Snakes in the Morrison Formation?

SEBASTIAN APESTEGUIA; MICHAEL CALDWELL; RANDALL NYDAM; FERNANDO FABIO GARBEROGLIO; ALESSANDRO PALCI

Colorado & Utah

Jornada; Mid-Mesozoic: The age of dinosaurs in transition; 2014

The currently oldest known snake fossils are isolated vertebrae from sediments in North Africa and North America dated as Albian to Cenomanian.  The origin of the clade Ophidia is broadly considered having occurred during the Jurassic, but fossil evidence supporting this hypothesis has remained elusive.  Here we report on fragmentary vertebral remains (LACM 120472) from the Fruita locality 4684, Morrison Formation, Mesa County, Colorado, U.S.A. (Upper Jurassic; Kimmeridgian), that we recognize as belonging to an Upper Jurassic ophidian.  The specimen preserves three vertebrae, two of which are articulated, and one isolated vertebra, with four additional fragments the identity of which remain inconclusive. The two articulated vertebrae, exposed in dorsal view, exhibit a wide neural arch with a very low neural spine framed by prominent arcual ridges on each side and above the interzygantral ridge.  The isolated vertebra possesses a low neural spine forming a low crest on the dorsal surface that disappears anteriorly at the zygosphenal platform. The zygosphenes are thick and well developed, as in all fossil and modern snakes. The zygosphenal facets are separated from the zygapophyseal facets by a non-articular area, and the zygosphenal tectum has a festooned anterior margin in dorsal view. The condyles and cotyles are circular, and both are offset from the ventral margins of the centrum.  In ventral view, the centrum is strongly rectangular in outline, with a squared margin immediately anterior to the condyle.  Parazygantral foramina are present on each side of neural arch, recessed in fossae lateral to the zygantra. These vertebrae show critical similarities to much younger Mesozoic snakes such as Coniophis, Dinilysia, Najash, Pachyrhachis, Simoliophis, etc., and to all extant snakes, making certain the identification of these vertebrae as those of an Upper Jurassic snake, and thus extending the fossil record of snakes by 45 million years, from the uppermost Albian, to the Upper Kimmeridgian