Postcranial Osteology of the North American Middle Eocene Protocetid Georgiacetus

Abstract

Archaeocete whales are a paraphyletic assemblage of species that represent the evolutionary intermediaries between modern, fully marine cetaceans (Odontoceti and Mysticeti) and their terrestrial ancestors, the Mesonychia (Van Valen, 1966; Fordyce and Barnes, 1994; Thewissen, 1994). For over 80 years the holotype specimen of Protocetus atavus from the middle Eocene of Egypt was the oldest known postcranial skeleton of a cetacean. Necessarily it figured prominently in studies on the early evolution of the postcranial skeleton of cetaceans (e.g., Kellogg, 1936). Several critical specimens collected over the past two decades have greatly improved this record. Notable among these are partial postcranial skeletons of Ambulocetus natans (Thewissen et al., 1994, 1996); Rodhocetus kasrani (Gingerich et al., 1994), Remingtonocetus harudiensis (Gingerich et al., 1993; listed therein as Indocetus ramani but partly reidentified by Gingerich et al., 1995), and Georgiacetus vogtlensis (Hulbert et al., 1998). A fifth specimen, the Habib Rahi whale, remains undescribed (Gingerich, 1991). All of these are more complete than the holotype of P. atavus and include some appendicular elements. The postcrania of Ambulocetus and Remingtonocetus have been thoroughly described and illustrated (Gingerich et al., 1993, 1995; Thewissen et al., 1996). The purpose of this report is to describe the postcranial remains of G. vogtlensis and compare them with those of other archaeocetes.