Adelobasileusfrom the Upper Triassic of West Texas: the oldest mammal

Adelobasileus cromptoni Lucas and Hunt, 1990 is based on an incomplete skull from the lower part of the Tecovas Member of the Dockum Formation near Kalgary, Crosby County, Texas. Its age is late Carnian (Tuvalian), about 225 Ma, based on palynostratigraphy, ostracods, vertebrate biochronology, and sequence stratigraphy. This indicates that Adelobasileus is at least 10 million years older than any previously described mammal. The cranium of Adelobasileus shares a large suite of basicranial apomorphies with Liassic mammals. The distribution of 36 characters among 13 taxa of non-mammalian cynodonts and mammals indicates that Adelobasileus, Sinoconodon, and other mammals form a monophyletic group. Adelobasileus is excluded from the monophyletic taxon consisting of Megazostrodon, Haldanodon, morganucodontids, triconodontids, multituberculates, monotremes, and therians. Partly because of incomplete knowledge of Adelobasileus, it is impossible to resolve the trichotomy of Adelobasileus, Sinoconodon, and other mammals. Some cranial features of Adelobasileus, such as the incipient promontorium housing the cochlea, represent an intermediate stage of the character transformation from non-mammalian cynodonts to Liassic mammals.