Therapsid

? Therapsids
Phthinosuchus, an early therapsid
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Superclass: Tetrapoda (unranked) Amniota Class: Synapsida Order: Therapsida Broom, 1905
Suborders
Biarmosuchia Dinocephalia Eotheriodontia Anomodontia Gorgonopsia Therocephalia Cynodontia

Therapsids (order Therapsida), which have been called "mammal-like reptiles," are a group of amniote synapsids that flourished from the Early Permian to the Late Triassic. Amniotes are animals whose embryos are surrounded by an amniotic membrane that encases it in amniotic fluid. Reptiles have traditionally been defined as including all the amniotes except birds and mammals. Synapsids are tetrapods (four-legged vertebrates) that are characterized by a pair of holes in their skulls behind the eyes.

The therapsids were the dominant large terrestrial animals during the latter half of the Permian, but had virtually died out at the end of the Triassic. However, it is the cynodont therapsids that are considered to have given rise to the mammals. Although synapsids have traditionally been referred to as reptiles, when the term is used cladistically the taxon also includes the mammals because of descent through the therapsids.

The order Therapsida is highly diverse and subdivided into six or seven suborders.

Mesozoic era (251 - 65 mya)
Triassic	Jurassic	Cretaceous

Classification

There are different classification schemes that have been developed involving the Order Therapsid. The following is one scenario.

• Series Amniota
  • CLASS SYNAPSIDA
    • Order Pelycosauria
      • Suborder Caseasauria
      • Suborder Eupelycosauria
        • Family Varanopseidae
        • Family Ophiacodontidae
        • Family Edaphosauridae
        • Family Sphenacodontidae
    • Order Therapsida
      • Suborder Biarmosuchia
      • Suborder Dinocephalia
      • Suborder Anomodontia
      • Suborder Gorgonopsia
      • Suborder Therocephalia
      • Suborder Cynodontia
        • Mammaliformes
          • Class Mammalia

or:

• Suborder Cynodontia
  • Family Probainognathidae
    • Superfamily Chiniquodontoidea

CLASS MAMMALIA

Evolutionary history

The first true "reptiles" or amniotes are categorized as anapsids. Anapsids (Anapsida) are vertebrates characterized by solid skulls without 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 in the fossil record. One led to the Anapsida, which did not develop holes in their skulls, and the other to the Diapsida (diapsids), which possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. Diapsids ("two arches") are a group of tetrapod animals that appeared in the fossil record about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and are considered to include all birds, crocodiles, lizards, snakes, and tuataras (and possibly even turtles). While some lost either one hole (lizards), or both holes (snakes), they are still classified as diapsids based on their assumed ancestry.

The earliest solid-skulled amniotes are also considered to have given rise to a separate line, the Synapsida (synapsids). Synapsids have a pair of holes in their skulls behind the eyes (similar to the diapsids); 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.

Archaeothyris and Clepsydrops were the earliest known synapsids. They belonged to a group called pelycosaurs and they 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.

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 latter 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 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 consists of 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 (e.g: Robertia), to large bulky herbivores a ton or more in weight (e.g: 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.

1. The extremely mammal-like family, Tritylodontidae, survived into the Early Cretaceous.
2. An extremely mammal-like family, Tritheledontidae, are unknown later than the Early Jurassic.
3. The third group, Morganucodon and similar animals, were stem-mammals.

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.

References

ISBN links support NWE through referral fees

• Benton, M. J. Vertebrate Paleontology. 3rd ed. Blackwell Science Ltd., 2004. ISBN 0632056371
• Carroll, R. L. Vertebrate Paleontology & Evolution. New York: W.H. Freeman & Company, 1988.
• Kemp, T. S. The Origin and Evolution of Mammals. Oxford University Press, 2005. ISBN 9780198507611
• Romer, A. S. Vertebrate Paleontology, 3rd edition. University of Chicago Press, 1933. ISBN 0226724883

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