Archosauromorpha Huene, 1946

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 maintains 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 juvenile 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 robust 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 ). However, the poor preservation of these elements offers any diagnostic features to pinpoint a taxon more inclusive than Archosauromorpha.