Phthinosuchus, an early therapsid
Therapsids (order Therapsida, class synapsid), are "mammal-like reptiles" that flourished from the Early Permian to the Late Triassic periods (c. 275 - 205 million years ago) and are thought to have been the precursors of mammals. Aside from the mammals, all the other lines of descent from the therapsid ancestors have become extinct. Therapids share with other orders of the class Synapsida the identity of being tetrapods (four-legged vertebrates) characterized by a pair of holes (one on each side) in their skulls behind the eye sockets.
The therapsids were the dominant large terrestrial animals during the later half of the Permian period. Although most of the therapsids went extinct at the end of the Triassic period, the cynodont ("dog-teeth") line of therapsids survived that mass extinction event and produced descendants thought to have become the multitude of mammalian life on Earth today. The order Therapsida is highly diverse and subdivided into the one extant suborder, Cynodontia, and five or six extinct suborders.
|Paleozoic era (542 - 251 mya)|
|Mesozoic era (251 - 65 mya)|
Although synapsids have traditionally been referred to as reptiles, when the term is used cladistically the taxon of synapsida also includes the mammals because of their descent through the therapsids. Different classification schemes have been developed for the Order Therapsid. The following is one scenario.
Being members of the class Synapsida, the therapsids are Amniotes, animals whose embryos are surrounded by an amniotic membrane that encases the embryo in amniotic fluid. Reptiles have traditionally been defined as including all the amniotes except birds and mammals.
The first true "reptiles" or amniotes are categorized as Anapsids (Anapsida), which are vertebrates characterized by solid skulls without extra openings near the temples, but with holes in the skull only for nose, eyes, spinal cord, and so forth. Turtles are believed by some to be surviving anapsids.
Shortly after the appearance in the fossil record of the first reptiles, two branches appeared. One continued to develop as the Anapsida, and further along gave rise to a separate line, the Synapsida (synapsids), which have a single pair of holes in their skulls (one on each side) behind the eyes; this feature has the advantage of lightening the skull and increasing the space for jaw muscles. As noted above, the synapsids are considered to have eventually evolved into mammals. The term "mammal-like reptiles" is most commonly used to describe the group Therapsida, although it can be also used more broadly to describe non-mammalian Synapsids.
The original second branch descended from the first reptiles became the Diapsida (diapsids), which possessed a two pairs of additional holes in their skulls (one of each pair on each side of the skull) roughly behind the eye holes. Appearing in the fossil record about 300 million years ago during the late Carboniferous period, the diapsids have dispersed geographically and morphologically unto today giving rise, it is thought to birds, crocodiles, lizards, snakes, and tuataras (and possibly even turtles).
Archaeothyris and Clepsydrops were the earliest known synapsids. They belonged to a group called pelycosaurs and lived in the Carboniferous Period. The pelycosaurs were the first successful group of amniotes, spreading and diversifying until they became the dominant large terrestrial animals in the latest Carboniferous and Early Permian periods (c. 309-289 million years ago).
The therapsids, a more advanced group of synapsids, appeared during the first half of the Permian and went on to become the dominant large terrestrial animals during the later half.
Therapsids' evolutionary track began in the Early Permian when a group of pelycosaurs, the Sphenacodontia, a lineage that gave rise to Dimetrodon and its family, is considered to have given rise to therapsids. The evidence was their anatomical features such as the skull, and the vertebrae. Therapsid temporal fenestrae (the paired extra holes in their skulls behind the eyes) were larger than those of the pelycosaurs. Endothermy (warm-bloodedness) in therapsids is speculated to have arisen by the Middle or Late Permian (Dinocephalians and anomodonts were probably warm-blooded). Therapsids probably had naked skin, like that of mammals, rather than scales as in reptiles and pelycosaurs. Early therapsids did not have fur, which developed in the Middle or Late Permian, in the theriodonts. Therapsids became the dominant land animals by the Middle Permian, replacing the pelycosaurs.
Therapsida comprises three major clades: the dinocephalians, the herbivorous anomodonts, and the mostly carnivorous theriodonts, with the carnivorous biarmosuchians considered as a paraphyletic assemblage of primitive forms. After a brief burst of evolutionary diversity, the dinocephalians died out in the later Middle Permian, but the anomodont dicynodonts and the theriodont gorgonopsians and therocephalians flourished, being joined at the very end of the Permian by the first Cynodonts. The Cynodonts were the most mammal-like of the therapsids.
Therapsids were by far the most diverse and abundant large animals of the Middle and Late Permian, including a diverse range of herbivores and carnivores, ranging from small animals the size of a rat (for example, Robertia), to large bulky herbivores a ton or more in weight (for example, Moschops).
After flourishing for many millions of years, these successful animals were all but wiped out by the Permian-Triassic mass extinction about 250 million years ago, the largest extinction in Earth's history. According to the fossil record, only a few therapsids and no pelycosaurs, survived the Permian extinction and went on to be successful in the new early Triassic landscape. The very successful gorgonopsians died out altogether and the remaining groups were represented by only one or two families of a few species, each surviving into the Triassic. Of these, the dicynodonts, now represented by a single family of large stocky herbivores, the Kannemeyeridae, and the medium-sized cynodonts (including both carnivorous and herbivorous forms), flourished worldwide, throughout the Early and Middle Triassic. They died out across much of Pangea at the end of the Late Triassic, although they continued for some time longer in the wet equatorial band and the south.
Some exceptions were the still further derived eucynodonts. At least three groups of them survived.
Dicynodonts are thought to have become extinct before the end of the Triassic, but there is evidence that they survived the extinction. Their fossils have been found in Gondwana. Other animals that were common in the Triassic also took refuge here, such as the Temnospondyls.
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