separates it from cusp D. D itself is crest-shaped and forms the lingual wall of the intermediate fossa; it is described as "very odd", and may in fact consist of two fused, triangular cusps. A crest issuing from D separates the back from the intermediate fossa, and another, larger crest separates the front from the back fossa and nearly reaches cusp B. Cusp E is triangular and separated from cusps F and D by valleys, which are bordered internally by crests connecting the cusps. F is rounded. The microstructure of the tooth enamel is not clearly recognizable, evidently because the tooth is degraded, though structures resembling enamel prisms (bundles of hydroxyapatite crystals) and Hunter-Schreger bands are recognizable. Identity Because of the complexity of the crown, Goin and colleagues identified the tooth as a mammal; although some non-mammalian groups, like crocodylians, may have complex teeth, none approach the level of complexity seen in LACM 149371. They could find no resemblance to australosphenidans including monotremes, metatherians including marsupials, xenarthrans, and some related groups. They did see some general resemblances to the upper premolars of the early South American ungulates, but the cusp arrangement is different from that of any ungulate. There are also some resemblances to the early rodents Ivanantonia from Asia and Nonomys from North America, but Ivanantonia has a central groove and lacks fossae, and Nonomys has a prominent cingulum (shelf) at the edges of the tooth and also lacks the fossae of LACM 149371. The tooth resembles multituberculates—a large group of extinct mammals with many-cusped teeth—in the shapes of the valleys and crests, but multituberculates lack fossae and usually have quadrangular teeth with two longitudinal rows of cusps separated by a central valley. In the same features, LACM 149371 resembles gondwanatheres, a small and enigmatic group of mammals from the Cretaceous through Eocene of the southern (Gondwanan) continents that may be related to multituberculates. In particular, Ferugliotherium from the late Cretaceous of Argentina has similarly formed cusps and also has crests that connect the cusps to the center of the tooth. However, the upper molars are unknown, and the low-crowned teeth of Ferugliotherium lack deep fossae. Members of the higher-crowned gondwanathere family Sudamericidae do have fossae. Goin and colleagues conclude that LACM 149371 most likely represents a member of the gondwanathere family Ferugliotheriidae; if so, it would be among the youngest known gondwanatheres. References Bibliography Campbell, K.E., Jr. 2004.