Appendicular myology of Skorpiovenator bustingorryi: A first attempt to reconstruct pelvic and hindlimb musculature in an abelisaurid theropod.

We present the pelvic and hindlimb musculature of the abelisaurid Skorpiovenator bustingorryi, constituting the most comprehensive muscle reconstruction to date in ceratosaur theropods. Using extant phylogenetic bracket method, we reconstructed 39 muscles that can commonly found in extant archosaurs. Through the identification of bone correlates, we recognized thigh and hindlimb muscles including knee extensors, m. iliofibularis, m. flexor tibialis externus, mm. caudofemorales, mm. puboischiofemorales, and crus muscles important in foot extension and flexion (e.g., m. tibialis anterior, mm. gastrocnemii). Also, autopodial intrinsic muscles were reconstructed whose function involve extension (m. extensor digiti 2-4), flexion (mm. flexor digitorum brevis superficialis), interdigital adduction (m. interosseus dorsalis) and abduction (m. interosseous plantaris, m. abductor 4). Abelisaurids like Skorpiovenator show a deep pre- and postacetabular blade of the ilia and enlarged cnemial crests, which would have helped increasing the moment arm of muscles related to hip flexion and hindlimb extension. Also, pedal muscles related to pronation were probably present but reduced (e.g., m. pronator profundus). Despite some gross differences in the autopodial morphology in extant outgroups (e.g., crocodilian metatarsus and avian tarsometatarsus), the present study allows us to hypothesize several pedal muscles in Skorpiovenator. These muscles would not be arranged in tendinous bundles as in Neornithes, but rather the condition would be similar to that of crocodilians with several layers formed by fleshy bellies on the plantar and dorsal aspects of the metatarsus. The musculature of Skorpiovenator is key for future studies concerning abelisaurid biomechanics, including the integration of functional morphology and ichnological data.

Hindlimb and pelvic anatomy of Caiman yacare (Archosauria, Pseudosuchia): Myology and osteological correlates with emphasis on lower leg and autopodial musculature.

The anatomy of the archosaurian pelvis and hindlimb has adopted a diversity of successful configurations allowing a wide range of postures during the evolution of the group (e.g., erect, sprawling). For this reason, thorough studies of the structure and function of the pelvic and hindlimb musculature of crocodylians are required and provide the possibility to expand their implications for the evolution of archosaurian locomotion, as well as to identify potential new characters based on muscles and their bony correlates. In this study, we give a detailed description of the pelvic and hindlimb musculature of the South American alligator Caiman yacare, providing comprehensive novel information regarding lower limb and autopodial muscles. Particularly for the pedal muscles, we propose a new classification for the dorsal and ventral muscles of the autopodium based on the organisation of these muscles in successive layers. We have studied the myology in a global background in which we have compared the Caiman yacare musculature with other crocodylians. In this sense, differences in the arrangement of m. flexor tibialis internus 1, m. flexor tibialis externus, m. iliofibularis, mm. puboischiofemorales internii 1 and 2, between Ca. yacare and other crocodylians were found. We also discuss the muscle attachments that have different bony correlates among the crocodylian species and their morphological variation. Most of the correlates did not exhibit great variation among the species compared. The majority of the recognised correlates were identified in the pelvic girdle; additionally, some bony correlates associated with the pedal muscles are highlighted here for the first time. This research provides a wide framework for future studies on comparative anatomy and functional morphology, which could contribute to improving the character definition used in phylogenetic analyses and to understand the patterns of musculoskeletal hindlimb evolution.

An exceptional neurovascular system in abelisaurid theropod skull: New evidence from Skorpiovenator bustingorryi.

Abelisaurids were one of the most successful theropod dinosaurs during Cretaceous times. They are featured by numerous derived skull traits, such as heavily ornamented bones, short and tall snout, and a strongly thickened cranial roof. Furthermore, nasals are distinctive on having two distinct nasal patterns: strongly transversely convex and heavily sculptured (e.g., Carnotaurus), and transversely concave, with marked bilateral crests and poorly sculptured surfaces (e.g., Rugops). Independently of the pattern, some abelisaurid nasals (e.g., Rugops) show a distinctive row of large foramina on the dorsal surface, which were in general associated to skin structures (scales). Skorpiovenator bustingorryi is a derived abelisaurid coming from the upper Cretaceous beds of northwestern Patagonia, represented by an almost complete skeleton including a well-preserved skull. Particularly, the skull of Skorpiovenator shows nasal bones characterized by being transversely concave, rimmed by lateral crests and with a conspicuous row of foramina on the dorsal surface. But more interesting is that the skull roof also exhibits a row of large foramina that seem to be continuous with the previous nasal foramina. CT scans made on the skull corroborates a novel feature within theropods: the nasal foramina on the external surface are linked to an internal canal that runs across the nasal bones. We compared this feature with CT scans of Carnotaurus and revealed that it also possess an internal system as in Skorpiovenator, but being notably smaller. The symmetry and disposition of the foramina in the nasal and skull roof bones of Skorpiovenator would indicate a neurovascular correlate (i.e., blood vessels and nerves), probably to the lateral nasal and supraorbital vessels and the trigeminal nerve. The biological significance of such neurovascular system can be conjectured from several hypotheses. A possible one involves an enhanced blood volume in these bones linked to a zone of thermal exchange, which may help avoid overheat of encephalic tissues. Another plausible hypothesis takes into account the presence of display skin structures in which blood volume nourished the mineralized skin, which would have a role in intraspecific communication. However, other more speculative explanations should not be discarded such as a correlation with integumentary sensory organs.

Skull Anatomy and Pneumaticity of the Enigmatic Coelurosaurian Theropod Bicentenaria argentina.

The enigmatic basal coelurosaur Bicentenaria argentina is a small theropod that comes from the Upper Cretaceous of Patagonia. It is constituted by more than 130 elements, including cranial remains. These are represented by an incomplete snout, palate, dermatocranium, and lower jaws still in articulation. The skull material of Bicentenaria was preliminarily studied by previous authors. In this study, we conducted a CT scan of the holotype of Bicentenaria, which allowed us to fully describe all preserved skull bones, including some previously unknown elements. The morphological analysis indicates that Bicentenaria shares many cranial features with other basal coelurosaurs as compsognathids, Ornitholestes, and tyrannosauroids. Otherwise, results of this analysis show that Bicentenaria exhibits a set of traits unique to this taxon. Furthermore, several pneumatic recesses were closely matched with those pneumatic features present in many derived coelurosaurs, particularly tyrannosauroids. This new information, together with future discoveries, will improve our knowledge about the muscular correlates of Bicentenaria and other coelurosaurs, and may shed light about the phylogenetic relationships of this group. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1884-1900, 2020. © 2019 American Association for Anatomy.