The Identity and Homology of the Postprotocrista and its Role in Molarization of Upper Premolars of Perissodactyla (Mammalia)

Two different crests have been identified in the upper cheek teeth of perissodactyls as the postprotocrista. Both occur in the upper premolars of some early perissodactyls, demonstrating that they are not homologous. The true postprotocrista extends distolabially from the protocone, often connecting the protocone and the metaconule, at least primitively. The other crest, here termed the endoprotocrista, extends less labially and more distally than the postprotocrista and does not connect to the metaconule. In some taxa, the hypocone arises from the distal end of the endoprotocrista. Thus, the endoprotocrista plays an important role in molarization of some perissodactyl upper premolars. Molarization is completed by separation of the hypocone from the protocone and the metaconule connecting to the hypocone, involving either migration or loss of the postprotocrista. The endoprotocrista is similar to the “Nannopithex fold” of North American adapid primates, but it is otherwise currently known only in some early perissodactyls. Perissodactyl upper premolars become molarized by at least two modes. One mode involves the development of the hypocone from the endoprotocrista, as described here. A second involves the enlargement and lingual migration of the paraconule, as inferred for the third upper premolar of early equids. In the upper molars, the hypocone appears to be derived from the cingulum. The present study highlights the difficulties in making inferences regarding the mode of premolar molarization from static morphology. This further reflects the uncertainty inherent in using modes of premolar molarization in phylogenetic analysis and concomitant problems in determining primary homology.