Cranial Anatomy of Csakvaromys bredai (Rodentia, Sciuridae, Xerinae) and Implications for Ground Squirrel Evolution and Systematics.

We provide the first detailed description of the cranial anatomy of an early marmotine ground squirrel Csakvaromys (= Spermophilinus) bredai, based on a well-preserved facial skull and associated specimens from the early late Miocene (11.1–9.7 Ma) Grytsiv fauna in western Ukraine. Notable cranial features of C. bredai include a shallow, broad, and elongated facial skull with a straight dorsal profile, massive rostrum, deeply concave lateral lip of the infraorbital foramen, shortened infraorbital canal, large lacrimal foramen, an elongated anteromedial edge of the zygomatic arch extending onto the premaxilla, a broad and flat interorbital region with closed supraorbital foramina, and non-elevated supraorbital margins. Phylogenetic analysis of a 136 craniodental character matrix reconstructs Csakvaromys, together with Palaeosciurus, Protospermophilus, and Sciurotamias, within the subtribal clade Sciurotamiina. We provide the first reliable morphological synapomorphies for Xerinae, and suggest a revised definition for a monophyletic Sciurotamiina, which is supported by a large lacrimal foramen ventrally contributed by the maxilla, the facial tuberosity with a longitudinal crest for the superficial masseter, rudimentary ectopterygoid crest, and the presence of an expansive attachment area for the anterior fibers of the anterior deep masseter muscle. The craniofacial anatomy reveals that C. bredai was a frugivore with a predominantly hard-nut diet and arboreal or semi-arboreal habit. [ABSTRACT FROM AUTHOR]