the beginning of the fall of oxygen, which probably began in earnest some 260 million years ago, in the middle and late part of the Permian period, the diapsids did little in the way of diversification or specialization. They remained small and lizard-like. They gave no indication that they would be the ancestors of the largest land animals ever to appear on Earth, in the form of the Mesozoic dinosaurs. If the time of highest oxygen stimulated insects to their greatest size, the same cannot be said of the diapsids.
The most pressing question is whether or not this group was warm-blooded.
The lineage that ultimately gave rise to turtles was very successful during the late Pennsylvanian but less so into the Permian. Anapsids did evolve into giant forms, including cotylosaurs and the even larger pareiosaurs. These were armored giants, surely slow moving and herbivores that lived right until the end of the Permian. Other anapsids were small and more lizard-like. It is very likely that the gigantic size of the earlier Permian anapsids was allowed by high oxygen. All modern anapsids use ectothermy; they are cold-blooded. Presumably the ancient forms used this system as well, but that is still controversial.
The third group of amniotes from this time, the synapsids, or mammal-like reptiles, are known in their most primitive form from Pennsylvanian rocks, and these ancestors of mammals had a lizard-like small shape and mode of life in all probability. It is assumed that these early synapsids were cold-blooded. They in turn gave rise to two great and largely temporally successive stocks, the pelycosaurs (or finbacks, like the early Permian Dimetrodon) and their successors, the therapsids (the lineage giving rise to mammals). It is this latter group that is also called the mammal-like reptiles.
Unlike the diapsids, the synapsids diversified during the oxygen high and at its peak became the largest of all land vertebrates. In the