Mammals: A polyphyletic origin
The environmental conditions that stimulated such changes may have produced similar adaptations in more than one group of animals. It is likely that mammalian traits were acquired by several separate reptilian groups. It was originally hypothesized that the line of reptiles from which the platypus and echidna stemmed was not necessarily the same as that which was to give rise to other mammals. In other words mammals had a polyphyletic origin (derived from more than one ancestor) rather than a monophyletic origin (derived from a single ancestor). Recent evidence based on the skull morphology of Probainognathus is argued for monophyletic origin for the mammals. Much of this debate depends on whether the advanced theriodonts were reptiles or represented the first mammals. What is certain is that monotremes diverged from the main mammalian line during the Triassic, whereas the other major division in the mammals, namely differential of placental and marsupial forms only occurred during the late Cretaceous period.