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Therapsida is a clade comprising a major group of that includes and their ancestors and close relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more underneath the body, resulting in a more "standing" quadrupedal posture, as opposed to the lower sprawling posture of many and .
Therapsids evolved from earlier synapsids commonly called "", specifically within the Sphenacodontia, more than 279.5 million years ago. They replaced the pelycosaurs as the dominant large land animals in the Guadalupian through to the Early Triassic. In the aftermath of the Permian–Triassic extinction event, therapsids declined in relative importance to the rapidly diversifying archosaurian (, and , etc.) during the Middle Triassic.
The therapsids include the , the group that gave rise to mammals (Mammaliaformes) in the Late Triassic around 225 million years ago, the only therapsid clade that survived beyond the end of the Triassic. The only other group of therapsids to have survived into the Late Triassic, the , became extinct towards the end of the period. The last surviving group of non-mammaliaform cynodonts were the Tritylodontidae, which became extinct during the Early Cretaceous.
containing what appear to be hairs have been found from the Late Permian. Though the source of these hairs is not known with certainty, they may suggest that hair was present in at least some Permian therapsids.
The closure of the pineal foramen in probainognathian cynodonts may indicate a mutation in the regulatory gene Msx2, which is involved in both the closure of the skull roof and the maintenance of hair follicles in mice. This suggests that hair may have first evolved in probainognathians, though it does not entirely rule out an earlier origin of fur.
Whiskers probably evolved in probainognathian cynodonts. Some studies had inferred an earlier origin for whiskers based on the presence of foramina on the snout of therocephalians and early cynodonts, but the arrangement of foramina in these taxa actually closely resembles lizards, which would make the presence of mammal-like whiskers unlikely.
Some exceptions were the still further derived eucynodonts. At least three groups of them survived. They all appeared in the Late Triassic period. The extremely mammal-like family, Tritylodontidae, survived into the Early Cretaceous. Another extremely mammal-like family, Tritheledontidae, are unknown later than the Early Jurassic. Mammaliaformes was the third group, including Morganucodon and similar animals. Some taxonomists refer to these animals as "mammals", though most limit the term to the mammalian crown group.
The non-eucynodont cynodonts survived the Permian–Triassic extinction; Thrinaxodon, Galesaurus and Platycraniellus are known from the Early Triassic. By the Middle Triassic, however, only the eucynodonts remained.
The therocephalians, relatives of the cynodonts, managed to survive the Permian–Triassic extinction and continued to diversify through the Early Triassic period. Approaching the end of the period, however, the therocephalians were in decline to eventual extinction, likely outcompeted by the rapidly diversifying Saurian lineage of , equipped with sophisticated respiratory systems better suited to the very hot, dry and oxygen-poor world of the End-Triassic.
Dicynodonts were among the most successful groups of therapsids during the Late Permian; they survived through to near the end of the Triassic.
Mammals are the only living therapsids. The mammalian crown group, which evolved in the Early Jurassic period, radiated from a group of mammaliaforms that included the docodonts. The mammaliaforms themselves evolved from , a lineage of the eucynodont suborder.
In addition to the six major groups, there are several other lineages and species of uncertain classification. Raranimus from the early Middle Permian of China is likely to be the earliest-diverging known therapsid. Tetraceratops from the Early Permian of the United States has been hypothesized to be an even earlier-diverging therapsid, but more recent study has suggested it is more likely to be a non-therapsid sphenacodontian.
Historically, carnivorous dinocephalians, including both anteosaurs and titanosuchids, were called titanosuchians and classified as members of Theriodontia, while the herbivorous Tapinocephalidae were classified as members of Anomodontia.
Evolutionary history
Therapsids evolution from a group of pelycosaurs called Sphenacodontia. Synapsid Classification & Apomorphies (2025). 9780253005335, Indiana University Press. ISBN 9780253005335 Therapsids became the dominant land animals in the Middle Permian, displacing the pelycosaurs. Therapsida consists of four major clades: the , the herbivorous , the carnivorous , and the mostly carnivorous . After a brief burst of evolutionary diversity, the dinocephalians died out in the later Middle Permian (Guadalupian) but the anomodont as well as the theriodont and therocephalians flourished, being joined at the very end of the Permian by the first of the .
Like all land animals, the therapsids were seriously affected by the Permian–Triassic extinction event, with the very successful gorgonopsians and the biarmosuchians dying out altogether and the remaining groups—dicynodonts, therocephalians and cynodonts—reduced to a handful of species each by the earliest Triassic. Surviving dicynodonts were represented by two families of disaster taxa (Lystrosauridae and Myosauridae), the scarcely known Kombuisia, and a single group of large stocky , the Kannemeyeriiformes, which were the only dicynodont lineage to thrive during the Triassic. They and the medium-sized cynodonts (including both carnivorous and herbivorous forms) flourished worldwide throughout the Early and Middle Triassic. They disappear from the fossil record across much of Pangea at the end of the Carnian (Late Triassic), although they continued for some time longer in the wet equatorial band and the south.
Classification
Six major groups of therapsids are generally recognized: Biarmosuchia, Dinocephalia, Anomodontia, Gorgonopsia, Therocephalia and Cynodontia. A clade uniting therocephalians and cynodonts, called Eutheriodontia, is well supported, but relationships among the other four clades are controversial. The most widely accepted hypothesis of therapsid relationships, the Hopson and Barghausen paradigm, was first proposed in 1986. Under this hypothesis, biarmosuchians are the earliest-diverging major therapsid group, with the other five groups forming the Eutherapsida, and within Eutherapsida, gorgonopsians are the sister taxon of eutheriodonts, together forming the Theriodontia. Hopson and Barghausen did not initially come to a conclusion about how dinocephalians, anomodonts and theriodonts were related to each other, but subsequent studies suggested that anomodonts and theriodonts should be classified together as the Neotherapsida. However, there remains debate over these relationships; in particular, some studies have suggested that anomodonts, not gorgonopsians, are the sister taxon of Eutheriodontia, other studies have found dinocephalians and anomodonts to form a clade, and both the phylogenetic position and monophyly of Biarmosuchia remain controversial.
Biarmosuchia
Biarmosuchia is the most recently recognized therapsid clade, first recognized as a distinct lineage by Hopson and Barghausen in 1986 and formally named by Sigogneau-Russell in 1989. Most biarmosuchians were previously classified as gorgonopsians. Biarmosuchia includes the distinctive Burnetiamorpha, but support for the monophyly of Biarmosuchia is relatively low. Many biarmosuchians are known for extensive cranial ornamentation.
Dinocephalia
Dinocephalia comprises two distinctive groups, the Anteosauria and Tapinocephalia.
Anomodontia
Anomodontia includes the dicynodonts, a clade of tusked, beaked herbivores, and the most diverse and long-lived clade of non-cynodont therapsids. Other members of Anomodontia include Suminia, which is thought to have been a climbing form.
Gorgonopsia
Gorgonopsia is an abundant but morphologically homogeneous group of saber-toothed predators.
Therocephalia
It has been suggested that Therocephalia might not be monophyletic, with some species more closely related to cynodonts than others. However, most studies regard Therocephalia as monophyletic.
Cynodontia
Cynodonts are the most diverse and longest-lived of the therapsid groups, as Cynodontia includes mammals. Cynodonts are the only major therapsid clade to lack a Middle Permian fossil record, with the earliest-known cynodont being Charassognathus from the Wuchiapingian age of the Late Permian. Non-mammalian cynodonts include both carnivorous and herbivorous forms.
See also
Notes
Further reading
(2025). 9789400768406 ISBN 9789400768406
External links
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