Mammals evolved from synapsids, a group of non-amphibian tetrapods, branching off from reptiles in the Carboniferous period. Pelycosaurs were the first terrestrial amniotes of large size, evolving into therapsids, which evolved into true mammals over 70 million years. After the extinction of dinosaurs, mammals diversified and became the dominant land vertebrates.
Mammals represent the last evolutionary stage of one of the two main groups of amniotes (non-amphibian tetrapods), the synapsids. Non-mammalian synapsids, often called therapsids (although mammals are also technically therapsids) branched off from the other main group, the sauropsids (reptiles), in the Carboniferous period. The earliest known synapsid was Archaeothyris, a small lizard-like creature that lived 320 million years ago. Like reptiles, synapsids evolved from Carboniferous amphibians.
Synapsids would then have developed into pelycosaurs, which would have been the dominant land vertebrate for about 40 million years, until the mid-Permian period, about 275 million years ago. Pelycosaurs are famous for being the first terrestrial amniotes of large size (1 meter to 3 meters or more). They had large tailsails, made with extended vertebral spikes, which are thought to have been used for regulating body temperature and for mating displays. Pelycosaurs, despite being synapsids, superficially resembled reptiles. This group was the most abundant land animal of the Early Permian.
Most pelycosaurs became extinct or evolved into other groups by the end of the Permian. One group, the sphenacodontids, evolved into the next major synapsid group, the therapsids. Therapsids, once called “mammal-like reptiles,” look proto-mammals to the casual eye. This group evolved at the beginning of the Permian and remained for about 150 million years, at the beginning of the Cretaceous. This overlaps the age of the dinosaurs by about 100 million years. Before the emergence of the dinosaurs, during the Late Permian, therapsids were the most numerous and successful land vertebrates. Unfortunately for the group, they were all but wiped out during the Permian-Triassic extinction event 251 million years ago.
In a gradual process that lasted 70 million years, from the mid-Permian to the mid-Jurassic, a group of therapsids, the cynodonts, evolved into true mammals. Information on early mammals of the Mesozoic Era is scarce, as most of these creatures were smaller than mice and lived in contexts where fossilization was difficult. True mammals are known to have emerged 125 million years ago, about 40 million years after the “mammaliaformes”, mammal-like therapsids. One of the earliest mammals was Hadrocodium, although whether this species was a true mammal is debated. The first monotremes, placentals and marsupials – the three main types of mammals – all appeared around the same time.
After the extinction of the dinosaurs at the end of the Cretaceous (65 million years ago) mammals rapidly diversified and occupied the niches for large and medium-sized animals vacated by extinct species. Today, mammals are the dominant land vertebrates, and reptiles crawl underfoot. Of course, the opposite was true during the Mesozoic Era.
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