by Many people assume that horses first came to America when Spanish explorers brought them here about 500 years ago. In fact, recent research has confirmed a European origin for horses associated with humans in the American Southwest and the Great Plains.
But those weren’t the first horses in North America. The family Equidae, which includes domesticated varieties of horses and donkeys as well as zebras and their relatives, is native to the Americas. The fossil record reveals the origin of horses here more than 50 million years ago, as well as their extinction across the Americas during the last Ice Age around 10,000 years ago.
We are paleontologists who focus our research on different types of fossils, including ancient horses. Our latest work used computer statistics to analyze gaps in the fossil record to understand more about which horse species did and did not inhabit one ancient habitat in Florida.
Horses evolved as ecosystems changed
Fossil horses have been collected by people across North America for centuries. Because horse fossils are abundant and widespread across the continent, scientists often point to the long span of the horse family as evidence of long-term evolutionary change.
Paleontologists like us, who study extinct mammals, rarely find complete skeletons. Instead, we focus on surviving fossil teeth, which help us understand ancient diets, and fossil limbs, which help clarify how these animals moved.
Machines are eating horses. In the wild today, they feed primarily on grass that provides little nutrition, so they must eat large quantities to survive. The large teeth of modern horses and their ancestors are mainly adapted for grazing on gritty grass. They replaced the smaller teeth of more primitive horses that were adapted to browsing on soft, leafy vegetation.
We know what horses ate millions of years ago by studying distinctive microscopic scratches, pits and other wear patterns on their teeth that were created as the ancient horses chewed plant food. And analyzes of carbon preserved in fossil teeth show that species of horses ate different plants together; some were browsing on leaves from trees and trees, some were grazing on grass, and some were still on a mixed diet.
The change in tooth shape traces the change in dominant vegetation types in North America, from tropical forests which then gave way to a large expansion of open prairie grasslands. As the climate and flora changed over millions of years, horses changed from being mostly forest browsers to mostly open country grazers. Their teeth and feeding patterns adapted to the environment.
Another adaptation can be seen in the legs of horses. Modern horses have one hooked toe on each foot. Many fossil horses – the ancient ancestors of today’s horses – were extinct through the tip of the leg. The single toe on each leg is well elongated for fast and sustained running to avoid predators and for long-distance seasonal migrants. The more ancient three-toed feet provided stability on unstable or wet ground. The adaptation from three toes to one was likely in response to changing habitats.
But even as the environment changed, one particular species did not replace another overnight. The fossil record in North America reports periods millions of years ago when multiple species of horse coexisted on ancient landscapes. Species were different sizes and equipped with teeth to chew different plants, so they did not compete directly for the same foods. It is likely that some species were better suited to forests in different habitats within these ancient ecosystems while others were better suited to grasslands.
Understanding Florida’s fossil record
Paleontologists have been collecting horse fossils in Florida for over 125 years. The Florida Museum of Natural History at the University of Florida, where we work, has more than 70,000 fossil horse specimens from more than a thousand sites across the state.
One of our most prolific fossil sites, Montbrook, offers a glimpse of an ancient stream bed 5.8 million years old. It preserved more than 30 extinct mammals, including rhinoceroses, elephants and carnivores, as well as hundreds of fossil horse bones and teeth.
Although six species of horse are known elsewhere in Florida, we have only found four so far at Montbrook. We were puzzled by this smaller number of horse species, so we decided to investigate. Did the two “missing” horse species really live in Montbrook, or have scientists not yet found their fossil remains?
We designed a theoretical model that compares Montbrook, which has only four horse species, to other fossil sites in Florida that contain all six. Using a statistical technique scientists call “bootstrapping,” our computer essentially simulated a continuous fossil collection over time. We generated 1,000 theoretical fossil collection events based on counts of fossil species from the sites where the six are present, to predict the probability of collecting the species currently missing at Montbrook.
Results from our simulation show that the two horse species missing at Montbrook were absent for different reasons. One of the real horses is probably missing; the other can still be found with further digging.
Examining ‘gaps’ in the fossil record
Knowing an absent species is just as important as knowing when one is present at a fossil site. Absences may be indicators of underlying ecological and biological drivers that alter population dynamics. In addition to other types of analyses, researchers can apply this type of predictive modeling to many fossil species and ancient landscapes.
Ever since Charles Darwin proposed his theory of evolution, scientists have known that the fossil record is incomplete, leaving gaps in our knowledge of the past and evolutionary change. Paleontologists are challenged to explain these gaps, including which species were or were not present at particular fossil sites.
Certain materials, such as teeth and shells, which are often more durable than porous bone, can create gaps, and fossilize better than others. Likewise, different chemical conditions during fossilization, and even the amount of time spent collecting fossils at a particular site, can contribute to the lack of information.
Fortunately, fossil horse teeth are well preserved and commonly found. As new discoveries are made, such as those from our ongoing excavations in Florida, they will help clarify and narrow gaps in the fossil record.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
Written by: Stephanie Killingsworth, University of Florida and Bruce J. MacFadden, University of Florida.
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Bruce J. MacFadden receives funding from the US National Science Foundation.
Stephanie Killingsworth does not work for, consult with, share in or receive funding from any company or organization that would benefit from this article, and does not she disclosed any relevant connections beyond their academic appointment.
