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Studies of fossil vertebrates belonging to the group Synapsida are central to understanding mammalian origins. Synapsida includes mammals and is one of the two major clades of amniotes (all fully terrestrial vertebrates). The other is Reptilia, which includes modern turtles, snakes, lizards, crocodiles, and birds. The therapsids, one major group of nonmammalian synapsids (historically but erroneously known as “mammal-like reptiles”) have been particularly important to understanding the acquisition of mammalian characteristics. One of the key features within the evolutionary history of synapsids is the morphological differentation of their dentition (teeth) over time (1). On page 1603 of this issue, Cisneros et al. (2) describe a new therapsid fossil from South America, Tiarajudens eccentricus, which displays a unique dentition, including broad chewing teeth on the palate and a pair of extremely long saber canines. The discovery provides novel insights into early tooth differentiation in synapsids and into the evolution of herbivory (plant eating) and its accompanying complex social interactions.