<P> Pliohippus arose from Callippus in the middle Miocene, around 12 mya . It was very similar in appearance to Equus, though it had two long extra toes on both sides of the hoof, externally barely visible as callused stubs . The long and slim limbs of Pliohippus reveal a quick - footed steppe animal . </P> <P> Until recently, Pliohippus was believed to be the ancestor of present - day horses because of its many anatomical similarities . However, though Pliohippus was clearly a close relative of Equus, its skull had deep facial fossae, whereas Equus had no fossae at all . Additionally, its teeth were strongly curved, unlike the very straight teeth of modern horses . Consequently, it is unlikely to be the ancestor of the modern horse; instead, it is a likely candidate for the ancestor of Astrohippus . </P> <P> Dinohippus was the most common species of Equidae in North America during the late Pliocene . It was originally thought to be monodactyl, but a 1981 fossil find in Nebraska shows some were tridactyl . </P> <P> Plesippus is often considered an intermediate stage between Dinohippus and the extant genus, Equus . </P>

Where did the ancestors of modern horses originated