Therapsida

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Therapsids
Temporal range: Early PermianHolocene 275–0 Ma (Range includes mammals)
Inostrancevia alexandri.JPG
Mounted skeleton of Inostrancevia alexandri, a gorgonopsian therapsid
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Sphenacodontoidea
Order: Therapsida
Broom, 1905
Clades

Therapsida is a group of synapsids, and includes mammals and their ancestors.[1][2] Many of the traits today seen as unique to mammals had their origin within early therapsids, including having their four limbs extend vertically beneath the body, as opposed to the sprawling posture of other reptiles. The earliest fossil attributed to Therapsida is Tetraceratops insignis from the Lower Permian.[3][4] Therapsids evolved from pelycosaurs (specifically sphenacodonts) 275 million years ago. They replaced the pelycosaurs as the dominant large land animals in the Middle Permian and were replaced, in turn, by the archosauromorphs in the Triassic, although one group of therapsids, the kannemeyeriiforms, remained diverse in the Late Triassic. The therapsids included the cynodonts, the group that gave rise to mammals in the Late Triassic around 225 million years ago. Of the non-mammalian therapsids, only cynodonts and dicynodonts survived the Triassic–Jurassic extinction event. The last of the non-mammalian therapsids, the tritylodontid cynodonts, became extinct in the Early Cretaceous, approximately 100 million years ago.

Characteristics[edit]

Illustration of Pristerognathus, a cat-sized therocephalian therapsid

While the early therapsids had skulls very similar to those of their pelycosaurian ancestors, morphological difference is exhibited within the post-cranial morphology.

Legs and feet[edit]

Their legs are positioned more vertically beneath their bodies than are the sprawling legs of reptiles and pelycosaurs. The feet were more symmetrical, with the first and last toes short and the middle toes longer, an indication that the foot's axis was placed parallel to that of the animal, not sprawling out sideways. This would have given a more mammal-like gait than the lizard-like gait of the pelycosaurs.[5]

Jaw and teeth[edit]

Therapsids' temporal fenestrae are greater than those of the pelycosaurs. The jaws of some therapsids are more complex and powerful, and the teeth are differentiated into frontal incisors for nipping, great lateral canines for puncturing and tearing, and molars for shearing and chopping food.

Evolutionary history[edit]

Raranimus, a primitive therapsid

Therapsids evolved from a group of pelycosaurs called sphenacodonts.[6][7] Therapsids became the dominant land animals in the Middle Permian, displacing the pelycosaurs. Therapsida consists of four major clades: the dinocephalians, the herbivorous anomodonts, the carnivorous biarmosuchians, and the mostly carnivorous theriodonts. After a brief burst of evolutionary diversity, the dinocephalians died out in the later Middle Permian (Guadalupian) but the anomodont dicynodonts as well as the theriodont gorgonopsians and therocephalians flourished, being joined at the very end of the Permian by the first of the cynodonts.

Like all land animals, the therapsids were seriously affected by the Permian–Triassic extinction event; the very successful gorgonopsians dying out altogether and the remaining groups - dicynodonts, therocephalians, and cynodonts - reduced to a handful of species each by the earliest Triassic. 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 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.

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. Many taxonomists refer to these animals as "mammals", though some 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 out competed by the rapidly diversifying Saurian lineage of diapsids, equipped with sophisticated respiratory systems better suited to the very hot, dry and oxygen-poor world of the End-Triassic.

Dicynodonts were long thought to have become extinct near the end of the Triassic, but there is evidence that they survived into the Cretaceous. Their fossils have been found in Gondwana.[8] This is an example of Lazarus taxon. Other animals that were common in the Triassic also took refuge here, such as the temnospondyls.

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 probainognathians, a lineage of the eucynodont suborder.

Taxonomy[edit]

Classification[edit]

Phylogeny[edit]

Synapsida 

Caseasauria


 Sphenacodontia 

Sphenacodontidae


 Therapsida 

Tetraceratops



 † Biarmosuchia 

Eotitanosuchidae



Phthinosuchidae



 Eutherapsida 
 † Dinocephalia 

Anteosauria



Tapinocephalia



 Neotherapsida 
 † Anomodontia 

Dromasauria



Dicynodontia



 Theriodontia 
 † Gorgonopsia 

Lycaenops



Inostrancevia



 Eutheriodontia 
 † Therocephalia 
 † Eutherocephalia 

Bauria




 Cynodontia 

Mammalia











See also[edit]

References[edit]

  1. ^ Romer, A. S. (1966) [1933]. Vertebrate Paleontology (3rd ed.). University of Chicago Press. 
  2. ^ therapsid (fossil reptile order) - Encyclopedia Britannica
  3. ^ M. Laurin & R. R. Reisz. 1996. The osteology and relationships of Tetraceratops insignis, the oldest known therapsid. Journal of Vertebrate Paleontology 16(1): 95-102.
  4. ^ J. Liu, B. Rubidge & J. Li, New basal synapsid supports Laurasian origin for therapsids, 2009, Acta Palaeontol. Pol., 54 (3): 393-400
  5. ^ Carroll, R. L. (1988). Vertebrate Paleontology and Evolution. New York: W. H. Freeman and Company. p. 698. ISBN 0-7167-1822-7. 
  6. ^ Synapsid Classification & Apomorphies
  7. ^ A. K. Huttenlocker, and E. Rega, 2012. Chapter 4: The Paleobiology and Bone Microstructure of Pelycosauriangrade Synapsids. Pp. 90–119 in A. Chinsamy (ed. [clarification needed]) Forerunners of Mammals: Radiation, Histology, Biology. Indiana University Press.
  8. ^ Thulbord, Tony; Turner, Susan (2003). "The last dicynodont: an Australian Cretaceous relict". Proceedings of the Royal Society of London B 270: 985–993. doi:10.1098/rspb.2002.2296. 

Further reading[edit]

  • Benton, M. J. (2004). Vertebrate Palaeontology, 3rd ed., Blackwell Science.
  • Carroll, R. L. (1988). Vertebrate Paleontology & Evolution. W. H. Freeman & Company, New York.
  • Kemp, T. S. (2005). The origin and evolution of mammals. Oxford University Press.
  • Romer, A. S. (1966). Vertebrate Paleontology. University of Chicago Press, 1933; 3rd ed.
  • Bennett, A. F., & Ruben, J. A. (1986). "The metabolic and thermoregulatory status of therapsids." In The ecology and biology of mammal-like reptiles. Smithsonian Institution Press, Washington, DC, 207-218.

External links[edit]