Therapsida is a major group of eupelycosaurian synapsids. It includes mammals and their ancestors and close relatives. Many defining traits of modern mammals had their origins within early therapsids. Changes in skeletal structure brought their limbs underneath their body mass which resulted in a more "standing" quadrupedal posture, as opposed to the lower sprawling posture of many reptiles and amphibians. Therapsid teeth began to differentiate into canines, incisors, and molars. Their necks also gained more flexibility allowing therapsids greater range of motion in moving their heads.

Therapsids split from their closest relatives within the "pelycosaurs", the carnivorous Sphenacodontidae (including the famous Dimetrodon) during the Late Carboniferous, prior to 299 million years ago. Their early evolutionary history during the Cisuralian (Early Permian), is obscure, and they lack confirmed fossil records during this time period leaving a substantial ghost lineage. They replaced the pelycosaurs as the dominant large land animals in the Guadalupian (Middle Permian) through to the Early Triassic. In the aftermath of the Permian–Triassic extinction event, therapsids declined in relative importance to the rapidly diversifying archosaurian sauropsids (pseudosuchians, dinosaurs and pterosaurs, etc.) during the Middle Triassic.

The therapsids include the cynodonts, 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 dicynodonts, 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.

Characteristics

thumb|left|Illustration of [[Alopecognathus, an early therocephalian therapsid]]

Jaw and teeth

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

Posture

Therapsid legs were positioned more vertically beneath their bodies than were the sprawling legs of reptiles and pelycosaurs. Also compared to these groups, the feet were more symmetrical, with the first and last toes short and the middle toes long, an indication that the foot's axis was placed parallel to that of the animal, not sprawling out sideways. This orientation would have given a more mammal-like gait than the lizard-like gait of the pelycosaurs.

Physiology

The physiology of non-mammalian therapsids is poorly understood. Most Permian therapsids had a pineal foramen, indicating that they had a parietal eye like many modern reptiles and amphibians. The parietal eye serves an important role in thermoregulation and the circadian rhythm of ectotherms, but is absent in modern mammals, which are endothermic. Fossilized facial skin from the dinocephalian Estemmenosuchus has been described as showing that the skin was glandular and lacked both scales and hair.

Evolutionary history

thumb|left|200 px|Holotype skull of [[Raranimus dashankouensis, the most basal-known therapsid]]

Therapsids are members of the clade Sphenacodontia, with their closest relatives within the primitive "pelycosaur" synapsids being Sphenacodontidae, including the famous Dimetrodon. The early evolutionary history of the group is obscure, with no confirmed records from the Early Permian (though Tetraceratops has been controversially suggested by some authors to be more closely related to therapsids than to sphenacodontids) leaving a substantial ghost lineage. Therapsids became the dominant land animals in the Middle Permian, displacing the pelycosaurs, though the nature of this transition is obscure due to a poor fossil record at the Early-Middle Permian transition ("Olson's Gap").]]

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 herbivores, 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.

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.

thumb|200 px|Reconstruction of [[Bonacynodon schultzi, a probainognathian cynodont related to the ancestors of mammals]]

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 diapsids, 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 probainognathians, a lineage of the eucynodont suborder.

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.

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,

Further reading

  • 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.
  • Ross, R.P., Ross, C.A. (2023). Permian Period, geochronology. Encyclopedia Britannica. <nowiki>https://www.britannica.com/science/Permian-Period</nowiki>
  • Padian, Kevin (2013-09). "A Review of "Forerunners of Mammals: Radiation, Histology, Biology"". Journal of Vertebrate Paleontology. 33 (5): 1250–1251.
  • "Therapsida: Mammals and extinct relatives" Tree of Life
  • "Therapsida: overview" Palaeos