The horse fossils found in the bottommost layer of rock at the site would be the oldest ones. Therefore, in this case, the species of horse fossils that would be found in the bottommost layer of rock are the Orohippus fossils.
Orohippus is an early ancestor of the horse that lived during the Eocene epoch, around 50 million years ago. It was small in size, about the size of a small dog, and had multiple toes on each foot, unlike modern horses.
So, when a scientist discovers Orohippus fossils in the bottommost layer of rock along with Mesohippus and Miohippus, it indicates that Orohippus is the oldest of the three species found at that site.
