Occlusal Pattern in Paulchoffatiid Multituberculates and the Evolution of Cusp Morphology in Mammaliamorphs with Rodent-like Dentitions

Abstract

Multituberculates developed a very complex masticatory apparatus during their long evolutionary history from the Jurassic to the Paleogene. Besides their rodent-like elongated incisors and diastemata, Cenozoic cimolodont Multituberculata display masticatory movements involving two distinct cycles in the mastication. An orthal slicing-crushing cycle associated with an enlarged lower fourth premolar precedes a palinal grinding cycle linked to upper molars with three longitudinal rows of cusps. With their plesiomorphic lower premolars and upper molars, the Late Jurassic/Early Cretaceous multituberculate family Paulchoffatiidae can provide the key for the understanding of the origin of the complex mastication of the Cimolodonta. Using for the first time propagation phase contrast Synchrotron X-Ray microtomography to perform both microwear and topographic analyses in order to characterize the mastication of Paulchoffatiidae, we digitized dental material from the Late Jurassic of the Guimarota Coal Mine (Leiria, Portugal) at the European Synchrotron Facility (Grenoble, France). Mastication in Paulchoffatiidae is characterized by a palinal grinding cycle. In contrast to Cimolodonta, no evidence of an orthal slicing-crushing cycle has been observed: the lower premolars mainly have a grinding function like the molars as they do exhibit buccal attrition facets bearing longitudinal striations. Nevertheless, the slightly oblique striations observed on the mesial part of the paulchoffatiid lower premolars possibly presage the orthal phase of the Cimolodonta. Our topographic analysis indicates that a strong relationship between individual cusp shape and direction of chewing is emphasized in rodents and rodent-like Mammaliamorpha such as Cimolodonta and Tritylodonta. Surprisingly, this relationship is not evident in Paulchoffatiidae. This unexpected result can be explained by the non-involvement in the attrition of many premolar cusps in Paulchoffatiidae, as indicated by our microwear analysis. The stronger the attrition, the more the direction of the masticatory movements influences the cusp morphology in Mammaliamorpha.