Mambachiton fiandohana Nesbiư & Patellos & Kammerer & Ranivoharimanana & Andre & Wyss & Flynn 2023, gen.et sp. nov.
Creators: name: Nesbiư, Sterling J.
name: Patellos, Emily
name: Kammerer, Christian F.
name: Ranivoharimanana, Lovasoa
name: Andre
name: Wyss, R.
name: Flynn, John J.
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Archosauria
Mambachiton
Mambachiton fiandohana

description: Mambachiton fiandohana gen.et sp. nov.   
 Etymology: Genus name is a combination of the Malagasy mamba, meaning crocodile, and Ancient Greek χιτών (khiton), which can refer to a suit of armour. A secondary reference to the molluscan chiton is also intended, based on the evocative superficial similarity in armour morphology between Mambachiton fiandohana and polyplacophorans. Species name is the Malagasy word meaning source or beginning, in reference to this taxon’s phylogenetic position near the crocodile–bird split, the node to which Archosauria is definitionally linked.   
 Holotype — UA 8-25-97-132, an articulated series of anterior to posterior cervical vertebrae and osteoderms, anterior cervical vertebra neural arch fragment, cervical rib, and disarticulated osteoderms (Fig. 1A). A right partial postfrontal was directly associated with the holotype.   
 Referred specimen— FMNH PR 5065, middle trunk vertebrae, posterior trunk vertebrae, sacral vertebra one, anterior caudal vertebra, right scapulocoracoid, less scapulocoracoid, right ilium, and right proximal portion of femur (Fig. 1B).   
 All of the referred elements were found near the holotype in the same horizon over a number of field seasons. We consider it highly probable that the holotype and the referred bones pertain to a single individual, based on consistency of size, consistency of character states for a taxon near the base of Archosauria, similar preservation, and the fact that none of the elements are duplicated with the holotype or each other. However, considering the lack of direct association with the cervical series and several years over which these elements were collected in the field, we refrain from including them in the holotype. Our hypothesis that all of these bones belong to Mambachiton fiandohana can only be tested with the discovery of another specimen showing association of these and diagnostic cervical elements; for now, justification for referring these non-holotype elements to Mambachiton fiandohana is given in the description.   
 Locality and age— The holotype and referred elements of Mambachiton fiandohana were collected within the ‘basal Isalo II’ beds/ Makay Formation, in a coarse-grained grey sandstone at Locality M-13, east of Sakaraha in the southern Morondava Basin of south-western Madagascar (precise locality information on file at the AMNH, FMNH, and UA). Other taxa recovered from the holotype’s locality include the traversodontids Menadon besairiei and Dadadon isaloi (Flynn et al. 1999, 2000), the rhynchosaur Isalorhynchus genovefae (Flynn et al. 1999), the lagerpetid Kongonaphon kely (Kammerer et al. 2020), and the other reptile remains described below. The age of the ‘basal Isalo II’ deposits is Ladinian–Carnian (Mid-to-Upper Triassic) based on the correlations and caveats provided in Flynn et al. (2000) and Kammerer et al. (2020).   
 Diagnosis —The holotype of Mambachiton fiandohana differs from all other archosauriforms by the presence of the following combination of character states (asterisks indicate autapomorphies): *small tuber present at the dorsal margin of the prezygadiapophyseal lamina on the lateral side of the prezygapophysis in the cervical vertebrae; epipophyses absent on the dorsal surface of the postzygapophysis; laterally expanded dorsal portion of the neural spine; tapering anterior process of the cervical osteoderms articulating with a distinct groove on the ventral surface of the preceding osteoderm; smooth, unsculptured osteoderms; *high number of osteoderms per cervical vertebra (five to eight, depending on position); staggered arrangement of osteoderms across the midline.   
Potential further diagnostic character states for this new taxon discernable from the referred material include: neural spines of the trunk vertebrae and first sacral vertebra laterally expanded dorsally; weakly developed hyposphene–hypantrum articulations between the trunk vertebrae; lateral articulation surface of sacral rib anteriorly and posteriorly constricted, resulting in an ‘I’-shape; coracoids with short postglenoid processes; distal end of the scapula expands anteroposteriorly more than the proximal end; rugose tuber occurs just distal to the glenoid of the scapula; ilium with notch on the articular surface for reception of the ischium.  
 Ontogenetic assessment— No histological sectioning was performed, but a number of co-ossifications throughout the skeleton suggest that the holotype and referred materials of Mambachiton fiandohana werenearskeletalmaturityattimeofdeath (see: Griffin et al. 2020). The neurocentral sutures are completely co-ossified throughout the preserved portions of the cervical series (the first preserved neural arch is broken) and the trunk, sacral, and anterior caudal vertebrae are completely or partially co-ossified at the neurocentral sutures; only one side of the neurocentral suture is fully co-ossified in the posterior trunk vertebra. Co-ossification of the scapulocoracoids is complex; anteriorly it is complete, but a clear suture occurs posteriorly on the less element. No co-ossification occurs in this region on the right element.   
 Nomenclatural acts— This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank Life Science Identifiers (LSIDs) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix zoobank.org/. The LSIDs for this publication are as follows: urn:lsid:zoobank. org:pub: 5CD3A1E5-2882-4004-8875-FCD534CF88AF.   
 Mambachiton: urn:lsid:zoobank.org:act: A15DFD4D-8C6D-4940-B708-464540433830.   
 Mambachiton fiandohana: urn:lsid:zoobank.org:act: 630DA554- FF99-4046-BBAE-42391392FB00.   
 Description   
 Postffontal   
 A largely complete left postfrontal (UA 8-25-97-132; Fig. 2), found directly associated with the holotype, represents the only cranial material potentially referrable to Mambachiton fiandohana. It is uncertain whether this bone actually pertains to Mambachiton fiandohana, or to an otherwise unknown reptile from the quarry. However, it is clear based on its small size and other features, that this element is not that of a rhynchosaur, the most common archosauromorph from the locality. Yet, we cannot exclude the possibility that it could pertain to a different avemetatarsalian or other, unknown archosauriform. This bone, which forms the posterodorsal portion of the orbit, bears a complex sutural surface with the frontal that angles anterolaterally. The dorsal portion of the orbital margin is raised slightly relative to the rest of the element and is faintly rugose. A fossa on the posteromedial portion indicates that the postfrontal would have participated in the supratemporal fossa, but a contact surface at the posterior edge (either for the postorbital or parietal) indicates that the postfrontal did not participate in the supratemporal fenestra.   
 Cervical series   
The cervical series (UA 8-25-97-132) consists of six complete vertebrae, the first of which articulates with its neural arch; a fragment of the postzygapophyses of an anterior cervical was also recovered (Figs 3–8). The articulated cervical vertebrae encompass most of the cervical series, but their exact positions within the complete series (e.g. vertebrae 3–9) is unclear. The last element of the series is possibly transitional between the cervical and trunk series, but the parapophysis of this element still lies entirely on the centrum.  
The centra of the cervical series shorten posteriorly (Table 2) to the fourth preserved vertebra, before lengthening again slightly. The lateral sides of the centra bear a shallow fossa (Figs 3, 4) between the articular facets; the depth of these fossae is constant throughout the series, although taphonomic crushing has distorted them on the less side of the sixth preserved vertebra and on the right side of the fissh (Fig. 8A–F). The amphicoelous centra have circular facets. A distinct but weakly extended ridge on the midline of the ventral surface (=ventral keel) is present on the first three preserved vertebrae (Figs 6, 7E) and on the anterior half of the fourth (Fig. 7K), but is absent on the fissh (Fig. 8E) and sixth. Paramedian ridges parallel the midline ridge on the first complete vertebra (Fig. 6). On the anterior cervical vertebrae, the posterior surfaces of the parapophyses are oval, the long axis oriented anteroposteriorly, whereas in the more posterior cervical vertebrae, the long axis is oriented posterodorsally. The anteroventral position of the parapophysis creates a concave surface medial to the structure, but lateral to the midline, in the first two preserved vertebrae. The parapophyses and diapophyses are separated throughout the series, though these structures converge posteriorly as the centrum shortens.  
 The neural aches preserve a series of distinct laminae and deep fossae as in other long-necked archosaurs (e.g. Arizonasaurus babbitti, Nesbiư 2005; Teleocrater rhadinus, Nesbiư et al., 2018). Laterally, a deep centrodiapophyseal fossa (sensu Wilson et al. 2011) lies medial and ventral to the diapophysis, and is roofed by a posterior centrodiapophyseal lamina (sensu Wilson 1999). A few small laminae of bone are present within this fossa in the more anterior cervical vertebrae (Fig. 7A–F). A shallow parapophyseal centroprezygapophyseal fossa is present in the first preserved vertebra, deepening posteriorly as the result of a more pronounced prezygadiapophyseal lamina. A small tuber is present at the dorsal margin of the prezygadiapophyseal lamina on the lateral side of the prezygapophysis. A postzygapophyseal centrodiapophyseal fossa is absent in the anteriormost preserved neural arch (Figs 3–5), but appears in the second preserved neural arch (first preserved complete vertebra; Fig. 7A, B) and deepens posteriorly on that vertebra as the posterior centrodiapophyseal lamina develops. The postzygapophyseal centrodiapophyseal fossa is a deep pit, with its anterior extent obscured by the posterior centrodiapophyseal lamina in the second completely preserved vertebra; this condition is similar to that of Teleocrater rhadinus (e.g. NMT RB505) and saurischian dinosaurs (Langer and Benton 2006).   
 The prezygapophyses project anterolaterally at an angle about 45 o from the anteroposterior plane, and a large gap separates the articular surfaces at the midline (Figs 3–8). The postzygapophyses are angled about 45 o laterally (from horizontal) in posterior view; a wide gap between the articular surfaces widens anteriorly. No epipophyses are present at the dorsal margin in any of the cervical vertebrae (structures present in most avemetatarsalians), but a slight swelling of the neural spine posteriorly occurs in the homologous position. A fossa between the pre- and postzygapophyses excavates the base of the neural spine.   
 The neural spine is mediolaterally thin and situated over the posterior two-thirds of the centrum. A shallow fossa occurs at the lateral base of the neural spine throughout the preserved series. Most of the neural spines are hidden under osteoderms, but (computed tomography) reconstructions permit a number of observations (Figs 8, 9). Neural spines are expanded laterally at their dorsal margins, while the dorsal margins are nearly flat. The neural spines are slightly shorter (measured from their bases) than the centra are long. The neural spines shorten anteroposteriorly as their corresponding centra shorten posteriorly. The anterior edge of the neural spine slopes slightly anterodorsally in the more anterior elements, but not to the steep degree seen in aphanosaurs (Nesbiư et al. 2018). The posterior margin of all the neural spines are nearly vertical.   
A single isolated rib, missing much of the shass, is preserved anteriorly in the cervical series (Figs 3–6). Its disarticulation prevents its position within the cervical series from being assigned with certainty, but it appears to derive from the posterior portion of the series based on the positions of the capitulum and tuberculum. A sheet of bone on one side of the capitulum and the tuberculum may represent an anterior process, as is present in most archosauromorphs. The capitulum is broken, but a thin web of bone links it to the tuberculum.  
 Cervical osteoderms   
 Osteoderms cover the dorsal margins of the cervicals throughout the preserved series (in the holotype UA 8-25-97-132; Figs 3–5, 9, 10). Twenty articulated osteoderms are preserved on the right side of the holotype and nine on the less (Fig. 9). The long (mediolateral) axes of the osteoderms are deflected ventrolaterally about 60 o to the horizontal in posterior view. Each osteoderm is thick medially and progressively thins laterally as the osteoderms become dorsoventrally compressed. The dorsal surface of the medial edge sometimes preserves a ridge, with a slight depression adjacent to it. Where exposed, the surface of the medial portions of the osteoderms is slightly rugose, but no interdigitating contact between elements is evident. In anterior view, the lateral portion of the osteoderms flexes slightly dorsally. In dorsal view, the lateral margin is rounded, with a greater lateral extent more posteriorly, and the posterior margin varies from rounded to straight; some osteoderm margins bear small projections, but the paưern is inconsistent. CT data show that each osteoderm possesses a round (in cross-section), anteriorly tapering process (Fig. 9B, C, G–L). This process makes up half the length of the medial edge; although it occurs near the midline, it does not form the midline in articulation. No clear dorsal articular surfaces mark where osteoderms overlap (Figs 9, 10). Dorsal surfaces are smooth and unsculptured, but at least seven non-serial osteoderms on the right side bear a small dimple, which differs in size and location between elements. The osteoderms are consistently shaped throughout the column; they become only slightly longer (anteroposteriorly) posteriorly (see analysis below).   
 In Mambachiton, the configuration of osteoderms relative to each other and to their associated vertebra is unique. They are arranged in two rows along the midline, but are staggered across the midline, as in some pseudosuchians (e.g. Prestosuchus chiniquensis; Nesbiư 2011; Nundasuchus songeaensis, Nesbiư et al. 2014). The osteoderms are imbricated, such that the posterior edge of the more anterior osteoderm overlaps the anterior edge of the subsequent one. Additionally, the tapering anterior process articulates with a distinct groove (Fig. 9J, N) on the preceding osteoderm, an arrangement unique among archosauriforms. The number of osteoderms per vertebra exceeds the typical ‘one paramedian pair to vertebra’ ratio typical in Archosauriformes (Nesbiư 2011). Nine osteoderms cover the longest cervical vertebra in Mambachiton, whereas six or seven osteoderms occur per vertebra in the posterior portion of the preserved vertebral series. The number of paramedian osteoderms per vertebra in Mambachiton is the highest known among any archosauriform.   
 Variation- Our principal components analysis (Fig. 11) show some clustering based on relative location of the osteoderms along the cervical column. PC1 shows the most variance along Landmarks 3, and 1 (respectively) which correlates with the angle between the anterior process and the rugose medial flare. PC2 shows the most variance along Landmark 5, 3, and 4 (respectively) which correlates with the medial edge of the osteoderm.   
The Principal Component 1 vs. Principal Component 2 graph shows a cluster of similar shapes among anterior and posterior osteoderms respectively (Fig. 11). The shapes of more central osteoderms do not cluster as closely, varying more broadly across both principal components. Anterior osteoderms form a cluster between 0.00 and 0.15 along PC1, and 0.05 and -0.05 in PC2. Posterior osteoderms form a cluster between 0.00 and -0.15 along PC1, and show less variability along PC2, in a cluster between 0.00 and -0.05. This indicates that anterior osteoderm shapes typically show more variation along both PC1 and PC2, whereas the shape of the posterior osteoderms tends to vary along only PC1, and are less variable along PC2. Central osteoderms show higher variability along both PC1 and PC2, more so than either the anterior or posterior osteoderms. Central osteoderms form a cluster mainly between 0.05 and -0.10 along PC1, and between 0.05 and -0.05 along PC2. Two major outliers (central osteoderms 5 and B) cause the central osteoderms to show high deviation in shape, occupying more of the graph space along PC1 and PC2.  
The PCA graph (Fig. 11) shows liưle apparent separation between right and less osteoderm variation, but this may be the result of a relatively smaller sample size for the right elements. Interestingly, mirrored paired osteoderms (taking into account the slight staggering of the less and right osteoderm columns) plot relatively distantly from each other in PCA space (by a similar length), despite being similar to one another in anatomy. In this case, the mirrored pairs are between osteoderms 3 and A, 5 and C, and 6 and D.  
These variations among the osteoderms are congruent with our morphometric analysis. Anteriorly positioned osteoderms appear more rounded, whereas posterior elements appear more angular and more parallelogram shaped. Central osteoderms lie somewhere between these two end members, though the morphometric analysis results shows no clear paưern that would indicate a gradual transition in shape from one extreme to the other. Rather, anterior and posterior osteoderms show liưle variability, and plot separately within the PCA space. Central osteoderms are much more variable, and can overlap in morphology with either the anterior or posterior osteoderms. Less and right paramedian osteoderms are nearly identical, but not when paired together.  
 Other osteoderms   
A variety of other osteoderms were recovered with the holotype (UA 8-25-97-132), but their position in the skeleton is unknown. A cluster of three disarticulated, but similarly oriented, osteoderms prepared in ventral view are the most similar to the osteoderms articulated along the cervical vertebral column (Fig. 10). These osteoderms may have belonged to the anterior portion of the neck dorsal to the vertebrae, given that this part of the neck is slightly disarticulated and the morphology of these osteoderms closely matches the more anterior osteoderms in the articulated cervical series. These osteoderms are triangular, and this triangular shape becomes posterolaterally extended anteriorly. These three osteoderms have long and tapered anterior processes near their medial edges; their posterolateral portions broaden, as in the articulated neck osteoderms. A deep, anteroposteriorly oriented groove lies near the medial edge, probably the articulation surface for the long anterior process, reminiscent of the groove on the ventral surface of the cervical osteoderms (observed through CT data) discussed earlier. The medial surface, thickest dorsoventrally, is covered by a system of ridges and grooves, probably marking the midline and contact surface with its antimere osteoderm.  
 A rectangular osteoderm also was found directly associated with the holotype (Fig. 11F, G). We are tentatively assigning it to the holotype given its close proximity to the cervical series and because no other taxa from the quarry are known to bear osteoderms. Nevertheless, we do recognize the possibility that another, currently unknown reptile taxon (e.g. an aetosauriform) could be the source of this single osteoderm. Given the highly divergent shape of this osteoderm relative to of all others known from this locality, its position and anatomical directions are unknown. Accordingly, we employ the anatomical orientation of a typical paramedian aetosaur osteoderm, given their general similarities (Parker 2007, 2008, Desojo et al. 2013). This rectangular osteoderm is about twice as wide as long, with a distinct bend just off the mediolateral centre. The presumed anterior edge of the dorsal surface bears a flat lamina across its entire edge. The dorsal surface is ornamented; small dimples are present near the apex, and the medial and lateral portions bear more elongated grooves and rounded ridges. This isolated osteoderm is consistently thick dorsoventrally throughout its body. Its ventral surface is smooth and concave.Overall, this osteoderm is aetosaur-like in form, the presence of an anterior articulation surface (=anterior bar, anterior lamina) being rather rare among archosauriforms. The ornamentation of this osteoderm also is similar to small-bodied aetosaurs (e.g. Small and Martz 2013).   
 Table 2. Continued   
Two other fragments found with the holotype also appear to be osteoderms, but this cannot be confirmed definitively (Fig. 10E). Both have compressed, tapered processes that could correspond to the anterior processes of the other osteoderms.  
 Referred vertebrae   
 Four trunk vertebrae (part of FMNH PR 5065) found near the holotype are referred to Mambachiton fiandohana based on similar size, consistency of character states, preservation, and the fact that none of the elements are duplicated with the holotype or each other; we consider it possible, and even likely, that all these vertebrae belong to the same individual as the holotype (Figs 12, 13). Two of these vertebrae are from the middle trunk (Fig. 12), and two from the posterior trunk (Fig. 13), judging from centrum shapes as well as diapophysis and parapophysis shapes.   
The well-preserved middle trunk vertebrae (Fig. 12) both are essentially complete. The centra are strongly waisted between the anterior and posterior articular facets, and a slight fossa is present on the lateral sides, just ventral to the closed neurocentral suture. The centra have a length (25 mm) to articular facet height (19 mm) ratio of 1.32, measured from the anterior articular facet. The amphicoelous articular facets are oval, with a slightly longer dorsoventral than mediolateral axis. Both centra lack a ridge on the midline ventrally.  
 The neural arches bear laterally extended transverse processes composed of the combined stalks of the parapophysis and diapophysis (Fig. 12). The more anteroventrally located parapophysis has a concave articular surface whereas the posterodorsally and laterally located diapophysis has a convex articular surface. A deep centrodiapophyseal fossa (sensu Wilson et al. 2011) lies between and ventral to the parapophysis and diapophysis, and is framed posteriorly by a posterior centrodiapophyseal lamina (sensu Wilson 1999). More posteriorly, a postzygapophyseal centrodiapophyseal fossa is bordered anteriorly by the posterior centrodiapophyseal lamina and posterodorsally by the postzygadiapophyseal lamina. An anterior, short, and thick centrodiapophyseal lamina is present. In dorsal view, the transverse process is triangular, with the most lateral point at the posterior edge. No parapophyseal centroprezygapophyseal fossa is present in these vertebrae, unlike in Teleocrater rhadinus (NMT RB 500).   
 The prezygapophyses are angled ~30 o from the mediolateral plane in anterior view, and a wide gap (2.5 mm) separates them at the midline (Fig. 12). The lateral edge of the prezygapophysis bears a thin lamina that terminates posteriorly at the anterior edge of the base of the neural spine. Medially, the prezygapophyses are well separated, a gap that is interpreted as a hypantrum. The bases of the prezygapophyses meet in a depression at the midline, slightly excavating the anterior part of the base of the neural spine. The postzygapophyses have slightly concave articular facets; a thin lamina lies at the medial edge of the facet. The postzygapophyses are separated by a ventrally flat piece of bone, similar to that of Teleocrater rhadinus (NMT RB 516; Nesbiư et al. 2018 text fig. 9); hence, we interpret this structure as a hyposphene, similar to those of other archosaurs (see below). A deep fossa lies between the postzygapophyses, and excavates the posterior portion of the base of the neural spine.   
 The neural spine lies dorsal to the posterior half of the centrum (Fig. 12). No dorsally facing fossa is present on the lateral side of the neural spine, in contrast to the one that occurs in Teleocrater rhadinus (NMT RB500). The anterior edge of the neural spine is dorsoventrally straight in lateral view, whereas the posterior edge is slanted posterodorsally at the point where the dorsal tip lies posterior to the postzygapophyses. The dorsal portion of the neural spine is expanded on the lateral sides, rounded but rugose on the dorsal surface, and slightly arched in lateral view. The posterodorsal tip is more rounded than the anterodorsal tip.   
The posterior part of the trunk is represented by two vertebrae (Fig. 13). Of the two, one (Fig. 13A–F) is considered more anterior, because the diapophysis and parapophysis are still slightly separated, whereas the two structures are combined into a synpophysis in the second posterior trunk vertebra (Fig. 13G– L). The amphicoelous centrum of the posterior trunk vertebrae bears rounded anterior and posterior articular surfaces. The centrum surfaces in the more posterior vertebra are mediolaterally wider than dorsoventrally tall. The centra of both posterior trunk vertebrae are shorter than those of the more anterior trunk vertebrae, yet a clear fossa is still present in each posterior trunk vertebra on the lateral side of the centrum just ventral to the neurocentral suture. The ventral surfaces of the centra are smooth at the midline.  
The neurocentral sutures are closed on the right side but open on the less in both posterior trunk vertebrae (Fig. 13). In the more posterior of these, the parapophysis and diapophysis both lie dorsal to the neurocentral suture on the neural arch. As in the more anterior trunk vertebrae, the diapophysis and parapophysis are both located on a transverse process. The articular surfaces of the diapophysis and parapophysis are convex and continuous in both (Fig. 13), the articular surfaces of the diapophysis and parapophysis not being differentiated.The homologous lamina surrounding the diapophysis and parapophysis in the more anterior trunk vertebrae (e.g. anterior and posterior centrodiapophyseal lamina) are not as laterally expanded and are distinctly rounded in these posterior trunk vertebrae. However, the fossae (centrodiapophyseal fossa, postzygapophyseal centrodiapophyseal fossa) are similarly deep. The more posterior vertebra (Fig. 13G–L) has a deep prezygapophyseal centrodiapophyseal fossa, unlike any of the other trunk vertebrae.  
 In lateral view, the prezygapophyses and postzygapophyses lie in the same horizontal plane, and slope about 35 o relative to the transverse plane in posterior view. Anteriorly, the articular surfaces of the prezygapophyses are separated by a clear gap that widens anteriorly; this gap develops posteriorly into a depression at the midline, where it excavates the base of the neural spine. This gap is similar to that of the hypantrum of the other trunk vertebrae of Mambachiton fiandohana and of other archosaurs (Stefanic and Nesbiư 2018, 2019). The thin lamina of bone on the lateral side of the postzygapophysis in more anterior vertebrae is absent in the posterior trunk vertebrae. Posteriorly, the articular surfaces of the postzygapophyses are slightly concave and are separated by a thin wedge of bone, like that of the more anterior trunk vertebrae. A deep fossa excavates the posterior edge of the neural spine and separates the postzygapophyses on the midline.   
The neural spine of the posterior trunk vertebrae lies dorsal to the posterior half of the centrum as in the other trunk vertebrae. The anteroposterior length of the neural spine is just short of the anteroposterior length of the centrum in the posterior trunk vertebrae. The mediolaterally thin neural spine expands laterally, anteriorly, and posteriorly at its dorsal margin. In dorsal view, the dorsal surface of the neural spine is nearly flat, and slightly rugose. The anterior edge of the neural spine is dorsoventrally straight, whereas the posterior margin is posterodorsally oriented. In the more posterior trunk vertebrae, the neural spine is taller and more anteroposteriorly restricted than in its anterior counterpart. In the more posterior element (Fig. 13G–L), a thin lamina of bone at the midline projects posteriorly.  
 Sacral vertebra. A single sacral vertebra (FMNH PR 5065) is known, here identified as the first sacral (Fig. 14). Much of the less sacral rib is broken away, but this vertebra is otherwise complete. The ‘C’-shaped articular surface of the sacral rib articulates with the medial side of the anterior process of the ilium (see below), indicating that this is primordial sacral vertebra 1, and the precise articulation indicates that these two elements are likely from the same individual. The centrum is waisted between the anterior and posterior articular facets, and a fossa is present on the lateral side of the centrum just ventral to the neurocentral suture, similar to that of Teleocrater rhadinus (NMT RB1395). The neurocentral suture is closed. The anterior articular surface is concave, whereas the posterior surface is flat.   
 The sacral ribs extend directly laterally, having a small ventral component. In ventrolateral view, the sacral rib has an ‘I-beam’ construction, with flat dorsal and ventral faces and concave anterior and posterior surfaces. The posterior concave surface is deep relative to its counterpart in Teleocrater rhadinus (NMT RB1395). The anterodorsal tip of the rib is expanded and curved anteriorly to produce a ‘C’-shaped articulation surface with the ilium. The anterior extension of the rib in Mambachiton fiandohana is greater than that of Teleocrater rhadinus (NMT RB1395) and more similar to that of early dinosauriforms (e.g. Silesaurus opolensis, Dzik and Sulej 2007). The posterolateral and ventral portions of the sacral rib are posteroventrally deflected, but not as posteriorly expanded as in Teleocrater rhadinus (NMT RB1395). The contact between the sacral rib and vertebra forms a strong ridge dorsally, posteriorly, and ventrally, but no clear suture is present between the two elements; this indicates that co-ossification is complete, similar to that of the only known sacral 1 of Teleocrater rhadinus (NMT RB1395). Furthermore, the sacral rib aưaches to the anterior half of the centrum, a diagnostic feature of the first primordial sacral vertebra in archosaurs (Nesbiư 2011). Sacral ribs are not shared between sacral vertebrae in Mambachiton fiandohana.   
 The neural arch is co-ossified to the centrum, with no sign of a neurocentral suture (Fig. 14). In anterior view, mediolaterally thin laminae frame the lateral sides of the neural canal; just lateral to these laminae, there are small but deep fossae bordered laterally by the raised contact with the sacral rib. The prezygapophyses are angled ~45 o to the mediolateral horizontal plane and are separated by a noticeable gap (5 mm in length). The processes meet medially at a fossa on the midline. Posteriorly, the postzygapophyses are angled ~45 o to the mediolateral horizontal plane and meet at the midline at a point; a deep fossa between the postzygapophyses penetrates the base of the neural spine. Like the anterior surface of the vertebra, mediolaterally thin laminae forming the lateral side of the neural canal border a fossa that extends to the sacral rib. The neural canal, the largest of all the vertebrae, is oval in outline, with a long, dorsoventrallyoriented axis. The ventral portion of the neural canal excavates the body of the centrum.   
The neural spine lies dorsal to the posterior half of the centrum. The anterior and posterior edges of the neural spine bear thin laminae at the midline. The dorsal end of the neural spine is more prominently expanded (in all directions) and dorsally rounded than in other preserved vertebrae. The surface of this expansion is rugose.  
 Caudal vertebra. There is a single caudal vertebra (part of FMNH PR 5065; Fig. 15), from the anterior portion of the series, as suggested by its large, well-developed transverse processes and the lack of chevron facets on the posteroventral portion of the centrum. The neural spine is broken at its base, but this vertebra is otherwise complete. This centrum is amphicoelous and longer than tall, with distinct fossae on the lateral surface ventral to the transverse processes. Ventrally, a small depression occurs on the midline near the posterior portion, but no depression is present anteriorly. A fused caudal rib/transverse process links the centrum with the fused neural arch. Here, a distinct rim circumscribes the transverse process, but no suture is evident; this rim marks where the caudal rib has completely fused to the rest of the vertebra. The wing-like transverse processes project laterally, having a small posterior component; in anterior view they curve very slightly ventrally towards their tips. The right transverse process appears to expand anteroposteriorly at its lateral margin. A rugose surface is present on the dorsal surface of both transverse processes about halfway to their lateral terminations. The rugose surface is larger anteriorly and decreases in size posteriorly.   
 The base of the neural spine is positioned over the posterior half of the centrum, and deep fossae lie between the prezygapophyses and postzygapophyses. The prezygapophyses project ~30 o from horizontal, and angle ~45 o anteriorly. There is a clear gap at the midline between the two articular surfaces. A broad, flat lamina lies between the postzygapophyses at the midline.   
 Pectoral girdle   
 Scapulocoracoid: The scapula and coracoid (parts of FMNH PR 5065) are co-ossified, with only remnants of a suture in both the less (Fig. 16 E–H) and right (Fig. 16 A–D) elements; a raised rim marks the junction of the scapula and coracoid. The main bodies of the scapulocoracoids retain much of their original three-dimensional form, but the distal portion of the right scapulocoracoid is crushed and slightly displaced. The scapular blade is incompletely preserved distally, and parts of the anteroventral edge of the coracoid are missing. The scapula is wide proximally, constricted anteroposteriorly dorsal to the glenoid, and it expands anteroposteriorly distally. The glenoid is formed 50% by the scapula and 50% by the coracoid. As in early-diverging avemetatarsalians (e.g. Teleocrater rhadinus NMT RB 480; Asilisaurus kongwe NMT RB 159) and some paracrocodylomorphs (Poposaurus gracilis; Schachner et al. 2020; Postosuchus spp., Peyer et al. 2008), the glenoid projects posteroventrally and the medial edge of the glenoid is only barely visible in lateral view(Fig.16A,E). A slight rim circumscribing the glenoid separates the humeral articulation surface from the body of the scapulocoracoid. A distinct, large, and posterolaterally expanded tuber is confluent with the dorsal margin of the glenoid. This tuber is mediolaterally compressed and covered in a rugose surface stretching from the dorsal margin to the glenoid. This tuber is the aưachment site for the scapular head of the M. triceps, as in Crocodylia (Mook 1921, Meers 2003). Most archosauriforms have a scar or tuber in the same location, but the large size of this feature in Mambachiton fiandohana is similar to that of the loricatan (pseudosuchian archosaur) Batrachotomus kupferzellensis (Gower and Schoch 2009, text fig. 3). However, the position of this tuber in Mambachiton fiandohana differs from that of the more distally shissed feature of Batrachotomus kupferzellensis and other pseudosuchians, and instead more resembles that of some avemetatarsalians (e.g. Asilisaurus kongwe NMT RB 159), or taxa with a slightly more distally shissed scar (e.g. Teleocrater rhadinus, NMT RB 478). The proximal portion of the scapula, anterior to the glenoid, is concave distal to the anteroposteriorly-oriented acromion ridge that is located on the anterodorsal edge of the proximal portion of the scapula, a character state present in proterochampsids and crown group archosaurs (Nesbiư 2011). The acromion ridge encircles the anterodorsal margin of the proximal portion of the scapula.   
 In lateral view, both the anterior and posterior edges of the scapular blade are distinctly concave, whereas the anterior edge of the less scapulocoracoid is much more concave. The anterior and posterior edges are rounded from the lateral to the medial surfaces. The posterior edge is thicker than the anterior one. No proximodistally-oriented ridge occurs on the posterior edge, in contrast with some early avemetatarsalians (e.g. Teleocrater rhadinus, NMT RB 480; Asilisaurus kongwe NMT RB 159) which have a clear ridge. The distal end of the scapular blade is greatly expanded relative to its midshass, much more so than in other early avemetatarsalians (e.g. Teleocrater rhadinus, NMT RB 480; Asilisaurus kongwe, NMT RB 159). The distal surface is incompletely preserved, but appears thicker mediolaterally than the mid-portion (as determined from the right element). The medial surface is concave; a mediolaterally thicker portion extends distally from the glenoid.   
 In ventrolateral view, the oval-shaped coracoid is largely complete (Fig.16A–D).Itssuturewiththescapulaisanteroposteriorly straight, and nearly continuous (as determined from the right side), lacking a clear notch. The orientation of the glenoid matches that of scapula, but the coracoid portion of the glenoid extends more posteriorly than the scapular component, but not as asymmetrically as in Asilisaurus kongwe (NMT RB159). The posteroventral portion of the coracoid expands posteriorly and ventrally to the glenoid, creating a clear postglenoid process. A cless separates the ventral rim of the glenoid from the ventral margin of the coracoid. A similar expansion and cless is present in early crown group archosaurs (e.g. Asilisaurus kongwe NMT RB 159; Batrachotomus kupferzellensis, Gower and Schoch 2009, text fig. 3). The posteroventral edge of the coracoid of Mambachiton fiandohana is thickened mediolaterally relative to the anterior half, as in other archosaurs, and the ventral edge is rounded in ventrolateral view. In ventral view, the thickened ventral edge bears a weakly defined groove that parallels the ventral edge. A similar groove is present in Postosuchus alisonae (Peyer et al. 2008) and crocodylomorphs (Nesbiư 2011). The cless between the glenoid and the ventral edge terminates anteriorly as a slight depression on the lateral surface. The coracoid foramen is located anterior to the glenoid, just posterior to the center of the coracoid. The anteroventral portion of the coracoid is mediolaterally thin.   
 Ilium: A right ilium (part of FMNH PR 5065; Fig. 17) is referred to Mambachiton fiandohana because the articular surface for the first sacral vertebra on the medial side of this element corresponds nearly perfectly with the articular surface of the first sacral vertebra found at the same locality. In lateral view, the ilium has a short anterior process and a posteriorly elongated posterior process, similar to that of archosauriforms (e.g. Euparkeria capensis; Ewer 1965) and a variety of archosaurs (e.g. ornithosuchids; von Baczko and Ezcurra 2013). The anterior process (=preacetabular process) is short and does not extend anterior to the pubic peduncle. The surface dorsal to the supra-acetabular crest is slightly convex and smooth; Mambachiton fiandohana does not bear a vertical crest dorsal to the supra-acetabular crest, in contrast to its presence in some pseudosuchians (Arizonasaurus babbitti, MSM P 4590; Batrachotomus kupferzellensis, SMNS 80268) or avemetatarsalians (e.g. Teleocrater rhadinus, NHMUK PV R 6795; Asilisaurus kongwe, NMT RB 159). The anteroventral margin of the anterior process is slightly thickened mediolaterally relative to the rest of the iliac blade, as in Teleocrater rhadinus; however, the anterior tip of the process is not curved medially as it is in Teleocrater rhadinus (NHMUK PV R6795). The dorsal margin of the iliac blade, from the anterior to the posterior process, is slightly convex in lateral view and the dorsal margin is tapered. In lateral view, there is a slight depression dorsal to the supra-acetabular crest, between the anterior and posterior processes. The posterior process tapers to a rounded posterior termination. The ventral edge of the posterior process is expanded medially into a shelf on the medial surface. In ventral view, this shelf is slightly concave and is reminiscent of the brevis fossa or shelf present in avemetatarsalians (e.g. Teleocrater rhadinus, NMT RB1039; Asilisaurus kongwe, NMT RB 159), but also is not that dissimilar to morphology of the same area in Batrachotomus kupferzellensis (SMNS 80268) and several other eucrocopod archosauriforms, such as Chanaresuchus bonapartei and Gracilisuchus stipanicicorum (Ezcurra pers. comm.). In posterior view, the ventral termination of the ilium is triangular, with a slightly concave ventral side.   
 The acetabulum is relatively deep compared to that of archosauriforms primitively; a well-developed supra-acetabular crest forms its dorsal portion. The sharp portion of the supra-acetabular crest fails to reach the articulation surface with the pubis. A weakly rimmed depression lies within the acetabulum (Fig. 17A), a feature also present in Teleocrater rhadinus (NMT RB1039). Just ventral to it lies an antitrochanter surface, which reaches the articular surface with the ischium. The acetabulum likely is closed, given that its ventral margin is ‘v’-shaped in lateral view. The ischial peduncle, thickest posteriorly, thins anteriorly; a slight rim marks the posterior boundary of its articulation with the ischium. As in other early-diverging avemetatarsalians (Nesbiư et al. 2017; character 414), a small notch occurs on the articulation surface with the ischium, slightly anterior to the main body of the ischial peduncle and the ventral point of the acetabulum (Fig. 17). The articulation between the ischium and ilium is longer than between it and the pubis. The articulation surface of the ischium meets the pubis at a point at an angle of ~50 o.   
 The medial surface of the ilium bears two distinct sacral rib scars corresponding to primordial sacrals one and two, judging from comparisons with other archosauriforms (Erythrosuchus afficanus; Gower 2003) and archosaurs (see: Nesbiư 2011). The first sacral rib articulates in the junction between the main body of the ilium and its anterior process. A distinct scar, as confirmed by the preserved first primordial sacral vertebra (see above), occurs on the anterior process of the ilium, nearly reaching the anterior end of this process. The posterior edge of the first sacral rib scar meets the anterior edge of the second sacral rib scar. The laưer tapers posteriorly and meets the anteroposteriorly oriented medial ridge. Based on this scarring, the second sacral rib appears to have articulated with both the dorsal and ventral surfaces of the medial ridge, the articulation extending to within 2 cm of the posterior end. This ridge meets the ventral edge of the posterior process, forming the brevis shelf. The medial ridge thins posteriorly. A large rim separates the sacral rib scars from the smooth medial face of the acetabular portion of the ilium.   
 Femur: The proximal portion of a less femur (part of FMNH PR 5065; Fig. 18) was recovered from the locality; the size of its head closely matches that of the acetabulum (FMNH PR 5065). Where present, the poorly preserved surface of the femur is striated proximodistally. Proximally, a straight groove follows the long axis of the femoral head, as in some early avemetatarsalians (e.g. Teleocrater rhadinus, Silesaurus opolensis) and some pseudosuchians (e.g. poposauroids; Nesbiư 2011). Three tubera are present proximally, a prominent anterolateral one, a weakly developed anteromedial one, and a rounded and broad posteromedial one. The medial edge of the posteromedial tuber bears a proximodistally oriented ridge. A faint line between smooth and fibrous bone divides the anterior portion of the anterolateral tuber; the proximal portion of this line marks the medial portion of the femoral head, which transitions smoothly from the shass as in Teleocrater rhadinus (Nesbiư et al. 2018, text fig. 21) and other aphanosaurs. Medially, much of the fourth trochanter is eroded off, but the remaining portion indicates that it was positioned low, more similar to Teleocrater rhadinus than lagerpetids (Dromomeron gregorii; Nesbiư et al. 2009), silesaurids (e.g. Silesaurus opolensis; Dzik 2003) or dinosaurs (e.g. Herrerasaurus ischigualastensis, Coelophysis bauri). A shallow depression lies anteromedial to the fourth trochanter. Anterolaterally, the proximal surface is slightly concave and bounded distally by a swelling (Fig. 18). This swelling appears to be homologous with the scar for the inferred M. iliotrochantericus caudalis scar in Teleocrater rhadinus (NHMUK PV R6795), and the anterior trochanter in ornithodirans (e.g. Dromomeron gregorii, silesaurids, and dinosaurs). The femoral shass is hollow, as indicated by the presence of a diagenetic calcite infilling of the core; the cortex is ~2.3 mm wide, and the maximum cross section width is 16.4 mm.
description: Published as part of Nesbiư, Sterling J., Patellos, Emily, Kammerer, Christian F., Ranivoharimanana, Lovasoa, Andre, Wyss, R. & Flynn, John J., 2023, The earliest-diverging avemetatarsalian: a new osteoderm- bearing taxon from the Triassic (? Earliest Late Triassic) of Madagascar and the composition of avemetatarsalian assemblages prior to the radiation of dinosaurs, pp. 327-353 in Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) (Zool. J. Linn. Soc.) 199 (2) on pages 329-345, DOI: 10.1093/zoolinnean/zlad038, http://zenodo.org/record/8426293