Arguably the oldest and most primitive known mammal is Adelobasileus, based on the back half of a skull from the
Late Triassic (late Carnian, about 225 Ma) of Texas (Lucas and Luo, 1993). This unique fossil predates the next oldest mammals by at least 10 million years, and shares with later mammals several derived features, including configuration of certain cranial foramina, morphology of the occipital condyles, and presence of a bony floor of the cavum epi-ptericum. The incipient development of a promontorium to house the cochlea is anatomically intermediate between the conditions in cynodonts and early mammals. Unfortunately, the fossil lacks teeth, a lower jaw, and other parts of the skeleton that might corroborate its mammalian status.
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