摘要：SummaryIn Carnivora, increases in body size often lead to dietary specialization toward hypercarnivory. Ursine bears (TremarctosandUrsus), however, are the only omnivorous Carnivora that evolved large body sizes (i.e., >50 kg). Traits contributing to their gigantism, and how those traits evolved, have never been studied. Here we propose that special dental characters of Ursinae (parallel buccal and lingual ridges) permit a sagittally oriented mastication associated with increasing emphasis on plant foods. This pattern can be traced back to a new early diverging bear of plant-dominated omnivorous diet,Aurorarctos tirawagen. et sp. nov. from the late Middle Miocene of North America, which was supported as the earliest known ursine bear by phylogenetic analysis. The anatomical transition to increased masticatory efficiency, probably together with the ability to hibernate, helped bears break prior ecological limitations on body size and led to the evolution of a distinctive lineage of herbivorous-omnivorous, large-bodied Carnivora.Graphical AbstractDisplay OmittedHighlights•We describe the earliest known ursine bear, supported by phylogenetic analyses•The new fossil bear has the earliest evidence of plant-dominated omnivory diet•We discuss how traits helped ursine bears breaking energetic limitations on body sizeBiological Sciences; Evolutionary Biology; Phylogenetics
