RECONSTRUCTING LOCOMOTION IN EXTINCT SCIURIDS

Extant sciurids encompass a wide range of locomotor styles, from specialized gliders (“flying” squirrels) to arboreal tree squirrels to fossorial ground squirrels. Sciurids are also well-represented in the fossil record from the Eocene onwards, making them an excellent study taxon for understanding the locomotor effects of the transition from forested to grassland environments across much of the globe. In order to understand squirrel locomotor evolution, informative fossil proxies for squirrel behavior must be identified. Humeral morphology holds a great deal of potential, not only because of the importance of the humerus to locomotion, but because humeri are common in the sciurid fossil record. We measured and calculated humeral functional indices for 42 species of extant sciurids across four locomotor categories (gliding, terrestrial, arboreal, and fossorial). We also compiled a data set of fossil sciurid humeri from the Mio-Pliocene of the Inland Northwest to test whether these indices could be used to reconstruct locomotion in extinct taxa. Principle components and discriminant analyses show that, with the exception of terrestrial squirrels, all other locomotion types can be confidently distinguished from one another on the basis of the relative mid-shaft thickness of the humerus and of the relative breadth of the distal epicondyles. Our discriminant analysis unequivocally identified the fossil specimens as fossorial, which is unsurprising given that they represent taxa related to extant ground squirrels and are all younger than 7 million years in age. Future analyses will include older specimens in the interest of identifying the timing and speed of the transition to burrowing in ground squirrels. A larger sample of extant terrestrial squirrels is also critical for understanding this transition and the forces that drove it.