Soumyasaurus aenigmaticus, Sarıgül & Agnolín & Chatterjee, 2018

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 situated 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, character 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, character 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 comparison 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 Ornithodira, 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.