Spermophilus variegatus, the rock squirrel, is native to southern Nevada, Utah, Colorado, the panhandle of Oklahoma, and south through Arizona, New Mexico, western Texas, and much of Mexico. Squirrels are interesting in that their skulls are highly conserved, having changed little since the oldest-known squirrel (Protosciurus) appeared in the late Oligocene. This can be easily seen by comparing S. variegatus to S. columbianus, the Columbian ground squirrel, Cynomys ludovicianus, the black-tailed prairie dog, and Sciurus niger, the eastern fox squirrel. For this reason, one could call squirrels 'living fossils'.
To date, the lack of significant variation between squirrel species has frustrated efforts to discover their phylogenetic relationships. However, DNA-hybridization data suggest that ground squirrels are closely related to marmots, having diverged from them in the late Miocene. CT scanning offers the opportunity to easily acquire information on the internal anatomy of squirrel skulls, and to apply morphometric tools to their study. It is hoped that this will contribute to the eventual resolution of the squirrel family tree.
About the Species
This specimen, an adult male, was collected near Portal, Cochise County, Arizona by V. H. Calahane. It was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Donald Swiderski of the University of Michigan Museum of Zoology. Funding for scanning was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin.
About this Specimen
The specimen was scanned by Matthew Colbert on 28 July 2000 along the coronal axis for a total of 421 slices, each slice 0.1483 mm thick, with an interslice spacing of 0.1483 mm.
Giboulet, O., P. Chevret, R. Ramousse, and F. Catzeflis. 1997. DNA-DNA hybridization evidence for the recent origin of marmots and ground squirrels (Rodentia: Sciuridae). Journal of Mammalian Evolution 4:271-284.
Nadler, C. F. 1966. Chromosomes of Spermophilus franklini and taxonomy of the ground squirrel genus Spermophilus. Systematic Zoology 15:199-206.
Oaks, E. C., P. J. Young, G. L. Kirkland, Jr., and D. F. Schmidt. 1987. Spermophilus variegatus. Mammalian Species 272:1-8.
Mammalian Species account of Spermophilus variegatus (American Society of Mammalogists)
The brain of Spermophilus tridecemlineatus (Comparative Mammalian Brain Collections website)
Spermophilus variegatus on The Mammals of Texas Online Edition