Emerging head first: the early fossil record of snake cranial evolution

CALDWELL, MICHAEL W.; RANDALL NYDAM; ALESSANDRO PALCI; SIMÕES, TIAGO R.; GARBEROGLIO, FERNANDO F.; APESTEGUÍA, SEBASTIÁN

calgary

Congreso; 77th Society of Vertebrate Paleontology Meeting; 2017

Society of Vertebrate Paleontology

The morphological disparity and species diversity of Late Mesozoic snakes indicate that they diverged from their closest squamate sister taxa through the early acquisition of a series of muque cranial features. The oldest (nlid Jurassic-Early CretaceotL ) snake fossil are sparse, but include infonnative cranial specimenof the "Parviraptor -like" snakes/stem snakes from Upper Jurassic and Lowermost CretaceotL localities in Laurasia and the newly recognized braincase from the Lowermost Cretaceous of South Africa that shows a "Dinilysia-like" morphology. These specimens portend the origin of later Mesozoic snake cranial morphologies from at least the Cenomanian to the Maastrichtian and de.arly demonstrate that the various dades of early snakes radiated around1specialization of the chondrocranial skeleton (braincase) and dermatocranium (jaws and elements of the stL pensorium). From the Early Cretaceous upward, two "groupings" ofancient snakes are recognizable from comparison of braincase and suspensoria ! anatomy: 1) "Dinilysia-Anilioid" condition, and, 2) the "Pachyrhachis-M acrostomatan", condition. The "Dini(ysia -Anilioid " condition is recognized from fossil skulls beginning íin the Valanginian (Lw. Cret.) through to the Maastrichtian (U. Cret.) from widely separated localities that were once part of the Gondwanan southern continental massif. 2)1The Pachyrhachis-M acrostomatan" condition is recognized from fossil skulls begimtingin the Cenomanian through to the Maastrichtian in spatially disparate marine environments of the Tethys and globally distributed SupraTethys Seaway. These two cranial and suspen orial c.onditions have been retained in descendant sister taxa and rernain recognizable amongst modern crown group snakes; these sinlllarities are phylogenetic, not fünctional (i.e., not convergent), and are diagnosed by character concepts describing the organization of the crista circumfenestralis, quadrate andlcolumella/extracolumellar anatomy, mandibular anatomy, and so on. It. is increasingly clear, as illunlinated by fossils from the earliest part of the snake fossil record, that the transition from the non-snake!izare! common ancestor to the first tme snake-lizard, was driven by the evolution of cranial anatomy.