Two fragmentary mammalian petrosals were collected in 1968 and 1969 from the Upper Jurassic Morrison Formation, Bone Cabin Draw, Wyoming, and from the Early Cretaceous Kelvin Formation near Coalville, Utah. The Morrison petrosal (AMNH 104830) came from a mammal with a petrosal as primitive as that of the morganucodonts. This specimen could belong to one of the Morrison tricondonts, a docodont, a very primitive therian, or one of the morganucodonts. The Kelvin petrosal (AMNH 95184) is extremely fragmentary, and its affinities within the Mammalia are uncertain. (No other mammals are known from this locality.)--Modified journal abstract.