Functional morphology and evolution of the adapiform dentition, with particular emphasis on the Asian Sivaladapidae

This study utilizes two-dimensional geometric morphometric techniques to address molar shape variation in extinct and extant prosimians using the relative orientation of selected homologous molar landmarks (paraconid, protoconid, metaconid, entoconid, hypoconid, hypoconulid, metaconulid, and the intersection of the cristid obliqua with the protolophid). Molar shape was studied using Thin-Plate Splines analysis to explore variation using shape variables simultaneously, rather than by comparing inter-landmark distances, as in traditional studies. The sampled extant taxa included members from the Malagasy lemur families Lepilemuridae, Lemuridae, and Indriidae, as well as Asian Lorisidae. Extant taxa were categorized using taxonomic, as well as dietary, categories to determine if intra-sample variation was correlated with either category. Results suggest that frugivores (and gramnivores) generally exhibit a relatively wide anterior talonid basin and little trigonid torsion (observed as the angle of the protolophid relative to the long axis of the molar). Folivores, on the other hand, generally exhibit a constricted anterior talonid and higher degrees of trigonid torsion. Lorisid omnivores were found to exhibit constricted anterior talonids, but little trigonid torsion.

The Thin-Plate Splines technique was also applied to a sample of three extinct adapiform families: Notharctidae, Adapidae, and Sivaladapidae. These comparisons were aimed at further exploration into dietary adaptation and diversity in extinct prosimians, and to specifically address the paleobiology of the Asian sivaladapids. Like their extant counterparts, adapiforms generally varied also in the relative orientation of the trigonid basin and in the dimensions of the talonid basin. In addition, the relative location of the paraconid also varied significantly within several adapiform families. Results of this study specifically highlight shape variation within several prosimian lineages, suggesting that a broader perspective of morphological diversity can be appreciated through analyzing adaptation within a particular family. For the sivaladapid genera, Sivaladapis and Hoanghonius, in particular, as well as North American Notharctidae, a molar shape similar to Nycticebus (an Asian loris) was noted. For Adapidae, a similarity with extant indriids was revealed. In summary, it is suggested here that multiple lines of morphological evidence should be employed to gain the broadest perspective of extinct primate adaptation.