Soumyasaurus aenigmaticus, Sarıgül & Agnolín & Chatterjee, 2018
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publication ID |
https://doi.org/10.5281/zenodo.15724747 |
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persistent identifier |
https://treatment.plazi.org/id/03EFE354-B424-FFE8-D18F-FB49FB86FF14 |
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treatment provided by |
Felipe |
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scientific name |
Soumyasaurus aenigmaticus |
| status |
gen. nov., sp. nov. |
Soumyasaurus aenigmaticus gen. nov., sp. nov.
Etymology. Species name represents the nature of the specimen, derived from the Latin word “aenigma” that means “enigma or riddle”.
Holotype. TTU-P11254 , partial left dentary. Type locality. Post Quarry ( MOTT 3624 ), Garza County, Texas.
Type horizon. Tecovas Formation (or the lower unit of the Cooper Canyon Formation sensu Martz, 2008), collected from the main thin, fossil-rich horizon that is situat- ed about 8 meters below the top of the formation ( Martz et al., 2013). The Post Quarry horizon corresponds to Norian ( Sarıgül, 2017b).
Diagnosis. A minute silesaurid distinguishable from all other members of the clade except Asilisaurus kongwe Nesbitt et al., 2010 by having smooth and conical dentary teeth that have no expansion or curvature above the root. S. aenigmaticus differs from A. kongwe in having a Meckelian groove restricted to ventral margin of the dentary. Because the anterior portion of TTU-P11254b is not preserved, it remains unknown whether the dorsal margin of the anterior dentary of S. aenigmaticus is convex as in that of A. kongwe . Similarly, apical sides of the preserved teeth of S. aenigmaticus are mostly obliterated and cannot be compared with the teeth of A. kongwe that possess weakly serrated carinae at the tip of each crown.
Description and remarks. The dentary fragment is a slender and transversely narrow element with an elliptical cross-section. The lateral side is almost featureless except for the presence of several neurovascular foramina ( Figure 5A View Figure 5 ). Few lingual pits at the alveolar margin are detected on the medial side, and a narrow Meckelian groove runs the length of the dentary on the ventral margin ( Figure 5B View Figure 5 ). Four erupted teeth are preserved, however, there are about 11 closely spaced alveoli, making the dentary tooth count of 15 or more ( Figure 5C View Figure 5 ). Characteristic grooves and foramina implying the presence of a keratinous beak in silesaurids (e.g., Dzik, 2003; Langer and Ferigolo, 2013) are lacking on the edentulous end and this portion is interpreted as the posterior end of the tooth row. The Meckelian groove also tapers towards the counter direction of the edentulous portion, indicating that TTU-P11254b probably represents a left side dentary ( Figure 5 View Figure 5 B-C).
Presence of ankylothecodont teeth, which means the teeth are fused at the base to the dentary bone ( Nesbitt, 2011, char- acter 174) is the main unambiguous synapomorphy shared by S. aenigmaticus and Silesauridae . Silesaurid teeth are ankylosed to their sockets by fibrous tissues which creates a collar-like structure around the tooth base as in Silesaurus , Sacisaurus and Diodorus ( Dzik, 2003; Kammerer et al., 2012; Langer and Ferigolo, 2013), contrasting the condition of other ankylothecodont archosauromorphs where the teeth are strongly attached to their base by bony ridges (e.g. Ezcurra, 2014). Another feature shared by S. aenigmaticus and other silesaurids except Asilisaurus kongwe is the Meckelian groove restricted to the ventral border on the medial side of the dentary ( Nesbitt, 2011, char- acter 152; Dzik, 2003; Ferigolo and Langer, 2007; Nesbitt et al., 2010; Kammerer et al., 2012). The anterior tip of the dentary is not preserved; thus, it is not possible to discern whether it is rounded or tapers anteriorly ( Nesbitt, 2011, character 155).
Technosaurus smalli is the only other silesaurid described from the same quarry with a holotype consisting of a premaxilla and an incomplete dentary (e.g., Chatterjee, 1984; Nesbitt et al., 2007) ( Figures 6 View Figure 6 A-C). T. smalli and S. aenigmaticus share the typical silesaurid synapomorphies of having a silesaurid-type ankylosed dentition and a ventrally restricted Meckelian groove. The tip of the dentary is missing in both taxa. Besides the obvious size difference, the major contrast between the two taxa is the dental morphology. The lower jaw dentition of T. smalli comprises triangular and possibly tricuspid teeth with unpronounced denticles on the dental edge and faint striations on crown surface; a structure which is clearly different from that of S. aenigmaticus .
Dentition of S. aenigmaticus is also very different from the other silesaurids with typical leaf-shaped teeth ( Dzik, 2003; Kammerer et al., 2012; Langer and Ferigolo, 2013). Although they differ in the position of the Meckelian groove, the dentary teeth of A. kongwe probably offers the best com- parison for the dentition of Soumyasaurus than any other silesaurid in both size and morphology. However, teeth of A. kongwe possess a serrated carina ( Nesbitt et al., 2010, Figure 1 View Figure 1 ), but this feature cannot be detected in those of S. aenigmaticus since the apical portions of the preserved teeth are either missing or severely damaged.
The inclusion of S. aenigmaticus in the data matrix of Nesbitt et al. (2017) resulted in its nesting within the basal dinosauriform clade Silesauridae . With the aim to test the robustness of tree topology, Bremer supports are calculated for each node. The support of major archosaur clades, as Or- nithodira, Dinosauriformes, and Crurotarsi is relatively low (Bremer support = 1), as previously recognized and discussed by Nesbitt (2011). The clade Silesauridae + Soumyasaurus also has a Bremer support = 1. The inclusion of Soumyasaurus within Saurischia, sister group to Dinosauria or Theropoda results in a tree of a length of 1392. This implies that a single step may change the position of S. aenigmaticus . Thus, S. aenigmaticus is attributed to Silesauridae , but with some degree of uncertainty.
Sauria McCartney, 1802, sensu Gauthier, Kluge and Rowe 1988
Archosauromorpha Huene, 1946, sensu Benton, 1985
Gen. et sp. indet.
Referred specimens. TTU-P 11254c, partial braincase; TTU-P 11254d, cervical vertebrae; TTU-P 11254e, left scapula.
Description and remarks. The braincase is poorly preserved, and the intimately fused bones complicate the demarcation of each element. The foramen magnum is obliterated under the collapsed roof of the braincase; the only putative feature visible at this area is a damaged foramen which might be related to a segment of the occipital vein ( Figure 7A View Figure 7 ). The occipital condyle is round in posterior view and the basioccipital probably forms most of the occipital condyle with limited contribution of exoccipitals as in most saurians ( Figure 7A View Figure 7 ). The condylar neck is ventrally constricted at the base, a condition that is also very apparent in lateral view, and then the basioccipital flares again to form a pair of medially wellseparated and anteroposteriorly long basal tubera ( Figures 7 View Figure 7 B-C). Each basal tuber displays slight excavations on the lateral side ( Figures 7 View Figure 7 C-D). The sphenoidal contribution to the basal tubera, if any present, cannot be detected.
The left lateral side comprise a large secondary tympanic opening (i.e., fenestra pseudorotunda) encapsulated by the fused exoccipital-opisthotic complex (oto-occipital or otoccipital), which is dorsolaterally pierced by a foramen that possibly transmitted a segment of the occipital vein, similar to what is identified on the opposite side of the braincase ( Figures 7 View Figure 7 C-D). The bony frame around the fenestra pseudorotunda is a distinguishing character of extant archosaurs, and TTU-P11254c represents few of the fossil examples in which this gracile structure is preserved ( Gower and Weber, 1998). The floor of the fenestra pseudorotunda main- tains a direct connection between the cranial cavity and the vagus foramen at the occipital side, from where the vagus (X) and accessory (XI) cranial nerves are carried along with the posterior jugular vein ( Figures 7 View Figure 7 C-D). In anterior view, the otic capsule possesses a distinct crescentic groove on its anteromedial border ( Figure 7E View Figure 7 ).
Although the fused exoccipital-opisthotic complex of TTU-P11254c is described in many archosauriforms (e.g., Gower and Sennikov, 1996; Currie, 1997; Gower and Weber, 1998; Gower, 2002), a posterior diversion of the vagus foramen evolved independently in crocodilians by the emergence of a secondary lamina ( Klembara, 2005), in neotheropods by the projection of the metotic strut (e.g., Currie, 1995; Sampson and Witmer, 2007; Fiorillo et al., 2009), and possibly in pterosaurs by the posterior ossification of the braincase (e.g., Bennett, 1991; Kellner, 1996). The posterior shift of the vagus foramen in TTU-P11254c is reminiscent to the condition described in neotheropods, where the presence of a well-developed metotic strut results in separation of the vagus nerve. However, this separation results in a laterally diverted transmission instead of a direct one from the endocranial cavity recalls that of non-avian theropods (e.g., McClellan, 1990; Currie and Zhao, 1993; Currie, 1995; Rauhut, 2004; Sampson and Witmer, 2007) rather than that of modern birds like Rhea and Aquila . It is also noted that the vagus nerve emerges from the occiput via a direct transmission from the braincase floor in one specimen referred to Troodon ( Fiorillo et al., 2009) . Moreover, the upper section of the fenestra pseudorotunda is topologically suitable for being the perilymphatic foramen, and the small foramen situated at the posterior side possibly represents the glossopharyngeal (IX.) nerve foramen. The glossopharyngeal nerve always leaves the braincase laterally from the metotic foramen or the fenestra pseudorotunda; however, a separate exit for this particular nerve is observed in ju- venile stages of some modern birds which turned into an ossified notch or a foramen in adult phase as in the subarctic bird genus Fulmarus ( Walker, 1985) .
Although the otoccipital of TTU-P11254c is highly comparable to that of non-avian neotheropods as mentioned above, this portion displays a clear contrast with the plesiomorphic state of the basal tubera. In non-avian theropods, the basal tubera are expanded ventrally and merged at the midline for the most part, if not completely (e.g., Chure and Madsen, 1988, figure 8; Sereno and Novas, 1993; Currie, 1995; Sampson and Witmer, 2007). A possible explanation for either TTU-P11254c represents a new type of theropod or another example of morphological convergence among Triassic archosauromorphs (e.g., Hunt, 1989; Nesbitt and Norell, 2006; Stocker et al., 2016) remains obscure because of the paucity of the available material. Recently, Piechowski et al. (2018) have suggested avian-like traits on the braincase of Silesaurus opolensis Dzik, 2003 based on ventrally directed paroccipital processes and reconstructed muscle attachments on the occipital side, even though the otoccipital of S. opolensis retains the plesiomorphic condition of having a laterally directed metotic foramen. Paroccipital processes of TTU-P11254c are not preserved, but the otoccipital is more derived than that of S. opolensis which may indicate a closer relation to avians if TTU-P11254c represents a dinosauriform. Nevertheless, TTU-P11254c might add to the large list of characters interpreted to occur among theropods later in the Mesozoic have already been convergently acquired by archosauromorph taxa during the Triassic.
The cervical vertebrae (TTU-P11254d) and the scapula (TTU-P11254e) bear a close resemblance to archosauromorph bones as well. The preserved cervical centra are anteroposteriorly elongate and transversely compressed, and they have a well-developed ventral keel ( Figure 8A View Figure 8 ). Presence of prominent hypapophyses on the cervicals is a plesiomorphic character that is lost in many archosaur groups ( Romer, 1956, Gauthier, 1986), but it is retained in the middle cervical vertebrae of Postosuchus spp. and Rauisuchus ( Nesbitt, 2011, character 192). The scapula is found attached to the cervicals; it possesses a ro- bust and dorsoventrally expanded morphology, differing from what is observed in lepidosauromorphs where the coracoid is the dominant element of the shoulder girdle ( Romer, 1956) ( Figure 8B View Figure 8 ). Howev- er, the poor preservation of these elements offers any diagnostic features to pinpoint a taxon more inclusive than Archosauromorpha.
Sauria McCartney, 1802, sensu Gauthier, Kluge and Rowe 1988
Gen. et sp. indet.
Referred specimens. TTU-P 11254f, distal end of a left femur; TTU-P 11254g, proximal end of left tibia; TTU-P 11254h, a fragmentary right tibia; TTU-P 11254i, two procoelous vertebrae and fragmentary undetermined shafts of bones.
Description and remarks. The thin-walled limb bones are extremely long and slender which is reminiscent to that of some basal ornithodirans such as Scleromochlus and Saltopus ( Benton, 1999; Benton and Walker, 2011), as well as that of pterosaurs ( Sereno, 1991a). But in a closer look, these limb bones are anatomically inconsistent with that of any typical archosaurian ( Figures 9 View Figure 9 A-G). The distal end of the femur does not bear a tibiofibular crest which is an archosauriform synapomorphy ( Nesbitt, 2011, character 322), whereas twin concave facets (i.e. cotyles) on the proximal tibia are comparable to that of lepidosaurians like Clevosaurus (Fraser, 1988) . Moreover, the procoelous state of the two severely damaged vertebrae ( Figure 9H View Figure 9 ) is considered to be a characteristic of a large number of squamates ( Romer, 1956), but also of many tanystropheids ( Pritchard et al., 2015) and the new basal archosauromorph Ozimek volans Dzik and Sulej, 2016 . The fragmentary shafts around the procoelous vertebrae are missing both ends but their morphology is identical to that of other described limb bones. Given that the procoelous vertebrae are attributed to the same taxon with the limb bones, these fragments may represent a small-sized lepidosauromorph taxon or a basal archosauromorph related to Sharovipterygidae . A tanystropheid affinity, on the other hand, is less like- ly since the basal forms possess a sigmoidal femur whereas more derived forms still retain the curvature at the distal end of femur ( Pritchard et al., 2015). All these elements are considered as saurian bones due to their incomplete and distorted condition which prevents a more detailed identification.
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