Therapsids were dominant during the Permian period and eventually evolved into mammals. They emerged during the Early Permian and are considered part of the first wave of amniotic diversity. There were at least three major clades of therapsids, including the ancestors of all modern mammals. Most were wiped out during the end-Permian extinction, but re-emerged as mammals after the extinction of non-avian dinosaurs.
Therapsids (meaning “beast face”) are a class of synapsids, dominant animals on Earth during the Middle to Late Permian period (about 300-251 million years ago). Synapsids themselves are one of two major groups of amniotes (egg-laying animals), the other being the sauropsids, or true reptiles. The non-therapsid synapsids, pelycosaurs, were actually more dominant during the Permian, but the therapsids are significant because they eventually evolved into mammals, survived the Cretaceous-Tertiary extinction that killed off the dinosaurs, and became the dominant group of terrestrial vertebrates, culminating in the evolution of Homo sapiens.
Animals of this class emerged during the Early Permian and are considered to be part of the first wave of amniotic diversity. Some of them, such as cynodonts (“dog’s teeth”), look remarkably like modern-day animals, including differentiated teeth, a swollen braincase, and upright walking (not bipedal but upright, characteristic of reptiles). The main difference is that all therapsids lay eggs. Historically, they were called “mammal-like reptiles,” but are now recognized as distinct from reptiles and are instead considered basal mammals. These animals are a very confusing area of vertebrate evolution and far less popular than dinosaurs or crown-group mammals, but necessary to understand the full picture of the evolution of life on Earth.
There were at least three major clades of therapsids: the dinocephali (“dinosaur heads”), the least advanced therapsids; the anomodont herbivores (beaked herbivores), which were the most diverse and numerous of the Permian herbivores; and the mostly carnivorous therodonts, the most mammal-like of the group, and the ancestors of all modern mammals. Another group, the carnivorous biarmosuchians, are either a paraphyletic clade – made up of several unrelated groups – or made up of a basal offshoot of other therapsids. As you can see, the situation is very confusing.
During the end-Permian extinction 251 million years ago, most of the therapsids were wiped out, along with 95% of all other animal species. For most of the next 190 million years or so, during the Mesozoic period, they would be lost in the shadow of the dinosaurs, the dominant life form on Earth. Only after all non-avian dinosaurs were killed did they re-emerge as mammals.
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