NEW FOSSILS FROM THE OLIGOCENE OF SOUTH CAROLINA CLARIFY THE IDENTITY OF THE GIANT DOLPHIN “SQUALODON” TIEDEMANI AND THE POSTCRANIAL EVOLUTION OF EARLY ODONTOCETES

The genus Squalodon has historically been treated as a wastebasket for heterodont odontocetes, with many species now reassigned to other clades (Allodelphinidae, Kekenodontidae, Waipatiidae) based on discoveries of more complete material. One such taxon is “Squalodon” tiedemani, represented by a partial rostrum of uncertain age dredged from the Wando River (Charleston, SC). Newly collected material including a partial skull and skeleton (CCNHM 103) and an isolated skull (CCNHM 220) indicate that “Squalodon” tiedemani is a stem odontocete with close affinities to “Genus Y”, an unnamed Agorophius-like giant dolphin from Oligocene Ashley and Chandler Bridge Formations of the Charleston area. CCNHM 103 includes a large cranium (CBL=100 cm) with prominent nuchal crests, distinct intertemporal constriction, large temporal fossae, incipient polydonty (13 upper teeth), limited heterodonty, and thickened cementum. Postcranial elements, including 90% of the vertebral column, numerous ribs, sternum, and much of the right flipper, shed critical light onto the poorly understood postcranial osteology of early odontocetes. Standard measures indicate that most vertebrae are nearly equidimensional similar to mysticetes, archaeocetes, and physeteroids (=Pattern 1 of Buchholtz) indicating undulation of the entire torso. Caudal vertebrae indicate the presence of a tail fluke, but wide mid-caudal vertebrae suggest the peduncle was poorly developed. The forelimb preserves well-ossified carpals and an unusual degree of fusion in the wrist; metacarpals and phalanges are elongate and cylindrical as in archaeocetes, suggesting a less rigid flipper than in extant Odontoceti. Digital articulations are convex and smooth, indicating synovial finger joints. These new discoveries highlight the stepwise evolution of the cetacean bauplan and gradual shift in locomotor ability in early odontocetes.