Davis, D. Dwight Formerly, Curator of Vertebrate Anatomy, Chicago Museum of Natural History, Chicago, Illinois.
Szalay, Frederick S. Department of Vertebrate Paleontology, American Museum of Natural History, New York, New York.
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One of the four subclasses of the class Mammalia, including all living mammals except the monotremes. The Theria were by far the most successful of the several mammalian stocks that arose from the mammallike reptiles in the Triassic. The subclass is divided into three infraclasses: Pantotheria (no living survivors), Metatheria (marsupials), and Eutheria (placentals). These were not strictly contemporaneous; the Pantotheria arose directly from mammallike reptiles, and the Metatheria and Eutheria in turn arose from pantotherelike forms during the Jurassic or Cretaceous, many millions of years later. Therian mammals are characterized by the distinctive structural history of the molar teeth. The fossil record shows that all the extremely varied therian molar types were derived from a common tribosphenic type in which three main cusps, arranged in a triangle on the upper molar, are opposed to a reversed triangle and basinlike heel on the lower molar. See also: Mammalia
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