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A new specimen of the tinodontid “symmetrodont” Eurylambda (Simpson, 1925a, 1929) from the Late Jurassic Como Bluff Quarry, Morrison Formation, is described. The specimen, a complete upper left molariform, is probably an M1.
The major crown cusps of Eurylambda show similarities to those of triconodontids on the one hand and to spalacotherioids on the other. Cusp B of basal mammaliaforms is tentatively proposed as homologous with the cusp traditionally described as a stylocone in Eurylambda and with cusp B′ of Peralestes. These homologies imply that the stylocone is ancestrally a small cusp in the lineage leading to Theria and that the development of a parastylar lobe or “hook” is a derived feature of post-tinodontid mammals. If accepted, this scenario results in a more complex origin for the therian upper molar than previously recognized. Wear facet 1 (Crompton, 1971) of holotherians would not be homologous between Kuehneotherium–Eurylambda–Zhangheotherium, on the one hand, and the therians, on the other.