Observations of vertebral suture closure pattern on Late Triassic rhynchosaurs of Brazil (Santa Maria Supersequence, Hyperodapedon Assemblage Zone)

CLARA HEINRICH; VOLTAIRE PAES NETO; AGUSTIN MARTINELLI; CESAR SCHULTZ; MARINA BENTO SOARES

Buenos Aires

Jornada; XXX Jornadas Argentina de Paleontología de Vertebrados; 2016

MACN

Neurocentral sincondrosis on vertebrae results in the fusion between neural arch and centrum, resulting in the ossification of the cartilaginous layer that articulates both units. In current crocodilians the sequence of vertebra fusion pattern occurs in caudal-cervical direction during growth, manifesting correlation with relative maturity and size. This criterion has been applied as a relative mature indicator to estimate the ontogenetic stage of many extinct archosauriform taxa. However, this pattern has been criticized due to a great deviation of the crocodilian-pattern in some groups. The exploration of the neurocentral fusion patterns among basal groups may elucidate how these processes worked in Archosauromorpha. Rhynchosauria comprise a diverse archosauromorph clade restrict to the Triassic, which, in the Santa Maria Supersequence (Hyperodapedon Assemblage Zone) of Brazil, is represented by Hyperodapedon and Teyumbaita. We analyzed the closure pattern of 241 neurocentral vertebral sutures (nearly 40 Hyperodapedon and three Teyumbaita individuals). Three phases for suture closure were observed: open, partially closed and closed; and four morphometric variables. We recognize eight adult-sized individuals with relatively more complete vertebral sequences, possibly indicating a cervical-caudal direction of fusion. In general, caudal vertebrae show more open neurocentral suture than dorsals and cervicals. However, some medium-sized individuals present open cervical vertebral sutures, which may indicate differences in closure timing. Nevertheless, the rhynchosaur pattern of suture closure seems to be incompatible with the crocodilian caudal-cervical one. We expect to link paleohistological information, acquiring further knowledge about the relation of the suture closure sequence, regarding the relative maturity and the evolutionary context of this process.