COMPARATIVE MYOLOGY OF THE FORELIMB OF SQUIRRELS (SCIURIDAE)

The musculature of the shoulder, arm, and forearm was studied in 19 ge nera of squirrels, representing the Pteromyinae (flying squirrels) and all 7 tribes of the Sciurinae (tree and ground squirrels). The object ive was to locate derived anatomical features of functional or phyloge netic significance and to determine how much morphological variation u nderlies the diverse locomotor behavior of squirrels, which includes t errestrial and arboreal bounding, climbing, digging, and gliding. The fossil evidence suggests that arboreality is primitive for squirrels, and in fact tree squirrels appear to represent the primitive sciurid m orphology. Ground squirrels are less uniform and exhibit a few derived features, including a clavobrachialis muscle not seen in other squirr els. Pygmy tree squirrels, which have evolved independently in three t ribes, exhibit convergence of forelimb anatomy, including the loss or reduction of several muscles in the shoulder and forearm. The forelimb anatomy of flying squirrels is the most derived and differs from that of tree squirrels in details of shoulder, arm, and forearm musculatur e. Some of these muscular differences among squirrels have phylogeneti c significance, being shared by closely related genera, but none has s ignificance above the tribal level. Many of the differences suggest a variety of changes in function that are amenable to further study. (C) 1997 Wiley-Liss, Inc..