TOXODONTIA, a sub-order of extinct South American Tertiary ungulate mammals typified by the genus Toxodon, so named from the bow-like curvature of the molar teeth. They all show signs of distant kinship to the Perissodactyla, as regards both limb-structure and dentition; while some exhibit resemblance to the Rodents and Hyraxes - resemblances which, however, are probably to be attributed to parallelism in development.
Under the sub-order Toxodontia may be included not only the typical Toxodon, but the more aberrant Typotherium (fig. I) of the Pleistocene of Buenos Aires and the smaller Pachyrucus and Hegetotherium of the Patagonian Santa Cruz beds. All the members of the sub-order have tall-crowned and curved cheek-teeth, some or all of which generally have persistent pulps, while at least one pair of incisors in each jaw is rootless. Thej bodies of the cervical vertebrae have flat articular surfaces, the bones of the two rows of the carpus alternate, and in the tarsus the navicular articulates with the calcaneum, which, as in the Artiodactyla, is articulated to the fibula, while the astragalus, which is slightly grooved above, is formed on the Perissodactyle plan. The number of toes varies between three and five, of which the middle one is the largest, and the femur may or may not have a third trochanter. The Typotheriidae and Pachyrucidae are remarkable among the Ungulates for ¦ Aiter Gervais.) FIG. I. - Skull of Typotherium cristatum, from the Pampas Formation of Buenos Aires. (4 nat. size.) the retention of clavicles, and for their curious approximation in dentition and certain characters of the skeleton to the Rodentia. The dental formula of Typotherium is i. 2, c. ° S, p. i, m. z; that of the smaller Patagonian forms differs by the larger number (3) of premolars. The toes were unguiculate rather than ungulate in character, except the hind ones (four in number) of Typotherium. Certain allied Patagonian forms, such as Argyrohyrax, have been supposed to be related to the Hyraxes.
The Toxodontidae differ from the preceding families by the loss of the clavicles and the reduction of the digits to three in each foot. The typical genus Toxodon is represented by animals the size of a (From British Museum [Nat. His.] Guide to the Fossil Mammalia.) FIG. 2. - Skeleton of the Toxodon (Toxodon platensis). From (About 3u nat. size.) rhinoceros, of which the entire skeleton is now known (fig. 2). The teeth, of which the formula is i. 3, c. ° p. a 4, m. 3, all grow from persistent pulps; those of the cheek-series are very tall, highly curved, and with a simplified crown-structure. In the older Nesodon, on the other hand, the cheek-teeth are shorter-crowned, and depart less widely from a generalized Perissodactyle type, the total number of teeth being forty-four, and there being scarcely any gap in the series. Very remarkable changes occur in the dentition as age advances, most of the teeth eventually developing roots, although the second pair of incisors in each jaw was rootless. The complete skeleton is not yet known, but it is ascertained that the femur differs from that of Toxodon in the retention of a third trochanter.
Toxodon is typified by T. platensis from the Pampean formation of Buenos Aires. Toxodontotherium and Xotodon are allied but rather older types. Nesodon is from the Santa Cruz beds of Patagonia, the typical N. imbricatus having a skull about a foot in length, but N. ovinus was a smaller animal, about the size of a sheep.
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