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Mesohippus
Mesohippus was one of the first horses to set foot on Earth. It had evolved from Eohippus. Mesohippus is also known as Orohippus.
In the late Eocene and the early stages of the Oligocene epoch (32–24 million years ago), the climate of North America became drier, and the earliest grasses began to evolve. The forests were yielding to flatlands, home to grasses and various kinds of brush. In a few areas these plains were covered in sand, creating the type of environment resembling the present-day prairies.
In response to the changing environment, equids, too, began to change. In the late Eocene, they began developing tougher teeth and becoming slightly larger and leggier, allowing for faster running speeds in open areas, and thus for evading predators in non-wooded areas. About 40 million years ago, the Mesohippus ("middle horse") suddenly developed in response to strong new selective pressures to adapt, beginning with the species Mesohippus celer and soon followed by Mesohippus westoni.
In the early Oligocene, Mesohippus was one of the more widespread mammals in North America. It walked on three toes on each of its front and hind feet (the first and fifth toes remained, but were small and not used in walking). The third toe was stronger than the outer ones, and thus more weighted; the fourth front toe was diminished to a vestigial nub. Judging by its longer and slimmer limbs, Mesohippus was an agile animal.
Mesohippus was slightly larger than Eohippus, about 610 mm (24") at the shoulder. Its back was less arched, and its face, snout, and neck were somewhat longer. It had significantly larger cerebral hemispheres, and had a small, shallow depression on its skull called a fossa, which in later horses became quite detailed, and serves as a useful marker for identifying an equine fossil's species. Mesohippus had six grinding "cheek teeth", with a single premolar in front—a trait all later equid species would retain. Mesohippus also had the sharp tooth crests of Eohippus, improving its ability to grind down tough vegetation. But this horse wouldn't stay for long. It soon developed into Merychippus, with a few other developments on the way, but Scientists normally say Mesohippus soon came to be Merychippus.
In the late Eocene and the early stages of the Oligocene epoch (32–24 million years ago), the climate of North America became drier, and the earliest grasses began to evolve. The forests were yielding to flatlands, home to grasses and various kinds of brush. In a few areas these plains were covered in sand, creating the type of environment resembling the present-day prairies.
In response to the changing environment, equids, too, began to change. In the late Eocene, they began developing tougher teeth and becoming slightly larger and leggier, allowing for faster running speeds in open areas, and thus for evading predators in non-wooded areas. About 40 million years ago, the Mesohippus ("middle horse") suddenly developed in response to strong new selective pressures to adapt, beginning with the species Mesohippus celer and soon followed by Mesohippus westoni.
In the early Oligocene, Mesohippus was one of the more widespread mammals in North America. It walked on three toes on each of its front and hind feet (the first and fifth toes remained, but were small and not used in walking). The third toe was stronger than the outer ones, and thus more weighted; the fourth front toe was diminished to a vestigial nub. Judging by its longer and slimmer limbs, Mesohippus was an agile animal.
Mesohippus was slightly larger than Eohippus, about 610 mm (24") at the shoulder. Its back was less arched, and its face, snout, and neck were somewhat longer. It had significantly larger cerebral hemispheres, and had a small, shallow depression on its skull called a fossa, which in later horses became quite detailed, and serves as a useful marker for identifying an equine fossil's species. Mesohippus had six grinding "cheek teeth", with a single premolar in front—a trait all later equid species would retain. Mesohippus also had the sharp tooth crests of Eohippus, improving its ability to grind down tough vegetation. But this horse wouldn't stay for long. It soon developed into Merychippus, with a few other developments on the way, but Scientists normally say Mesohippus soon came to be Merychippus.
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