Hi,i want to ask a question again,but i really dont know how to describe it so i apologize if you guys have a difficulty understanding my question.so here is the question.sometimes bird also regarded as reptiles if we look from their clade,not their class,aren't they?then,howcome we dont regard mammal as a reptile too base on their ancestor since modern mammals are the descendant of proto-mammals from pelycosaur and therapsid group,aren't they?isn't there a clade that include mammals and reptile inside like the avemetarsalia clade that include bird and reptiles(dinousaur in their case)?thanks and sorryfor the bad english

Your question is right in principle -- yes, we do say that birds are reptiles. But none of the ancestor groups of mammals are descended from reptiles. The broader clade that includes mammals is Synapsida, which includes the pelycosaurs -- but Synapsida is the sister clade to Sauropsida, the great clade that contains all reptiles, including dinosaurs and birds. (These are the two branches of Amniota, the clade that unites reptiles and mammals to the exclusion of amphibians).

The situation is made more complex for two reasons. First, basal synapsids, including pelycosaurs, used be known informally as "mammal-like reptiles". This was never correct; "reptile-like mammal" would have been a better term.

Second, the most primite amniotes -- whether in the synapsid or sauropsid or branch -- looked superficially like lizards. So it's easy to see why someone would informally talk about mammals having evolved from "reptiles", meaning the lizard-like animals at the base of Synapsida. But that's not really right: only animals on the sauropsid branch are technically reptiles.