Niche modeling reveals lack of broad-scale habitat partitioning in extinct horses of North America

The classic traits of modern horses (Equidae) large body size, high-crowned teeth (hypsodonty), and a single toe (monodactyly) are often considered adaptations to grassland environments. However, extinct horses that varied in these three traits overlapped geographically for millions of years during the Miocene and Pliocene. It has been hypothesized that co-occurring horse species partitioned habitats, with large-bodied, hypsodont, and monodactyl equids dominating open grasslands, while equids with different combinations of traits lived in more wooded areas. We tested for the presence of broad-scale habitat partitioning by compiling a large database of North American horse fossil occurrences with data on trait state (body size, hypsodonty index, and toe number) and paleoenvironment (derived from paleovegetation records). Null modeling of niche overlap in each of the North American Land Mammal Ages of the Miocene and Pliocene revealed that taxonomic and trait-based groups show no differences in habitat occupancy. Cluster dendrograms visualizing niche overlap showed that some ecological guilds shared derived traits, but derived traits were not associated exclusively with grassland habitats. Trait values are not predicted by the proportion of grassland habitats in time bins. Further, the three traits show no co-evolution across the equid tree when corrected for phylogenetic relatedness. Together, these results suggest that the evolution of large body size, hypsodonty, and monodactyly in equids was not due to a shared selective regime in response to expanding grassland habitats; instead, these traits may have evolved separately, likely due to a variety of small-scale selective pressures acting across the variety of habitats present in the Miocene and Pliocene.