Locomotor Behavior and Body Mass of Paramys delicatus (Ischyromyidae, Rodentia) and Commentary on Other Early North American Paramyines

Paramyine ischyromyids are one of the first ancestral rodent groups to appear in North America. Studying ecological indicators of these extinct animals enables us to better understand how they integrated into North American mammalian communities. In this study we reassess the locomotor behavior of a nearly complete skeleton of a paramyine, Paramys delicatus (AMNH FM 12506), using functional limb indices and living squirrels as extant analogues. We then used the results of the functional limb index study to select an appropriate locomotor group for body mass estimations of Paramys delicatus and other early North American (Wasatchian-Bridgerian) paramyines. This was done because body mass is strongly tied to locomotor patterns and more reliable body mass estimates can be generated from an extant sample that functionally resembles the fossils being studied. Functional limb indices were calculated for three locomotor groups (arboreal, semifossorial, and gliding) of living sciurids. Comparisons among arboreal, semifossorial, and gliding sciurids show that the functional indices related to mechanical advantage of muscles and limb robusticity enable distinction among locomotor groups; however, there is considerable overlap between arboreal and semifossorial taxa. Paramys delicatus was found to have generally greater mechanical advantages and limb robusticity than most living squirrels, including semifossorial taxa. As these traits are associated with semifossorial squirrels that frequently use scratch-digging, this suggests that Paramys delicatus and perhaps other early paramyines were likely proficient scratch-diggers. However, indices reflecting limb proportions of paramyines suggest that these early rodents may have used more hind limb dominated locomotion than do living squirrels. Body mass estimations for early paramyines were therefore derived from a semifossorial squirrel sample. Statistical comparisons suggest that many of the most reliable estimators for body mass in Paramys delicatus and other paramyines are those derived from humeral dimensions, with the most reliable estimate being humeral head superoinferior breadth. Using these estimators, individual body mass estimates of early paramyines range from 3391 to 4005 g for Paramys delicatus, 1137–1329 g for Paramys copei, 1291 g for Paramys taurus, and 3357 g for Notoparamys costilloi. All body mass estimations derived from postcranial elements are substantially larger than previously published estimates derived from the dentition, which may be because postcranial elements play a larger role in supporting body weight.