The evolution of equids (horses) has been extensively studied and is often used as an example of how evolution occurs, and has many times been used to prove that evolution does in fact occur. Horses appear in North America early in the Eocene at about 55-50 Ma. The story of horse evolution in North America focuses mainly on how horses have adapted to changes in the environment. Before grasslands emerged horses are thought to have been browsers, meaning that their diet consisted mostly of soft leafy vegetation or fruits. As grasslands started to replace forests during the Miocene (23-5.3 Ma), it is believed that the main food source for horses became the abrasive grasses found in the grasslands. This meant that horses would benefit from higher crowned teeth that would not wear down as easily while eating these abrasive grasses.
Recently, multiple studies have questioned this simple evolutionary story as evidence has been found that horses were still browsers long after they evolved hypsodont teeth.
In this study, mesowear analysis was performed on the teeth of 22 specimens from the Ashfall Fossil Bed site in Antelope County, Nebraska. These specimens represent 4 different genera that were alive and living together in this area around 11.8 Ma.
Even though there were multiple species of horses competing for the same food source, and, in contrast to other recent studies, the results support the traditional horse evolution story. The horses at Ashfall were mainly grazers, and there was little variation between the different genera.
More data from other sites in Nebraska would provide validation to this study. Also, mesowear analysis data from a site in Oregon that is similar in age to Ashfall would give a good set of comparison data and show if a different environment with a different variety of food sources would yield different results, results in which the hypsodont horses were actually browsers.
Nicole Werling, ’15
Cedar Rapids, IA
Sponsor: John Orcutt