Fossil Horse

Associated Smithsonian Expert: Anna K. Behrensmeyer, Ph.D.

Dr. Kay Behrensmeyer working on describing strata at Olorgesailie, Kenya

Photo by Rick Potts, Smithsonian

Dr. Anna Katherine “Kay” Behrensmeyer grew up in western Illinois and hunted fossils with her brothers in bluffs of Paleozoic marine limestones along the Mississippi River. Kay started undergraduate school as an art major, but after a geology course at Washington University, St. Louis and then Indiana University’s Geological Field Camp in Montana, she decided to become a geologist/ paleontologist. She earned her doctorate in vertebrate paleontology and sedimentology from the Department of Geological Sciences, Harvard University. Kay’s Smithsonian career has involved paleontological and geological research in the field and laboratory, with a particular focus on the ecological context of human evolution in East Africa. She is co-director of the Evolution of Terrestrial Ecosystems program, which is working with ecologists to understand natural communities of plants and animals and how these change over time. Kay also is working on paleontological and archeological field projects in Kenya and Arizona, and she also continues research on Pakistan fossil vertebrates. One of her favorite research areas is Amboseli National Park, Kenya, where she continues long-term (35-year) research on processes that affect bone destruction and preservation, a sub-field of paleontology known as taphonomy.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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Fossil skeleton of small odd-toed ungulate (Hyracotherium vasacciensis), now extinct
Photo by Chip Clark, Smithsonian, National Museum of Natural History

About Odd-toed Ungulates (Order Perissodactyla): Paleobiology

Evidence for the relatedness between odd-toed ungulates, including horses, rhinos, and tapirs, is the structure of their lower limbs, especially the large middle toe on each hoof. They also have distinctive teeth and are “hind-gut fermenters” with a digestive system that can process large amounts of low-quality vegetation. While they are not diverse today, these animals were abundant and diverse for millions of years. They first appear in the fossil record 58 million years ago (late Paleocene). During the 15 million years that followed, they diversified into many species, including ancestral horses that were the size of dogs, tapirs similar to those living today, and a variety of rhinos. An extinct rhino (Paraceratherium sp.) was the largest land animal that has ever lived. It weighed as much as 20 tons (eight times as much as an elephant). Odd-toed ungulates declined though the Oligocene and Miocene (about 34-5.5 million years ago), while even-toed ungulates rose to dominance, for reasons that are debated among paleontologists and paleoecologists.

Depiction of Triassic small mammal (Thrinaxodon) in habitat
Photo by Mary Parrish, Smithsonian, National Museum of Natural History

About Mammals (Class Mammalia): Paleobiology

On Earth today, mammals, particularly humans, are a common life form in many different habitats. But mammals were not always common or diverse. The first mammals, which evolved during the middle Triassic (about 240 million years ago), were small compared to many of their contemporaries, including the earliest dinosaurs. No larger than shrews or mice, these early mammals probably lived in burrows or other refuges when dinosaur and other reptilian predators roamed the Earth. Though they remained relatively small, mammals evolved to fulfill many ecological roles during the long reign of the dinosaurs. When dinosaurs (except birds) disappeared at the extinction event 66 million years ago, mammals survived and began to expand into ecological spaces (niches) vacated by the larger dinosaur herbivores and predators. By about 55 million years, ago, all modern groups of mammals had evolved. The Cenozoic (the Era in which we live) is known as the Age of Mammals, thanks to their successful radiation into a huge variety of habitats on Earth.