EXAMINATION OF CLAW RETRACTION IN A RELATIVELY COMPLETE SPECIMEN OF MESOCYON

Mesocyon temnodon is an Oligocene canid species that lived in localities such as South Dakota, North Dakota, Wyoming, Colorado, and Nebraska approximately 35.4 million years ago. While the specifics of its evolutionary relationships and phylogenetic position are unclear, it is a member of the early Canid subfamily Hesperocyoninae. It has been suggested that these primitive dogs may have some anatomical similarities to felids, especially in post-cranial skeleton. One of these possible similarities is the ability to retract the claws.

The purpose of this study was to evaluate claws from a relatively complete Mesocyon temnodon specimen from North Dakota (NDGS 64) in order to determine the plausibility of claw retraction. The materials used for this process were the NDGS 64 specimen and specimens of modern Felids and Canids. This test was conducted by examining three lines of evidence: the morphology of the middle phalanx, depth of arc in the distal phalanx, and the sites of attachment for the extensor and flexor muscles. In specimens capable of claw retraction, the middle phalanx has a curvature on the lateral side, making the bone asymmetrical in shape. Specimens incapable of claw retraction have a middle phalanx that is symmetrical in shape. Retractable claws have deeper arcs in the claw on the inferior surface, whereas non-retractable claws have a shallower depth of the arc. In addition to this, retractable claws have differing attachment sites of the extensor and flexor muscles compared to non-retractable claws. In all three of these tests, it was found that the observed specimen, NDGS 64, was incapable of claw retraction.