LITOPTERNA, a suborder of South American Tertiary ungulate mammals typified by Macrauchenia, and taking their name ("smooth-heel") from the presence of a flat facet on the heel-bone, or calcaneum for the articulation of the fibula. The more typical members of the group were digitigrade animals, recalling in general build the llamas and horses; they have small brains, and a facet on the calcaneum for the fibula. The cheekdentition approximates more or less to the perissodactyle type. Both the terminal faces of the cervical vertebrae are flat, the femur carries a third trochanter, the bones of both the carpus and tarsus are arranged in linear series, and the number of toes, although commonly three, varies between one and five, the third or middle digit being invariably the largest.
Of the two families, the first is the Proterotheriidae, which exhibits, in respect of the reduction of the digits, a curious parallelism to the equine line among the Perissodactyla; in this feature, as well as in the reduction of the teeth, it is more specialized than the second family.
The molar teeth approximate to the Palaeotherium type, but have a more or less strongly developed median longitudinal cleft. The three-toed type is represented by Diadiaphorus, in which the dental formula is i.-,c .; p. 5 i m. sand the feet are very like those of Hipparion. The cervical vertebrae are of normal form, the orbit (as in the second family) is encircled by bone, the last molar has a third lobe, the single pair of upper incisors are somewhat elongated, and have a gap between and behind them, while the outer lower incisors are larger than the inner pair, the canines being small. The skull has a short muzzle, with elongated nasals. Remains of this and the other representatives of the group are found in the Patagonian Miocene. In Proterotherium, which includes smaller forms having the same, or nearly the same, dental formula, the molar teeth differ from those of Diadiaphorus by the deeper median longitudinal cleft, which completely divides the crown into an inner and an outer moiety, the two cones of the. inner half being united. According to the description given by Argentine palaeontologists, this genus is also three-toed, the single-toed representative of the family being Thoatherium, in which the lateral metapodials, or splint-bones, are even more reduced than in the Equidae. In the second family - Macraucheniidae - the dentition is complete (forty-four) and without a gap, the crowns of nearly all the teeth being of nearly uniform height, while the upper molars are distinguished from those of the Proterotheriidae by a peculiar arrangement of their two inner cones, and the elevation of the antero-posterior portion of the cingulum so as to form an extra pit on the crown. To describe this arrangement in detail is impossible here, but it may be stated that the two inner cones are closely approximated, and separated by a narrow V-shaped notch on the inner side of the crown. The elongated cervical vertebrae are peculiar in that the arch is perforated by the artery in the same manner as in the llamas.
In the Santa Cruz beds of Patagonia the family is represented by the generalized genus Oxyodontotherium (in which Theosodon may apparently be included). It comprises animals ranging up to the size of a tapir, in which the nostrils were more or less in the normal anterior position, and the cheek-teeth shortcrowned, with the inner cones of the upper molars well developed and separated by a notch, and the pits of moderate depth. The last upper premolar is simpler than the molars, and the canine, which may be double-rooted, is like the earlier premolars. The radius and ulna, like the tibia and fibula, are distinct, and the metapodials rudimentary. On the other hand, in Macrauchenia, which was a much larger llama-like animal, the skull is elongated and narrow, with rudimentary nasals, and the aperture of the nose placed nearly on the line of the eyes and directed upwards, the muzzle not improbably terminating in a short trunk. Deep pits on the forehead probably served for the attachment of special muscles connected with the latter. Very curious is the structure of the cheek-teeth, which are high-crowned, with the two inner cones reduced to mere points, and the pits on the crown-surface large and funnel-shaped. In fact, the perissodactyle type is almost lost. The cervical vertebrae and limb-bones are very long, the radius and ulna being completely, and the tibia and fibula partially, united. The typical M. patagonica is a Pleistocene form as large as a camel, ranging from Patagonia to Brazil, but remains of smaller species have been found in the Pliocene (?) of Bolivia and Argentina.
The imperfectly known Scalabrinia of the Argentine Pliocene appears to occupy a position intermediate between Oxyodontotherium and Macrauchenia, having the nasal aperture situated in the middle of the length of the skull, and the crowns of the cheek-teeth nearly as tall as in the latter, but the lower molars furnished with a projecting process in the hinder valley, similar to one occurring in those of the former., In this place may be mentioned another strange ungulate from the Santa Cruz beds of Patagonia, namely, Astrapotherium., sometimes regarded as typifying a suborder by itself. This huge ungulate had cheek-teeth singularly like those of a rhinoceros, and an enormous pair of tusk-like upper incisors, recalling the upper canines of Machaerodus on an enlarged scale. In the lower jaw are two large tusk-like canines, between which are three pairs of curiously-formed spatulate incisors, and in both jaws there is a long diastema. The dental formula appears to be i. 3 i C., p ., m. 3.
Next Astrapotherium may be provisionally placed the genus Homalodontotherium, of which the teeth have much lower crowns, and are of a less decidedly rhinocerotic type than in Astrapotherium, and the whole dentition forms an even and unbroken series. The bodies of the cervical vertebrae are short, with flattened articular surfaces, the humerus has an enormous deltoid crest, suggestive of fossorial powers, and the femur is flattened, with a third trochanter. According to the Argentine palaeontologists, the carpus is of the alternating type, and the terminal phalanges of the pentedactyle feet are bifid, and very like those of Edentata. Indeed, this type of foot shows many edentate resemblances. The astragalus is square and flattened, articulating directly with the navicular, although not with the cuboid, and having a slightly convex facet for the tibia. From the structure of the above-mentioned type of foot, which is stated to have been found in association with the skull, it has been suggested that Homalodontotherium should be placed in the Ancylopoda, but, to say nothing of the different form of the cheek-teeth, all the other South American Santa Cruz ungulates are so distinct from those of other countries that this seems unlikely. It may be suggested that we have rather to deal with an instance of parallelism - a view supported by the parallelism to the Equidae presented by certain members of the Proterotheriidae. (R. L. *)
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