MORPHOLOGY OF THE SACRAL REGION OF CRICOSAURUS ARAUCANENSIS (CROCODYLOMORPHA: THALATTOSUCHIA): INSIGHTS INTO METRIORHYNCHIDS REPRODUCTIVE STRATEGIES

HERRERA Y.; FERNÁNDEZ M.S.; GASPARINI Z.; CAMPOS, L.

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

Metriorhynchids were a successful group of Mesozoic marine crocodylomorphs. Their fossil record is abundant and most of it has been recovered from the European margins of the Western Tethys as well as from the South American margins of the Eastern Pacific. The evolutionary history of these crocodylomorphs, characterized by extensive skeletal and physiological adaptations to a pelagic lifestyle, ranges from the Middle Jurassic to Early Cretaceous. After their extinction, no crocodile has developed such lifestyle. Adaptations to marine lifestyle resulted in certain morphological traits that parallel those of other marine reptile predators, namely limbs transformed into paddles, ?osteoporotic-like? pattern in some bones, lack of heavy armour and hypocercal tail. Along with these traits, the pelvic region was also modified. In this study, the pelvic region of Cricosaurus araucanensis (holotype, MLP 72-IV-7-1) was reconstructed. As in all metriorhynchids, in C. araucanensis there are two sacral vertebrae with the sacral ribs strongly ventrally deflected, a feature unique among crocodylomorphs. Due to this particular morphology, the ilium is ventrally displaced in relation to the vertebral column in such a way that the space between the vertebral column, dorsally, and the pubis and ischium, ventrally, is maximized. In this sense, the transverse section of the pelvic region is higher than wider, and resulted in the increase of the available space for the abdominal cavity at the level of the pelvic region. The described morphology is convergent with that of basal ichthyosaurs (e.g., Chaohusaurus) and basal sauropterygians (e.g., Keichousaurus hui). Noteworthy, both clades are known for developing a convergent evolution of live-bearing reproduction (i.e., giving birth to autonomous, free-living offspring, whatever their state of maturity). In metriorhynchids, this reproductive strategy has been proposed since the 70s by some authors based on postcranial characteristics, such as the absence of osteoderms, the paddle-like forelimbs and the hypocercal tail. We propose that the peculiar morphology of the metriorhynchid sacral region was functionally associated with live-bearing reproduction.