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Various lines of paleontological evidence support the theory, first emphasized by McKenna (1975), that eutherians primitively had at least five premolars. The most likely transformation to the four premolar number in many modern eutherians involved the loss of a premolar in the middle of the series, perhaps at the P3 locus. Transformational hypotheses invoking the retention of a last premolar and the loss of the last molar to arrive at the four premolar, three molar formula seem less plausible. More primitive dental formulae are apparent in the Cretaceous leptictid relative, Gypsonictops, Eocene sirenians, and a number of Mesozoic non-therians. The marsupial dental pattern, however, seems anomalous, and specialized in a way that contrasts strongly with the eutherian condition. Standard dental comparisons often conceal the problem of homology between corresponding teeth in different taxa. Such comparisons are strongly affected by the particular hypothesis of dental ontogeny and transformation assumed.
Journal of Vertebrate Paleontology © 1986 The Society of Vertebrate Paleontology