COMPARATIVE ANALYSIS OF BODY SIZE EVOLUTION WITHIN POCKET GOPHERS (RODENTIA, GEOMYIDAE)
We use upper and lower toothrow lengths from 266 fossil specimens and body mass data from museum collections and the literature for extant specimens. We use a published phylogenetic framework of geomyines and an unpublished phylogenetic tree of entoptychines to explore the mode of evolution of body size in the two groups of rodents. The results of our analyses show that the evolution of body size in Entoptychinae follows an Ornstein Uhlenbeck model whereas body size evolution within Geomyinae follows a directional model. The ancestral entoptychine was a mid-sized rodent. Within Entoptychinae, small body size evolved at least three times across three different genera. Within Geomyinae, the genus Thomomys remained small whereas several genera (e.g., Heterogeomys, Zygogeomys) independently evolved large sizes.
We also explored body size variation within Entoptychinae. There is no significant difference in body size among the genera Entoptychus, Gregorymys, and Pleurolicus; the rare genus Ziamys is significantly smaller. Our results show that species from the western United States and Mexico are larger than those found in the Great Plains. There also appears to be a consistent pattern of association between a large and a small species living contemporaneously within a given region. Ongoing analyses will help refine the timing of the evolution of body size in Geomyidae and help shed light on its possible association with locomotion and heterochronous environmental changes across North America.