The milk‐molars of Perissodactyla, with remarks on molar occlusion.

Abstract

SUMMARY. A survey is made of the milk‐molars of Perissodactyla and Condylarthra with a view to determining the evolutionary trends. Molarization of the milk‐molars takes place in a variety of ways, which may be grouped into three main types: (a) the condylarthran type, found also in Plagiolophus, (b) the type found in the Equidae and Brontotheriidae, and (c) that found in the Tapiroidea and Rhinocerotoidea. Each of these types has a characteristic order of development of the cusps. The condylarthran type is considered to be the most primitive. From a study of the wear of the teeth, it is concluded that the occlusal movement in all Perissodactyla is transverse (ectal). The characteristic features of occlusion in the various families are discussed. During molarization the talonid becomes intercalated between two trigonids, the intercalation taking place at a relatively late stage in type (a), earliest in type (b) and at an intermediate stage in type (c). The buccal parts of the trigonid and talonid become functional before the lingual parts. The Palaeotheriidae (excluding Pachynolophus and Propachynolophus) are regarded as a distinct family that retains condylarth features of the milk dentition to a late date, and cannot be derived from Hyracotherium. With the exception of Plagiolophus, they show in their milk‐molars transitional stages leading to the conditions found in other perissodactyls. The hypothesis is put forward that the order of development of the cusps depends upon their different sensitivities to the graded morphogenetic conditions in the developing jaw. Correlated evolution of the upper and lower teeth suggests that occlusion has a morphogenetic basis which affects both jaws. The concepts of homology due to descent from a common ancestral structure, homology due to parallel evolution, and serial homology are considered to be closely related and not sharply separable in practice.

Mechanical modelling of tooth wear

“…Also almost all initial polishing striations were removed. This supports the hypothesis that occlusal wear facets are caused by attrition [37] and that non-abrasive diets can be recognized from the facets caused by attrition-dominated wear [38]. The extent to which such pure attrition resembles the wear that occurs in natural chewing cannot be further explored here, but differences are likely to occur because natural attrition will always be tempered to some extent by the presence of food.…”

Mechanisms and causes of wear in tooth enamel: implications for hominin diets

“…Studied intensively for 60 years [4], wear patterns reflect the diet of living mammals and are capable of predicting diet in extinct forms [5 -8]. This has been studied at various scales of measurement [9].…”

A New Astrapothere (Mammalia, Meridiungulata) from La Meseta Formation, Seymour (Marambio) Island, and a Reassessment of Previous Records of Antarctic Astrapotheres

“…The cingulum is broad, rather expanded anteriorly, and forms a basal shelf more conspicuous than that of Trigonostylops. The preserved parts of the protocone and protoloph are essentially the same as in the Patagonian genus, but the main wear facet is on the apex and posterior slope of the protocone (facet 9, lingual phase of Butler, 1952), whereas in Trigonostylops, Tetragonostylops, and other astrapotheres with cuspidate molars the wear is consistently more intense on the protoloph. An additional tooth fragment of an astrapothere (MLP 90-I-20-6) reported by Marenssi et al (1994: fig.…”