Reconstructed masticatory biomechanics of Peligrotherium tropicalis, a non-therian mammal from the Paleocene of Argentina

Abstract

The large, bunodont, mammal Peligrotherium tropicalis is an enigmatic member of the earliest Paleocene fauna of Punta Peligro, Argentina. While being a contemporary of many of the earliest large-bodied “archaic ungulates” in the Northern Hemisphere, P. tropicalis is a remnant of an endemic Mesozoic non-therian lineage. The interpretation of P. tropicalis as an omnivore/herbivore has therefore been difficult to evaluate, given its phylogenetic placement outside of the therian clade, and lack of many of the molar characteristics thought to be essential for the forms of mastication seen in marsupials and placentals. Here we present a three-dimensional generalization of the classical “bifulcral” biomechanical model of bite force and joint force estimation, which is capable of accommodating the wide range of mediolateral force orientations generated by the muscles of mastication, as estimated by the geometry of their rigid attachment surfaces. Using this analysis, we demonstrate that P. tropicalis is more herbivorously adapted (viz. shows a greater Group 2 relative to Group 1 jaw adductor advantage for producing postcanine orthal bite forces) than even the hypocarnivorous carnivorans Procyon lotor and Ursus arctos , and is similar to the ungulates Sus scrofa and Diceros bicornis . This similarity also extends to the mediolateral distribution of relative muscle group advantage, with Group 1 muscles (responsible for effecting the initial adduction of the working-side hemimandible into centric occlusion) having greater orthal bite forces labially; and Group 2 muscles (those responsible for producing occlusal grinding motions) being more powerful lingually. Finally, we show that P. tropicalis preserves relatively little of its orthal bite force magnitude at high gape, suggesting that large-object durophagy would not have been a likely feeding strategy.