A new alligatorid from the lower Eocene Green River Formation of Wyoming and the origin of caimans

Abstract

ABSTRACT Tsoabichi greenriverensis, tax. nov., is based on cranial and postcranial remains from the lower Eocene (Wasatchian) Green River Formation of Wyoming. Casts of additional material in private collections include the complete skeleton of an immature specimen. It has abrupt supratemporal fenestral rims, extensive supraoccipital exposure on the skull table, and bipartite ventral osteoderms. The nasals are reduced to a thin anterior process as they approach the external naris, and the rim of the naris extends beyond the dorsal surface of the snout as a thin crest. There was no ‘spectacle’ between the orbits, but there are three distinct craniocaudally elongate ridges between the orbits. The parietal extends to the posterior margin of the skull table on either side of the large V-shaped dorsal exposure of the supraoccipital. Some of the dorsal osteoderms are mediolaterally elongate and bear two dorsal keels. A close relationship with caimans is supported by a parsimony analysis, although its precise position is unclear and some of the characters supporting a caimanine relationship also occur in poorly known Late Cretaceous and Early Tertiary alligatoroids. A possible close relationship with Paleosuchus (the living dwarf caimans) would imply a very complex biogeographic scenario with multiple dispersals between the Americas during the Tertiary.