Orohippus pumilus is a critical transitional fossil representing one of the earliest known ancestors to the modern horse. Named by Marsh in 1872, Orohippus pumilus appears in the middle Eocene (45 million years ago) of North America, and is best known from the badlands of Wyoming (Kitts, 1957). This small horse is considered a transitional fossil, because it shows many characteristics of both the modern horse, and its earlier ancestors. This is best demonstrated by the skeleton, which still resembles the Eocene fauna. For example, it has four toes with claws, rather than the single toe and hooves characteristics of modern horses. However, the teeth are becoming increasingly similar to those of modern horses (Benson, 1940). Orohippus pumilus resembled a fox or a raccoon in size and stature, and lived in a warm, humid climate with lush meadows and grasslands where it browsed for grasses (Gunnell, 1998).
The skull of Orohippus pumilus is unique in that it has begun to show features like modern horses. However, it is the differences that cement its place as a transition fossil. Orohippus pumilus had a proportionately shorter skull than modern Equus, as well as a shorter snout (Korth & Evander, 1982). The braincase is notably smaller. However, by far its most distinctive characteristic is that the bones forming the eye socket are not completely enclosed as in modern horses(Korth & Evander, 1982).
The teeth belonging to Orohippus pumilus are especially unique, and are in fact the most diagnostic feature (partly because the type specimen is comprised of mostly teeth). In total, Orohippus pumilus possessed 44 teeth. Most notably, the premolars have grown quite close together and have begun to resemble the molars (Marsh, 1874). This is critical because it would have given the animal an extra grinding tooth, and begins to resemble modern equine dentition. The teeth are also unique in that the incisors bear no “marks” traditionally used to age the animals in modern horses, and the canines are well developed in both sexes, indicating a lack of sexual dimorphism. Also of note is that all of the teeth possess very low crowns (Granger, 1908). The crown is the portion of the tooth that extends above the gum line. Low crowned teeth are characteristic of animals feeding on softer vegetation.
The post cranial elements of Orohippus pumilus indicate that it stood about 13.5 inches tall and closely resembled a fox or a raccoon in size. The front feet possessed four well-developed toes, while the back feet possessed only three. However, in all four feet, the middle toe is by far the most developed and is beginning to dwarf the others in size, indicating an evolutionary precursor to the modern equines. Orohippus pumilus is known to have had a separate radius and ulna in the fore limbs, as well as a distinct tibia and fibula in the hind limbs. The latest studies recreate Orohippus pumilus with a short neck, an arched back, and short legs (Marsh, 1874).
Modern recreations of the Eocene of Wyoming place Orohippus pumilus in a warm habitat that would have been both very wet and very humid. The result would have been a combination of luxurious forests with lush, sprawling meadows and grasslands being fed by bountiful lakes and streams. It is unsurprising that Orohippus pumilus transitioned to the modern equine feeding style in this habitat, as grass would have been both plentiful and nutritious in the warm, wet environment. This is consistent with the similarity between the premolars and molars, indicating that Orohippus pumilus was adapting to have an extra grinding tooth as it transitioned to a diet of more grass (Gunnell, 1998).
