Prosotherium

This animal was similar to rabbits, in aspect and in size. Compared to its relatives, it would have been about 20% larger than Propachyrucos, Pachyrukhos, and Paedotherium, but slightly smaller than Medistyllus.^[2] Its hind legs were particularly long.

Its skull was light and thin, notably in the posterior part. The tympanic part of the temporal bone was particularly developed, even more than its relative Pachyrukhos, and it is probable that its auricle was quite large, similar to the ears of a hare. The maxilla and the mandible were high and deep ; the mandible had a subtle coronoid process. The teeth were characterized by incisors pointing inward, molars and premolars covered by a thin layer of dental cementum, generally on the outside part of the upper teeth and on the inside part of the lower teeth.^[1]

The skull had a complete dentition, but the second and third incisors, as well as the canines and the first lower premolars, were vestigial. The first upper incisors were very large, and showed considerable development.^{[citation needed]}

The humerus was large and thin, like the ulna and radius. Metacarpals and phalanges indicates that the hand was small and delicate. The pelvis was elongated, slender and lightly build, while the femur was equipped of a small head separated from the main bone. The femur was even longer than in Pachyrukhos, and had a small trochanter curiously pushed back in the posterior part. Contrary to Pachyrukhos, the tibia and fibula were completely separated. The talus bone was likewise characteristic, with the trochlear nerve extant covering entirely the dorsal part of the bone, and a low and flat condylar crest. The calcaneus was relatively small, with an articulated facet for the cuboid bone slightly concave and occupying the entire distal part of the bone. The metatarsal was robust and shorter than in Pachyrukhos. The distal phalanx was high and narrow, flattening and widening towards the terminal part.^[1]

Its body was slender, with a very short tail and strong forelegs, although much shorter than the hind legs, equipped with long metatarsals.

The tibia and fibula were not fused, and the forearm and lower leg bones were respectively shorter than the humerus and femur, but the hind legs were much longer than the forelegs. The second and third fingers of the hand were of similar length, and the fourth was smaller ; the fifth finger was small, even smaller than in its relative Pachyrukhos. The hand seems more specialized than in Pachyrukhos, and shares similarities with that of Paedotherium. The metatarsals were elongated, especially the third.^{[citation needed]}