu - lated with vertebrae, but some are isolated and are relatively well preserved (Fig. 9D, E). They have a relatively straight outline along their entire length, lacking pronounced posterior flexion. The ribs are dorsoventrally broad anteriorly, but they gradually narrow distally. The tuberculum and capitulum have straight or more rounded tips, whereas the distal tips are pointed. The capit - ulum is very elongate anteriorly in the preserved ribs.
DISCUSSION
The assignment of Ymboirana acrux to Gymnophiona is sup - ported by several uniquely derived features, such as the pres - ence of pseudangulars, pseudodentaries, and amphicoelous vertebrae bearing medially constricted centra, pronounced hypapophyseal keels, and anterior basapophyseal processes (Wake 1980, Wilkinson and Nussbaum 2006). The Late Triassic Funcusvermis gilmorei Kligman et al. 2023, Early Jurassic Eocaecilia micropodia Jenkins and Walsh 1993, Early Cretaceous Rubricacaecilia monbaroni Evans and Sigogneau -Russell 2001, and Oligocene Ymboirana acrux represent the only named fossils of gymnophionomorphs with preserved skulls and postcranial elements known so far. Based on the anatomical comparisons detailed below, we conclude that Ymboirana acrux should be considered a member of Typhlonectidae and is thus the only taxon among the aforementioned fossil species belonging to crown group Gymnophiona.
The skull roof
A major variation in caecilian skulls is the presence or ab - sence of distinct upper temporal fenestrae: conditions termed zygokrotaphy and stegokrotaphy, respectively (Figs 10, 11). Most modern caecilians are more stegokrotaphic than zygokrotaphic, with at most a very narrow temporal gap between parietals and squamosals (Figs 10A, 11A). In zygokrotaphic rhinatrematids (Figs 10B, 11B), the ancestral jaw -closing muscles pass through the temporal fenestra and extend to the sagittal crest at the mid - line of the skull roof. In the other strongly zygokrotaphic caecil - ians, the jaw -closing muscles do not pass through the temporal fenestra or extend on to the skull roof (Figs10C, 11C; Supporting Information, Figs S3, S 4). Rhinatrematid zygokrotaphy is con - sidered primary (ancestral for caecilians, see: Nussbaum 1977) with zygokrotaphy in other caecilians (i.e. typhlonectids, Scolecomorphus, and Geotrypetes) secondary and having inde - pendently evolved at least three times (e.g. Nussbaum 1977, Wilkinson 1997, Kleinteich et al. 2012).
The fragmentary condition of the skull of the unique spe - cimen of Ymboirana, particularly the poor preservation of the squamosals, precludes determination of whether Ymboirana was stegokraphic or zygokrotaphic. Well -developed sagittal crests on the parietals, a feature typically associated with primarily zygokrotaphic skulls (Nussbaum 1977, Wilkinson et al. 2021), are not present in Ymboirana, and the parietals are not expanded laterally as in the secondarily stegokrotaphic Crotaphatrema (Nussbaum 1985), but it is not possible to rule out primary stegokrotaphy or secondary zygokrotaphy based solely on the morphology of the parietals. The parietals of Ymboirana have a median flattened portion and a lateral ventrally sloping region (Fig. 12A), which resembles secondarily zygokrotaphic caecil - ians (e.g. typhlonectids and scolecomorphids) but is also found in some more stegokrotaphic taxa (e.g. caeciliids).
Compared with typhlonectids, the parietal shape in Ymboirana is very similar to Chthonerpeton and Nectocaecilia (Figs 10C, 11C; Supporting Information, Figs S3A, S 4A) re - garding the degree of ventral sloping, whereas in Potomotyphlus kaupii and Typhlonectes (Supporting Information, Figs S3B, C, S 4B, C) the slope is less -pronounced and in Atretochoana eiselti it is completely absent (Wilkinson and Nussbaum 1997). Despite the uncertainty with respect to the skull roof shape of Ymboirana, its parietal morphology allows us to rule out putative taxonomic affinities with rhinatrematids and is at least consistent with the morphology present in typhlonectids.
Maxillopalatines, nasopremaxillae, and septomaxillae
The anterior portion of the skull is severely damaged making the identification of nasopremaxillae and maxillopalatines tentative and based mainly on their positions and the presence of tooth pedi - cels. In our reconstruction of the fossil skull (Figs 10D, 11D), we also assume that there is no separate septomaxillae in the snout.One could argue that there is no evidence for the lack of septomaxillae in Ymboirana, as well as for the fusion of maxillae with the palatines, and the nasals with the premaxillae. However, in all adult modern caecilians the maxillae are fused to the palatines (e.g. Taylor 1969), and separate nasals, premaxillae, and septomaxillae are known ex - clusively in rhinatrematids, ichthyophiids, scolecomorphids, and, with the exception of the septomaxillae, in Herpele (e.g. Taylor 1969, Müller et al. 2005, Wilkinson et al. 2011), which all differ from Ymboirana in several other respects (for more details, see Diagnosis). Thus, in the absence of evidence favouring the con - trary, we assume that Ymboirana exhibits the same skull configur - ation as all other known typhlonectids.
Quadrates
The quadrates of Ymboirana seem to lack distinct otic pro - cesses, as expected given the narrow and delicate morphology of the bone tentatively interpreted as the stapes. This condition is common among typhlonectids where, with the exception of Chthonerpeton and Nectocaecilia, the otic process is limited to a faint bony projection or may even be completely absent (Wilkinson and Nussbaum 1997). Another distinct preserved portion of the quadrates in Ymboirana comprises the ptery - goid process. Caecilians vary in the form of articulation be - tween the quadrate and the maxillopalatine (Wake 2003). In rhinatrematids and ichthyophiids, these bones are separated ventrally by a large pterygoid, whereas in many teresomatan cae - cilians the quadrates bear a well -developed, anteriorly directed pterygoid process and there is a small bone, which has been variously named, that we interpret as a reduced pterygoid (for a review, see: Müller et al. 2005) that lies between, or lateral to, the pterygoid process of the quadrate and the maxillopalatine. In some other teresomatan caecilians, including typhlonectids, the pterygoid process of each quadrate is particularly well developed (at least in some cases incorporating an embryonic pterygoid; see: Ramaswami 1948), and there is no separate element be - tween the quadrate and the maxillopalatine in adults. Ymboirana exhibits a large pterygoid process of the quadrate and there are no signs of an isolated bone between the quadrate and the pre - served portions of the maxillopalatine, which is consistent with typhlonectid morphology, although the damaged and fragmen - tary nature of the fossil necessarily limits confidence in the ab - sence of such an element.
Stapes
All caecilians, except adult scolecomorphids, have stapes (e.g. Nussbaum 1985, Müller et al. 2009). Caecilian stapes comprise a typically broad footplate that fills the foramen ovalis and a nar - rower and elongate stapedial style that usually articulates with the otic process of the quadrate (e.g. Wilkinson and Nussbaum 1997). Caecilians vary in the presence or absence of a stapedial foramen (or groove) in the base of the stapedial style (Fig. 13), a feature present in rhinatrematids, ichthyophiids, chikilids, and most herpelids, but absent in all other taxa (e.g. Wilkinson et al. 2011, Kamei et al. 2012, personal observation). The frag - ment tentatively identified as the stapes in Ymboirana is short and narrow, with no sign of a stapedial foramen. Compared to typhlonectids (Fig. 13B, C), the stapes in Ymboirana closely re - sembles the condition present in Potomotyphlus, as in both taxa this element is limited to a short, rod -like style and a small foot - plate (Wilkinson and Nussbaum 1997).
Os basale
The azygous os basale is a distinctive compound bone of cae - cilians that forms the posterior, posterolateral walls, and floor of the cranial cavity (Sarasin and Sarasin 1890). This element includes the otic capsules and bears numerous processes (e.g. parasphenoid and basipterygoid processes) and foramina (e.g. foramen ovalis, jugular foramen, and antotic foramina; see: Maddin 2011). In the holotype of Ymboirana, the os basale is represented by scattered fragments, few of which are sufficiently large to enable identification of features that might help inform the affinities of Ymboirana, but a well -developed basipterygoid process places it within Teresomata.
Lower jaws
Caecilian mandibles each have only two bones, named by Eifertinger (1933): a toothed pseudodentary anteriorly and an adentigerous pseudangular posteriorly that articulates with the upper jaw and has a pronounced and distinctive retroarticular process (e.g. Nussbaum 1977, 1983). The left pseudangular is one of the best -preserved bones of Ymboirana acrux, and is virtually complete, except for its anteriormost part. The retroarticular process is robust, posteriorly elongate, and re - curved, unlike the narrow, straight, and horizontal condition observed in rhinatrematids (Nussbaum 1977, 1983). A distinct processus internus, a feature common to almost all caecilians (except scolecomorphids; Nussbaum 1985), is also present in the pseudangular of Ymboirana.
The orientation of the posterior end of the retroarticular process is variable among caecilians (Fig. 14). In some taxa, the distal tips aredirectedalmostcompletelydorsomedially(e.g. Scolecomorphus, Uraeotyphlus, and Atretochoana), whereas in most species they are directed posterodorsally (e.g. caeciliids, ichthyophiids, and most typhlonectids). In Ymboirana it is oriented posteriorly, a condition found in some, but not all, individuals of Typhlonectes natans and T. compressicauda. The taxonomic variation regarding this feature remains poorly documented and understood among caecilians, but the shape of the retroarticular process of Ymboirana is at least compatible with our interpretation of it as a typhlonectid.
Caecilian pseudangulars typically have a well -developed and dorsally projecting processus condyloides, posterior to the articular surface (Fig. 11B–D, B 1–D1; Supporting Information, Fig. S4). However, the processus condyloides of Ymboirana acrux is small and faint. Additionally, as far as is known, in all caecilians (except Atretochoana; Wilkinson and Nussbaum 1997) the fossa at the opening of the canalis primordialis is immediately anterior to the articular surface of pseudangular. In Ymboirana acrux, the dorsal region of the pseudangular, anterior to the articular surface, is flat and without signs of such a fossa or a distinct canalis primordialis. We assume this fossa bearing the canalis primordialis should presumably be located in the unpreserved anteriormost portion of the pseudangular, as its complete ab - sence is unknown among caecilians and would imply a complete rearrangement in the skull and lower jaw.
The dentition
Caecilian teeth are pedicellate, mono or bicuspid, and arranged in two upper and one or two lower tooth rows (e.g. Wake and Wurst 1979). Adult tooth crown morphology is fairly simple among caecilians but there are some important variations that make tooth morphology helpful for interpreting the affinities of Ymboirana acrux, including the number of cusps, degree of size heterodonty within and between tooth rows, degree of recur - vature, the extent to which teeth are slender or robust, the shape of the distal tip, and the presence or absence of blade -like lateral flanges, crenulations, or serrations (e.g. Wake and Wurst 1979, Greven 1986, Wilkinson 1991, Wilkinson et al. 2013). Structures resembling pedicels located near the mandibular symphyses are tentatively interpreted as forming an inner mandibulary tooth row in Ymboirana acrux that allows us to exclude it from the Siphonopidae -crown, which lack inner mandibular teeth (Wilkinson et al. 2011).
Bicuspid teeth are considered primitive within caecilians, and are found (with or without monocusped teeth) in all rhinatrematids, ichthyophiids, chikilids, and grandisoniids, dermophiids (except Dermophis), the siphonopids Microcaecilia and Brasilotyphlus, and the herpelid Boulengerula (Wake and Wurst 1979, Nussbaum 1988, Wilkinson 1997; M.W. personal observation). Like caeciliids, typhlonectids, siphonoform siphonopids, scolecomorphids, Herpele, and Dermophis, Ymboirana has only monocuspid teeth. There appears to be little variation in the size of individual teeth either within or between toothrowsin Ymboirana,whichissimilartoaquatictyphlonectids (Atretochoana, Typhlonectes, and Potomotyphlus) and different from the more semi -aquatic typhlonectids Chthonerpeton and Nectocaecilia, as well as many non -typhlonectids (including scolecomorphids, caeciliids, and herpelids). Judged by their pedicels, teeth of the inner rows are slightly smaller than those of the outer rows, which is typical of aquatic caecilians.
Wilkinson (1991) characterized typhlonectid teeth as mostly conical, recurved, and bearing lateral flanges. Exceptional morphologies include Typhlonectes compressicauda, which dif - fers from the other typhlonectids in having dilated, spoon, or chisel -shaped tooth crowns (Greven 1986), and the monotypic Potomotyphlus, which has very slender teeth that are more uni - form and less conical proximally. None of the preserved teeth in Ymboirana have the distinctive morphologies described for T. compressicauda or Potomotyphlus, and they also seem to lack lat - eral flanges, but these structures are diminutive and consequently especially difficult to observe, given the available resolution in our CT -scans and the fact that no tooth crowns were preserved in the surface of the slabs to allow direct observations.
The vertebral column
Variation in the vertebral morphology of caecilians exists (e.g. Peter 1894, Estes and Wake 1972, Taylor 1977, Wilkinson and Nussbaum 1997), but is very incompletely known and its utility in systematics is underdeveloped. Many postcranial elements, including the atlas and the second vertebrae, are preserved in the holotype of Ymboirana acrux. Because the posterior region of the body was not preserved, and some portions of the ver - tebral column are damaged or compressed, it is not possible to determine the exact number of vertebrae of Ymboirana acrux (a total of 67 are recognized). Based on the shape of the preserved posteriormost ones, and comparisons with other caecilians, we conclude that approximately one -half of the specimen’s total body length is preserved. In modern caecilians, the known range in vertebral number is from 68 to 307 (Lowie et al. 2022b), but it is well -established that vertebral counts vary intraspecifically (e.g. Parker 1958, Taylor 1968). Given the constraints on interpreting this character, we only made limited use of it, allowing us to rule out putative affinities between Ymboirana and abbreviated taxa (e.g. some species of Hypogeophis; see: Maddock et al. 2018).
The neural arch of the atlas in Ymboirana lacks a distinct neural spine. Among extant typhlonectids, well -developed neural spines in the atlas are present in Chthonerpeton and Nectocaecilia (Fig. 15B 2, D2), whereas in the remaining typhlonectids (e.g. Typhlonectes and Atretochoana) they are limited to faint ridges or even completely absent, such as in Potomotyphlus (Fig. 15C 2). Ymboirana trunk vertebrae have the typical pattern of cae - cilian vertebrae, including the presence of medially constricted amphicoelous centra bearing well -developed hypapophyseal keels and large, anteriorly projecting basapophyseal processes (e.g. Wake 1980). Morphological variations within this general pattern are virtually unknown for most caecilians, hampering our interpretations; however, Typhlonectidae are the only family in which differences in the postcranial osteology have been docu - mented in any detail (e.g. Wake 1980, Azpelicuelta et al. 1987, Wilkinson and Nussbaum 1999).
The obligate aquatic typhlonectids (Atretochoana, Potomotyphlus, and Typhlonectes) have distinctive elongate and narrow trunk vertebrae compared to other caecilians (Wilkinson and Nussbaum 1997, 1999). Additionally, their neural spines are limited to faint ridges or are absent from most vertebrae (Wilkinson and Nussbaum 1999). The latter condition is also observed in the similarly elongate trunk vertebrae of Ymboirana, in which the ridge is hardly distinguished dorsally. Ventral ridges on the basapophyseal processes constitute another distinctive feature of typhlonectids other than Chthonerpeton (Wilkinson and Nussbaum 1999). These ventral ridges are particularly well - developed in trunk vertebrae of Ymboirana.
The holotype of Ymboirana acrux also preserves several ribs along its vertebral column, but most are badly damaged. Two ribs, preserved near a trunk vertebra located at the middle por - tion of the specimen, remain almost fully preserved. These ribs are relatively straight, and also dorsoventrally expanded at their anteriormost portion, narrowing distally as in the more aquatic typhlonectids (i.e. not Chthonerpeton or Nectocaecilia; see: Wilkinson and Nussbaum 1999). The capitula of these ribs are elongate anteriorly, a feature previously reported as an apomorphy of Typhlonectes natans (Wilkinson and Nussbaum 1999).
The taxonomic affinities of Ymboirana acrux
According to the most recent classification proposal of Gymnophiona, representatives of the family Typhlonectidae can be differentiated from all other caecilians by having fused, sheet - or sac -like embryonic gills (Wilkinson et al. 2011). This minimal diagnosis, although useful for the taxonomy of living groups, cannot be applied to the case of Ymboirana given that early life -history stages are unknown. Wilkinson and Nussbaum (1999) have provided an extended diagnosis of Typhlonectidae and listed 28 features that are considered derived among teresomatan caecilians. However, the conditions of most of these are also unknown in Ymboirana (either because they are features of soft anatomy or of unpreserved regions of the skeleton). Of these characters, only the presence of monocusped teeth can be verified reliably in Ymboirana, and this feature is known in several non -typhlonectid caecilians too. Thus, unambiguous synapo - morphies of Typhlonectidae are not known in Ymboirana.
But the typhlonectid affinities of Ymboirana are evidenced by features it shares with some, but not all, living representatives of Typhlonectidae. For example, the presence of ventral ridges in the basapophyseal processes is reported in all typhlonectids except Chthonerpeton, and the presence of elongate and narrow trunk vertebrae and strongly distally tapering ribs are known exclusively in the subgroup of fully aquatic typhlonectids (i.e. Typhlonectes, Potomotyphlus, and Atretochoana; see: Wilkinson and Nussbaum 1999). Further, the presence of a strong ven - tral sloping on the parietals, as well as the straightness of the ribs and the lack of posterior flexure, are at least reminiscent of Chthonerpeton and Nectocaecilia (Wilkinson and Nussbaum 1999). Taken together these features suggest that Ymboirana is more closely related to the fully aquatic typhlonectids than it is to Chthonerpeton or Nectocaecilia, and is possibly its sister -group, a hypothesis requiring further evaluation through formal phylo - genetic analysis.
Taphonomy
During the collection of the Ymboirana acrux holotype skel - eton, taphonomic procedures, such as the definition of the azi - muthal orientation or the top and the bottom of the fossiliferous assemblage, were not followed (e.g. Holz and Simões 2005). Because the fossil was originally considered mining tailings, it was stored near other similar materials in a spoil pile exposed to weathering. Furthermore, there are still no studies focused on the taphonomy of caecilian remains. Therefore, taphonomic in - terpretations are naturally restricted. It is also worth mentioning that because lacustrine deposits routinely preserve remains of allochthonous terrestrial animals, our conclusions remain only tentative and based on indirect evidence. Thus, they should be taken with caution.
Tremembé Formation is interpreted as having been depos - ited in a playa -lake system developed during the Oligocene (Riccomini 1989). Lacustrine environments are character - ized by a poorly oxygenated bottom and low energy available for the transport of sediments (e.g. Wetzel 2001) and, conse - quently, biological remains. These two factors contribute to the fossilization process in lacustrine systems and both were present in the Tremembé palaeolake (Riccomini 1989, Bergue et al. 2015). The other lissamphibian found in Tremembé Formation, a neobatrachian anuran, was described based on damaged and disarticulated remains (Barcelos and Santos 2023). To explain the damaged condition of the specimen, it was proposed that the remains pass through a ‘bloat and float’ stage, which is common for vertebrates fossilized in aquatic environments (e.g. Syme and Salisbury 2014).
Despite being damaged and compressed, several elements of the specimen assigned to Ymboirana acrux are still articulated, including some vertebrae, but the skull bones and some ver - tebrae at the middle portion of the fossil are scattered (i.e. not aligned with the rest of the vertebral column). In this scenario, it is plausible that the fragmented condition of some elements oc - curred during the ‘bloat and float’ stage, whereas the presence of still articulated elements could be related to the low intensity of transport events between death and deposition of the remains. Additionally, the scattered vertebrae at the central portion of the specimen may be related to reworking made by scavengers or abdominal rupture, one of the most common causes of local - ized disarticulation (see: McNamara et al. 2012, and references therein).
The identification of Ymboirana acrux as a representative of Typhlonectidae, a group traditionally known by their aquatic or semi -aquatic behaviour, is another piece of evidence corrobor - ating the taphonomic interpretation here proposed. Regarding extant typhlonectids, species of Chthonerpeton were usually considered to be less adapted to the aquatic environment, and have been found in wet soil as well as in rivers, small ponds, and flocked marshes (Taylor 1968, Tanner 1971, Gudynas et al. 1988, da Silva et al. 2003; M.W. personal observation). Chthonerpeton indistinctum and C. noctinectes have been kept in aquaria, sug - gesting that representatives of this genus are indeed well adapted to freshwater environments too (Barrio 1969, da Silva et al. 2003). The remaining extant typhlonectid taxa are more adapted to a fully obligate aquatic lifestyle (e.g. Wilkinson and Nussbaum 1997, Gorzula and Señaris 1998, Maciel and Hoogmoed 2011). This finding also corroborates the hypothesis proposed by Barcelos and Santos (2023), who suggested that the fossil record of am - phibians (including caecilians) in South America may be biased favouring the preservation of aquatic and well -ossified taxa.
description: Published as part of Santos, Rodolfo Otávio, Wilkinson, Mark, Ribeiro, do Couto Graziela, Carvalho, Alberto B. & Zaher, Hussam, 2024, The first fossil record of an aquatic caecilian (Gymnophiona: Typhlonectidae), pp. 1-23 in Zoological Journal of the Linnean Society 202 (2) on pages 3-20, DOI: 10.1093/zoolinnean/zlad188, http://zenodo.org/record/14333139