Postnatal developmental aspects in a Cretaceous snake, Dinilysia patagonica

CARLOS A. SCANFERLA & BARTH ANJAN S. BHULLAR

Neuquén, Argentina

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008

Studies on postnatal patterns of osteogenesis have demonstrated its important role as potential source of information in squamate systematics (Maisano, 2001, 2002). However, we virtually ignore these processes that affect the skeletal development in one of the major clades of living squamates, the snakes. Here, we report a new small cranial material of the Late Cretaceous snake Dinilysia patagonica Smith-Woodward, recovered from the University campus locality, Bajo de la Carpa Formation (Santonian), Neuquén, Argentina. The specimen MACN-N 106 consists of a partial skull that includes neurocranium (until the trigeminal foramen) with both quadrates and fragmentary jaws in articulation. Also, we use new information provided by another juvenile specimen (MLP 71-VII-29-1) referred to Dinilysia patagonica by Caldwell and Albino (2002), that consist of an incomplete neurocranium. These specimens share with other juvenile snakes the globous general aspect of the braincase, and the poor/null development of the sagital crest of the parietal and the sagital and transversal crests of the supraoccipital (well-developed in adult specimens of Dinilysia). The skull roof is fully ossified, and the exoccipital-opisthotic fusion is complete. In the adult specimen MACN-RN 1014, the distal (medial) tip of the suprastapedial process is in contact with the ball-shaped structure that connects the distal tip of the collumelar shaft of the stapes with the suprastapedial process (Caldwell and Albino, 2002). The distal (medially) tip of the suprastapedial process of the quadrate exhibits an ossification/calcification separated from it by means a suture. This structure, that covers the posterior extreme of the supratemporal, would represent the ossification/calcification of the intercalary (stylohial) cartilage, suggesting that the ball-shaped structure is an intermediate element and not the ossification/calcification of the intercalary (stylohial) cartilage (contra Caldwell and Albino, 2002). Notably, the posterior and lateral growth of the parietal bone generates some changes that are present in the adult forms. In juvenile forms, the supratemporal does not contact the parietal. The dorsal exposure of the prootic, represented in adults by a sliver surrounded by supratemporal, parietal (supratemporal process), otooccipital and supraoccipital bones, is widely exposed. In adult specimens, the occipital posterolateral projections exhibit an important longitude, surpassing widely the posterior end of the occipital condyle. However, in the specimens reported here, the end of the paraoccipital projections of the neurocranium are in the same line that the posterior end of the occipital condyle. Hence, these observations suggest remarkable changes in some characters during the postnatal ontogeny of this relevant snake.