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

MICHAEL WAYNE CALDWELL; RANDAL L. NYDAM; ALESSANDRO PALCI; TIAGO R. SIMOES; FERNANDO GARBEROGLIO; SEBASTIAN APESTEGUIA

Calgary

Congreso; Society of Vertebrate Paleontology 77th annual meeting; 2017

Society of Vertebrate Paleontology

The morphological disparity and species diversity of Late Mesozoic snake indicate that they diverged from their closest squamate sister taxa through the early acquisition of a series of unique cranial features. The oldest (mid Jurassic?Early Cretaceous) snake fossils are sparse, but include informative cranial specimens of the ?Parviraptor-like? snakes/stem snakes from Upper Jurassic and Lowermost Cretaceous 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 origins of later Mesozoic snake cranial morphologies from at least the Cenomanian to the Maastrichtian and clearly demonstrate that the various clades of early snakes radiated around specializations of the chondrocranial skeleton (braincase) and dermatocranium (jaws and elements of the suspensorium). From the Early Cretaceous upward, two ?groupings? of ancient snakes are recognizable from comparisons of braincase and suspensorial anatomy: 1) ?Dinilysia-Anilioid? condition, and, 2) the ?Pachyrhachis-Macrostomatan? condition. The ?Dinilysia-Anilioid? condition is recognized from fossil skulls beginning in the Valanginian (Lw. Cret.) through to the Maastrichtian (U. Cret.) from widelyseparated localities that were once part of the Gondwanan southern continental massif. 2) The Pachyrhachis-Macrostomatan? condition is recognized from fossil skulls beginning in the Cenomanian through to the Maastrichtian in spatially disparate marine environments of the Tethys and globally distributed SupraTethys Seaway. These two cranial and suspensorial conditions have been retained in descendant sister taxa and remain recognizable amongst modern crown group snakes; these similarities are phylogenetic, not functional (i.e., not convergent), and are diagnosed by character concepts describing the organization of the crista circumfenestralis, quadrate and columella/extracolumellar anatomy, mandibular anatomy, and so on. It is increasingly clear, as illuminated by fossils from the earliest part of the snake fossil record, that the transition from the non-snake lizard common ancestor to the first true snake-lizard, wasdriven by the evolution of cranial anatomy.