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Evolution of the Tribosphenic Molar Pattern in Early Mammals, with Comments on the “Dual-Origin” Hypothesis

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Abstract

Development of the tribosphenic molar was a fundamental event that likely influenced the rise of modern mammals. This multi-functional complex combined shearing and grinding in a single chewing stroke, and provided the base morphology for the later evolution of the myriad dental morphologies employed by mammals today. Here a series of morphotypes are presented that represent stepwise acquisition of characters of the molar crown, in an effort to clarify homologies and functional analogies among molars of tribosphenic and tribosphenic-like mammals, as well as their putative sister groups. This is accomplished by evaluation of wear features, which provide direct evidence of occlusal function, and mapping these features on molars of the various morphotypes demonstrates their utility in determining homology. The original singular lower molar talonid cusp is homologous with the hypoconid, and upper molar cusp C in early mammals is homologous with the metacone (cusp “C” is a neomorph with variable occurrence). The lingual translation of the metacone to a position more directly distal to the paracone (as in Peramus) creates an embrasure for the lower molar hypoconid, and is accompanied by the development of the hypoconulid and a new shearing surface. Lastly, the Gondwanan radiation of tribosphenic-like mammals, the Australosphenida (including monotremes), is determined to be functionally non-tribosphenic. The Tribosphenida are restricted to Laurasian taxa, with an origin at or just prior to the Jurassic-Cretaceous boundary.

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Acknowledgments

I would like to thank my dissertation committee, Rich Cifelli, Nick Czaplewski, Cindy Gordon, Rick Lupia, Laurie Vitt, and Steve Westrop (all University of Oklahoma, Norman, USA). Access to specimens was provided by Jerry Hooker, Andy Currant, and Pip Brewer (Natural History Museum, London, UK), Guillermo Rougier (University of Louisville, Louisville, USA), Bill Simpson (Field Museum, Chicago, USA), and John Flynn (American Museum of Natural History, New York, USA). I benefited from very useful discussions with Rich Cifelli and Guillermo Rougier. Thoughtful reviews of the manuscript were provided by Guillermo Rougier and an anonymous reviewer. This project was possible through funding by a Stephen J. Gould Grant from the Geological Society of America, as well as funding from the College of Arts and Sciences, Department of Zoology, Graduate Student Senate, and a Robberson Research Grant from the Graduate College, University of Oklahoma.

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  1. Sam Noble Oklahoma Museum of Natural History and Department of Zoology, University of Oklahoma, 2401 Chautauqua Ave., Norman, OK, 73072, USA

    Brian M. Davis

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Davis, B.M. Evolution of the Tribosphenic Molar Pattern in Early Mammals, with Comments on the “Dual-Origin” Hypothesis. J Mammal Evol 18, 227–244 (2011). https://doi.org/10.1007/s10914-011-9168-8

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