Within a few million years, two important amniote lineages became distinct: mammals' synapsid ancestors and the sauropsids, from which lizards, snakes, crocodilians, dinosaurs, and birds are descended.
In the Jurassic, their quadrate and articular bones evolved into the incus and malleus bones in the middle ear. Kermack and his co-authors argued for drawing the line between mammals and earlier synapsids at the point where the mammalian pattern of molar occlusion was being acquired and the dentary-squamosal joint had appeared.
The criterion chosen, they noted, is merely a matter of convenience; their choice was based on the fact that "the lower jaw is the most likely skeletal element of a Mesozoic mammal to be preserved." The first fully terrestrial vertebrates were amniotes — their eggs had internal membranes that allowed the developing embryo to breathe but kept water in.
Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation as these soft tissues are not often preserved in the fossil record.
Most research concerning the evolution of mammals centers on the shapes of the teeth, the hardest parts of the tetrapod body.
Among the other large pelycosaurs were Dimetrodon grandis and Edaphosaurus cruciger.
Therapsids descended from pelycosaurs in the middle Permian and took over their position as the dominant land vertebrates.
The times of origin are difficult to know, because vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been considerably earlier than the first fossil.
The synapsid pelycosaurs included the largest land vertebrates of the Early Permian, such as the 6 m (20 ft) long Cotylorhynchus hancocki.
The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes.