Abstract:
The early history of the Sciuridae is not well known, squirrels being generally poorly represented in the Tertiary fossil record. A nearly complete skeleton, recently discovered in early Oligocene deposits of Wyoming, represents what may be the oldest fossil squirrel known. For the first time, this early squirrel can be compared fully with its extant relatives. The specimen, assigned to Protosciurus jeffersoni, retains the primitive protrogomorphous zygomasseteric structure, as in other known Protosciurus, but the masseteric fossa of the mandible is farther forward than in most nonsciurid protrogomorphs. The auditory region of the skull has derived squirrel characters, but it is in the postcranial skeleton where similarities to extant squirrels are most apparent. Except for minor differences in joint construction, the skeleton is strikingly similar to that of Sciurus niger, the living fox squirrel. It differs from extant ground squirrels in the more gracile proportions of its long bones and asymmetry of foot construction. This early member of the squirrel family was clearly an arboreal squirrel, with morphology, and presumably habits, very similar to those of extant Sciurinae.
