 The tribosphenic design appears primitively in all groups of mammals.Some paleontologists believe that it developed independently in monotremes (or australosphenidans), rather than being inherited from an ancestor that they share with marsupials and placentals (or boreosphenidans); but this idea has critics and the debate is still going on.For example, the dentition of the Early Cretaceous monotreme "Steropodon" is similar to those of "Peramus" and dryolestoids, which suggests that monotremes are related to some pre-tribosphenic therian mammals, but, on the other hand, the status of neither of these two groups is well-established.