Studies of large herbivorous mammal (ungulate) communities have revealed an association between increased tooth crown height (hypsodonty) and a shift to more arid environments over the Cenozoic. Ecometric analyses have also been used to examine the relationship between current climate conditions and crown height of these communities, and very successfully used to estimate past precipitation. Small herbivorous mammals (rodents and lagomorphs) have been shown to similarly adapt to changing habitats over the Cenozoic, but did so millions of years earlier than larger mammals. Here, we have utilized ecometric methods to examine the relationships between rodent and lagomorph crown height and diversity with current climate conditions, finding strong correlations of community structure parameters with climate, particularly mean annual temperature, maximum temperature of the warmest month, and minimum temperature of the coldest month. We applied the resultant regression equations to well-sampled fossil localities from North America, spanning over 37 million years, allowing estimation of climate conditions in deep time. As expected, estimates show a general pattern of decrease in both precipitation and temperature across North America from 37 Ma to the present. Overall, these findings provide a new broadly-applicable proxy for estimation of Cenozoic climate conditions at a local scale.

Through the last 60 million years (Ma) there were well-documented climate and habitat changes across North America. Continental and regional scale studies on ungulate (hooved mammals) tooth crown heights in relation to climate and habitat changes have documented an increase in tooth height as environments became more arid to combat a highly abrasive diet of fibrous plants and grit. Based on the relationships between crown height and climate, several studies have used taxon free methods to estimate past precipitation from fossil ungulates. Small mammals have also been shown to adapt to changing habitats, with specializations for life in open habitats appearing millions of years earlier than larger mammals. Utilizing taxon free methods, this study compared the crown heights of rodents and lagomorphs (rabbits and pika) from across North America to climate variables, including both temperature and precipitation. Studying small mammals allows examination of local changes to climate and can be informative of what is to be expected in regional communities if modern warming trends continue. Using ArcMap, modern climate data and mammal faunal lists for 100 random points were compiled and generated across North America. Crown heights of species were categorized and mean crown height for the community was calculated for each site. Linear regressions were used to examine the relationship between crown height and climate variables. Using ArcMap and ordinary kriging, maps were generated of predicted climate for all of North America and compared to known climate data. In general, the North-South gradient of temperature and the Northwest to Southeast gradient of precipitation was captured in predicted maps showing agreement with known climate maps. Regressions were also used to predict climate for 72 well-known fossil sites from the late Eocene (~37 Ma) to Pleistocene (~1.9 Ma) of North America, generating estimates of temperature and precipitation for many sites with no prior climate estimates. Results show strong correlation between rodent and lagomorph crown heights and temperature and precipitation variables. Temperature variables were more strongly correlated to tooth crown heights than precipitation. Overall, a decrease in precipitation and temperature was found across North America from 37 Ma to the present, consistent with prior paleoclimate records from other regional and global proxies. Application to detailed regional records from the Mid Miocene Climatic Optimum (around 15 Ma, during the early Barstovian land mammal age), which was a pronounced period of warming comparable to the warming we see today, showed regional variation in responses to warming. Tooth crown height increased in California and Nebraska before Oregon, showing climate did not change uniformly across North America similarly to what would be expected in modern warming. Using the method proposed in this study, past and future regional climate trends, as well as biotic responses to those changing climate trends can be better understood.

Continental scale studies on ungulate crown heights in relation to climate and habitat changes have revealed a correlation between increasing hypsodonty and a shift to more arid environments. Small mammals have been shown to adapt to changing habitats millions of years earlier than larger mammals. In this study I examined fossil localities throughout the last 37 Ma across North America. Diversity of rodents and lagomorphs were analyzed through this time period, with examination of community structure characterized by relative percentages of taxa with different crown heights. Overall, a decrease in precipitation and temperature was found across North America from 37 Ma to the present. The Mid Miocene Climatic Optimum (around 15 Ma) was an pronounced period of warming, comparable to the warming we see today. Nebraska and California showed increases in crown height before Oregon. Overall, these findings help demonstrate how communities react at different time scales to climate change.