Changes in Body Mass over Evolutionary Timescales in Gophers
(Rodentia: Geomyidae: Entoptychinae)
Abstract: “Body mass is a critical property of animals that is linked to many ecological characteristics. It can also easily be estimated, including in the fossil record. In this study, I quantified the body size of 33 species of the subfamily Entoptychinae, one of the most species-rich groups of burrowing rodents of the Oligocene. Between 30 and 20 million years ago, these animals were one of the dominant members of mammal communities across North America. I used measurements of the upper and lower toothrows of 222 fossil specimens to track the evolution of body size in entoptychines during the Oligocene and Miocene. I also assessed variation among and within each of the four genera.
My results show that Ziamys is significantly smaller than the other three entoptychine genera: Entoptychus, Gregorymys, and Pleurolicus. Interesting patterns of body size variation also emerge within each genus. For example, it appears that geographically overlapping species of Gregorymys are significantly different in body size. Additionally, within Pleurolicus, the largest species are found in the western intermontane region whereas significantly smaller species are restricted to the Great Plains.
The results of my analysis of body size changes through time show that the largest entoptychine taxa were alive early in the evolution of the group and went extinct around 23 million years ago. At the same time, the median and mean body sizes decreased and the smallest species of the subfamily show up in the fossil record. This drop in body size may be linked to a potential increase in fossoriality. Indeed, the decrease in body size within Entoptychinae happens at the time when open grass environments ideal for burrowing become more prevalent across the western United States where the fossil gophers are abundant. My results also show a relatively stable body size disparity throughout the range of the subfamily. Future analyses will investigate this pattern and its possible connection to changes in locomotion through time within the clade.”