Following closely on the appearance of the first anapsids was the rise of a group of reptiles that would one day lead to the development of all mammals—the Synapsida. True mammals did not appear until the Late Triassic Period, but evolutionary developments leading to them as a line of animals distinct from reptiles began as far back as the Early Permian.
Synapsids include two groups of animals. The pelycosaurs were large animals; they are first known from Late Carboniferous fossils. By the Late Permian, they were joined by a mixed group of predatory and herbivorous synapsids collectively known as therap-sids. All of the synapsids were quadrupeds—four-footed animals.
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