A new madtsoiid snake (Squamata, Ophidia) from the Upper Cretaceous of Patagonia

GÓMEZ, R.O.; BÁEZ, A.M.

San Juan

Congreso; XXII Jornadas Argentinas de Paleontología de Vertebrados; 2006

Instituto de Energía Eléctrica de la Universidad Nacional de San Juan

Several madtsoiid species, all represented by few isolated, incomplete vertebrae, are known from Cretaceous rocks of Patagonia to date. A new madtsoiid taxon from the lower member of the Allen Formation (Malargüe Group, Neuquén Basin), Late Campanian-Early Maastrichtian in age, at Bajo de Trapalcó, Río Negro Province is described herein. It is based on nearly 200 isolated vertebrae from all regions of the column, allowing detailed descriptions of its vertebral anatomy, as well as its intracolumnar and intraspecific variation. In addition, inmature individuals have been identified based on vertebral features that are similar to those of most juvenile snakes. All vertebrae have typical parazigantral foramina, a putative derived character of Madtsoiidae, as well as features that are shared with all other madtsoiids, such as lack of prezigapophyseal processes, presence of paracotylar foramina, and width between diapophyses comparable to that between prezygapophyses. This material represents a tiny madtsoiid species, with mid-trunk vertebrae having a subtriangular centrum with a low trifid haemal keel, and a moderately vaulted neural arch bearing a well-developed, long neural spine. Several vertebrae, which belong to different axial regions, show dorsal paired tubercles near the posterior margin of the neural arch and anterior tubercles between the cotyle and the paradiapophyses. Two traits of the new taxon might be phylogenetically significant: hypapophysis with an unfused intercentrum on the anteriormost trunk vertebra and conspicuous paired pedicels on caudal vertebrae for the attachment of posteroventral elements. Within snakes, the former was previously described only in Dinilysia Woodward, whereas the latter has been observed in other madtsoiids and Eupodophis Rage and Escuillié. These characters are reminiscent of the conditions in mosasaurs, dolichosaurs, and varanoid lizards and, thus, agree with the hypotheses of the basal position of madtsoiids and of the close relationship of snakes with mosasauroid-varanoid lizards.