Litopterna

LITOPTERNA, an extinct order of hoofed mammals of the Tertiary and Pleistocene of South America, including two families, the Macraucheniidae, which parallel the camels of the northern world in general proportions, but with three toes on each foot, and the Proterotheriidae which resemble the horses in reduc tion of the toes to a single median digit. The name ("smooth heel") alludes to the facet on the heel-bone or calcaneum for articulation of the fibula, distinguishing them from the Perissodac tyla, with which they agree in the median symmetry and odd toed reduction of the hind foot.

Macrauchenia, the largest and best known genus, is as large as a camel, with long limbs, feet narrow and elongate with three digits of nearly equal size, short phalanges and probably padded toes. Radius and ulna are united but the ulnar shaft is broad, not reduced as in most ungulates, and the carpus is nearly serial. Tibia and fibula are partly consolidated. The neck is as long as in the camel, and shares with that animal the peculiarity of the vertebral artery passing inside the neural arch instead of per forating its wall as in all other mammals. The skull is rather small, with long face and the nasal aperture retracted almost to the middle of the skull, with facial pits thought to have lodged muscles for control of a proboscis as in the tapir. The dentition is unreduced, the simple front teeth grading uniformly into the larger and more complex molars. All the anterior teeth are spaced apart and the points of the upper and lower teeth alternate or interlock when the jaw is closed, instead of meeting evenly. The molars have double outer crescents, and transverse crests and cones that wear into a dentinal surface with deep round enamel pockets. The posterior premolars are molariform, the teeth in front of them progressively simpler.

Macrauchenia is found in the Pleistocene of Argentina. Theo sodon of the Miocene (Santa Cruz) is smaller with less elongate limbs and neck, proboscis less developed, ulna and radius separate, vestiges of the first and fifth digits in the fore foot and other primitive characters. The Proterotheriidae are characteristic of

the Oligocene, Miocene and Pliocene of Argentina, the Miocene genera being best known. Proterotherium, about the size of a sheep, has limbs and feet proportioned as in the later three-toed horses, with lateral digits complete but small and not reaching the ground ; the carpus and tarsus are nearly serial and inadaptive ly reduced, the metapodials are shorter and the phalanges longer than in the three-toed horses. The skull has the proportions of primitive Perissodactyla with moderately long face and unre duced nasals, and the cheek teeth are short-crowned, having two outer crescents and two imperfect, obliquely transverse crests on each upper molar; molariform premolars; but the anterior in cisors are enlarged into tusks instead of the canines as in most northern ungulates.

Diadiaphorus is similar to Proterotherium but larger, with lateral digits less reduced; Thoatherium is smaller and is remark able for its complete monodactyly, the lateral digits being re duced to very small nodules of bone at the back of the proximal end of the central metapodial. The three genera are found to gether in the Santa Cruz Miocene and in the Pliocene a three-toed genus survives, Epitherium much like Diadiaphorus. More primi tive genera from the older Tertiaries of Patagonia are assigned to this family, but the characters of their feet are not known.

The resemblance of the proterotheriid feet to those of the Equidae is a remarkable example of parallel specialization ; that of Macrauchenia to the camel is hardly less so, although in a different way. The perissodactyl resemblances throughout the or der are so far-reaching that it was included in the Perissodactyla by most of the older writers ; but Scott shows them to be wholly distinct.

See Burmeister, Anales del Museo Publico de Buenos Aires, t. i. (1864) ; R. Lydekker, Anales del Museo de la Plata t. ii. (1893) W. B. Scott, Reports of the Princeton Expedition to Patagonia (Iwo).