Patterns of tooth eruption and replacement in multituberculate mammals

Abstract

Dentitions of juvenile multituberculate mammals are rarely preserved in the fossil record. A previously undescribed skull and jaws of a juvenile individual of Taeniolabis taoensis contains in situ both upper and lower deciduous incisors, lower fourth premolars, and upper and lower first molars, all of which are just beginning to erupt. Based on this and other specimens, patterns of tooth eruption and replacement in at least some multituberculates can be reconstructed. All known multituberculates exhibit a pattern of diphyodonty similar to that seen in most placental mammals. Tooth eruption apparently occurred in an anteroposterior sequence. The primary generation P₄ (perhaps all the primary generation lower premolars) erupted by anterodorsal rotative eruption. The P₄ of most (perhaps all) Late Cretaceous-Tertiary multituberculates (ptilodontoids, taeniolabidoids, cimolomyids) appears to be monophyodont, in contrast to the diphyodont P₄ found in at least one paulchoifatiid plagiaulacoid. Comparisons of eruption patterns suggest that the adult P₄ of Late Cretaceous-Tertiary forms is homologous to the dP₄ of diphyodont taxa; accordingly, the “adult” P₄ of monophyodont forms is not comparable to the “adult” tooth in diphyodont taxa. The suppression of replacement at the P₄ locus may have evolved as a result of selection for a single, enlarged blade-like tooth in the lower dentition. The P⁴ also appears to have been monophyodont in Late Cretaceous-Tertiary multituberculates. Because suppression of replacement is theoretically possible at any polyphyodont or diphyodont tooth locus, the study of development (e.g., tooth eruption and replacement) is crucial for correct assessment of dental homologies in nonmammalian cynodonts and early mammals.