Cenozoic comparisons of diversity and disparity in the context of tectonic regime: A case study in North American rodents

Abstract

The tectonic evolution of landscape topography is an important driver of biotic evolution throughout Earth's history. Studies of extant diversity have shown a higher diversity of mammals in topographically complex regions, suggesting that landscape complexity plays a role in generating taxonomic diversity. However, this relationship is poorly understood, especially over geologic time scales. Previous work has posited that heterogeneous topography should lead to increased species diversity as a result of new niche spaces into which lineages may speciate. This is in comparison to homogeneous landscape topography, which offers fewer and less varied niche spaces and thus less ecological opportunities for diversification. It is also thought that barriers to species and individual movement created by mountainous landscapes may divide populations, leading to speciation by restricting gene flow. These two models can be distinguished by their effects on morphological disparity. If niche differentiation is important to the process, we would expect increased morphological disparity in association with heterogeneous landscape topography, whereas the allopatry model predicts no greater morphological disparity over the complex landscape, only taxonomic diversification. We explore these models in the tectonically active and passive regions of North America in rodents from 25 to 0 Ma. We investigate diversity and disparity trends and examine the effects of climate, landscape heterogeneity, and environmental shifts on these patterns. We find that morphological and taxonomic evolution are not impacted by topography in a predictable way, and find that homogeneous landscapes host taxonomic and morphological diversity surpassing that of heterogeneous landscapes for much of the last 19 million years in North America. Furthermore, diversity and disparity trends are both coupled and decoupled during this period, signifiying that taxonomic and morphologic increase and decrease are discordant through time. Our results support the finding that the relationship between landscape heterogeneity and species diversity has not remained constant through geologic time.