Abstract: A skull of Sinoconodon youngi (Triconodonta, Mammalia) is scanned with a Toshiba TCT 80A CT scanner. On the computed tomography of the mesosagittal plane of the skull, an image between the braincase cavity and the nasal cavity indicates the existence of a cribriform place in the earliest mammals. The presence of a cribriform plate in living placenta Is and the marsupials and absence in monotremes may imply that mammals are diphyletic and the origin of cribriform plate took place within the Mammalia. But the conclusion came from the living mammals only, and has not been supported by palaeontological evidence. Its appearance in one of the oldest and the most primitive mammals, Sinoconodon, possibly suggests that the cribriform plate may occur earlier, i.e. in the time of the transition from the advanced mammal-like reptiles, Cynodontia, to the true mammals, and that the distribution of the cribriform plate in the Theria and the Atheria contradicts with the diphyletic of mammals.