Adelobasileus cromptoni is a species of an extinct genus of proto-mammals from the Late Triassic (Carnian), about 225 million years ago. It is poorly represented in the fossil record and is known only from a partial skull recovered from the Tecovas formation in western Texas.
Adelobasileus predates the most advanced non-mammalian cynodonts (tritylodonta and trithelodonta) and all other known Mammaliaformes by 10 million years. In fact, distinct cranial features, especially the housing of the cochlea, suggest that Adelobasileus is a transitional form in the character transformation from cynodonts to Triassic mammals. For this reason, it is thought to be the common ancestor of all modern mammals or a close relative of the common ancestor.