The postcranial skeleton of Brasilodon quadrangularis (Cynodontia, Prozostrodontia) from the Late Triassic of Brazil.

AGUSTIN MARTINELLI; MARINA BENTO SOARES

Congreso; 74th Annual Meeting Society of Vertebrate Paleontology; 2014

The probainognathian cynodont Brasilodon quadrangularis and its closest allied Brasilitherium riograndensis (although considered as junior synonym by some authors; Liu & Olsen, 2010), from the early Norian of southern Brazil, are the sister-taxa of Mammaliaformes (e.g., Bonaparte et al., 2003, 2005, 2012; Abdala, 2007; Liu & Olsen, 2010). Differing from other cynodonts, they have slender and low zygomatic arch; muzzle longer than temporal portion of the skull; lack of prefrontal and postorbital; wide primary palate, posterior to choana; separated fenestra rotunda and foramen jugular; fused opisthotic and prootic (petrosal) with well-defined promontorium; foramen for the prootic sinus vein; quadrate with well-developed stapedial process; dentary reaching squamosal; position and relative size of the main crown cusps similar to morganucodontids with cusps e and g on middle postcanines; lower postcanines with a tongue (by cusp d) and groove-like (by cusps b and e) system, among others. Postcranial bones of these taxa are by far poorly known and only a few remains have been briefly described yet. We focus our study on associated postcranial bones referred to a single specimen of Brasilodon which include partial right scapula, left humerus, radii and ulnae, partial right pelvis, femora, partial right tibia and fibula, and phalanges, rib and vertebral fragments. The scapula has a poorly developed acromion, placed higher in comparison to the glenoid, as in Morganucodon. The infraspinous fossa is large, the posterior edge is inflexed with a posterior postscapular fossa. The humerus is slender with hemispheroidal head and distinctive greater and lesser tuberosities, lacking ectepicondylar foramen. The ulnar (smaller) and radial (larger) condyles are well-developed, subesphericals, mainly disposed on the anterior surface and the radial condyle also with great contribution on the posterior one. The ulna has well-ossified olecranon process. The pelvis has a large obturator foramen, with an apparent elongated iliac blade. The femur has a notorious hemispherical head, dorsomedially reflected, with greater and lesser trochanters separated by notches from the head. All these postcranial bones have a similar configuration to that of early mammaliaforms (e.g., Morganucodon, Eozostrodon). As many other cranio-dental features, the postcranial anatomy of Brasilodon reveals a unique pattern closer related to mammaliaforms than to any other cynodonts, with similar ecological strategies (such as fossoriality).