About the Animals: Horses
It's common to think life forms evolve in neat, connected stages, progressing from one variation to the next in an orderly ladder that leads from the original ancestor to its present-day counterpart.
But sometimes the family tree turns out to be more of a bush, with branches that end relatively quickly, or develop in similar ways without being connected.
Thanks to the work of generations of paleo sleuths and the fossils they’ve uncovered, we now know that horses have undergone this less-than-orderly style of evolution.
Horses began as leaf-eaters, with four toes on the forefoot and three on the hindfoot, and were as small as a Siamese cat. Now, they live as single-toed, long-legged grass-eaters that could easily outrun their ancestors.
Migration of the Horse
The Horse Evolutionary Tree
See how the ancestral horse lines evolved in sometimes-parallel lines of browsers and grazers who grew steadily taller and faster to become today’s horse.
Nebraska’s Pieces of the Puzzle
This state has supplied many pieces of the puzzle that help form the picture of changing climate and habitat, and its relationship to horse evolution.
Though some have been found in nearby Wyoming, so far Nebraska’s fossil record lacks specimens of the earliest horses. However, paleo sleuth Mike Voorhies discovered a deposit of sediment from the early Eocene in Knox County, Nebraska. This small deposit included "Hyracotherium," the earliest horse genus, though not the earliest species. The find is the only evidence in Nebraska of the lush early Eocene forests that grew from 66 to 45 MYA. Historical work was done here by paleo sleuth O.C. Marsh as well as by Mike Voorhies.
At Toadstool Park, a site with specimens from the late Eocene and Oligocene (about 37-28 MYA), primitive horses have been found -- only low-crowned browsers who ate leaves. There are no high-crowned grass eaters. All mammals that lived there are long extinct. It was warm and temperate in the late Eocene but drier and more open in the Oligocene. Historical work was done here by paleo sleuths by O.C. Marsh, Erwin Barbour and Mike Voorhies. Joseph Leidy also did work involving specimens from Toadstool Park. Secord, along with graduate student Grant Boardman, has published recent research on the mammal habitats at Toadstool.
At Norden Bridge, a Middle Miocene site from about 14 MYA, the climate at this time was warm and temperate with riparian (river-related) forest along stream channels. It had nearly frost-free winters (indicated by the presence of species like tortoises that could not tolerate long periods of freezing). It was home to both high-crowned horses (grazers) and low-crowned horses (browsers), indicating a mixed habitat. Historical work was done here by paleo sleuths Morris Skinner and Mike Voorhies.
Fossils from five kinds of horses were found at Ashfall Fossil Beds, where many communities of animals perished and were rapidly buried by a cloud of volcanic ash over the course of one to two months. This Middle Miocene site from 12 MYA was nearly frost-free at that time, with warm grasslands and riparian forests. Historical work was done here by paleo sleuth Mike Voorhies.
At Big Springs, a site from the Pleistocene (roughly 1.5 MYA), many large horse specimens (including zebras) have been found. The specimens suggest this was a warm interglacial interval of the Pleistocene (Ice Age). Historical work was done here by paleo sleuth Mike Voorhies. Recent research was published by Secord and his student, Zachary Kita, on the diets and habitats of horses, camels, elephants, and peccaries.