| Therapsida | |
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| From top to bottom and left to right, several examples of non-mammalian therapsids:Biarmosuchus (Biarmosuchia),Moschops (Dinocephalia),Lystrosaurus (Anomodontia),Inostrancevia (Gorgonopsia), alycosuchid (Therocephalia) andChiniquodon (Cynodontia) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Sphenacodontia |
| Clade: | Pantherapsida |
| Clade: | Sphenacodontoidea |
| Clade: | Therapsida Broom, 1905[1] |
| Clades | |
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Therapsida[a] is a clade comprising a major group ofeupelycosauriansynapsids that includesmammals 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 manyreptiles andamphibians.
Therapsids evolved from earlier synapsids commonly called "pelycosaurs", specifically within theSphenacodontia, more than 279.5 million years ago. They replaced the pelycosaurs as the dominant large land animals in theGuadalupian through to the Early Triassic. In the aftermath of thePermian–Triassic extinction event, therapsids declined in relative importance to the rapidly diversifyingarchosauriansauropsids (pseudosuchians,dinosaurs andpterosaurs, etc.) during the Middle Triassic.
The therapsids include thecynodonts, 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 theTriassic. The only other group of therapsids to have survived into theLate Triassic, thedicynodonts, became extinct towards the end of the period. The last surviving group of non-mammaliaform cynodonts were theTritylodontidae, which became extinct during theEarly Cretaceous.
Therapsids'temporal fenestrae were larger than those of the pelycosaurs. The jaws of some therapsids were more complex and powerful, and theteeth were differentiated into frontalincisors for nipping, great lateralcanines for puncturing and tearing, andmolars for shearing and chopping food.
Therapsid legs were positioned more vertically beneath their bodies than were the sprawling legs ofreptiles 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'saxis was placed parallel to that of the animal, not sprawling out sideways. This orientation would have given a moremammal-like gait than thelizard-like gait of the pelycosaurs.[2]
The physiology of therapsids is poorly understood. Most Permian therapsids had a pineal foramen, indicating that they had aparietal eye like many modern reptiles and amphibians. The parietal eye serves an important role in thermoregulation and thecircadian rhythm of ectotherms, but is absent in modern mammals, which areendothermic.[3] Near the end of the Permian, dicynodonts,therocephalians and cynodonts showparallel trends towards loss of the pineal foramen, and the foramen is completely absent in probainognathian cynodonts. Evidence from oxygen isotopes, which are correlated with body temperature, suggests that most Permian therapsids were ectotherms and that endothermy evolved convergently in dicynodonts and cynodonts near the end of the Permian.[4] In contrast, evidence from histology suggests that endothermy is shared across Therapsida,[5] whereas estimates of blood flow rate and lifespan in the mammaliaformMorganucodon suggest that even early mammaliaforms had reptile-like metabolic rates.[6] Evidence for respiratory turbinates, which have been hypothesized to be indicative of endothermy, was reported in the therocephalianGlanosuchus, but subsequent study showed that the apparent attachment sites for turbinates may simply be the result of distortion of the skull.[7]
The evolution of integument in therapsids is poorly known, and there are few fossils that provide direct evidence for the presence or absence of fur. The most basal synapsids with unambiguous direct evidence of fur aredocodonts, which are mammaliaforms very closely related to crown-group mammals. Two "mummified" juvenile specimens of the dicynodontLystrosaurus murrayi preserve skin impressions; the skin is hairless, leathery, and dimpled, somewhat comparable to elephant skin.[8][9] Fossilized facial skin from the dinocephalianEstemmenosuchus has been described as showing that the skin was glandular and lacked both scales and hair.[10]
Coprolites containing what appear to be hairs have been found from theLate Permian.[11][12] 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.[13] This suggests that hair may have first evolved in probainognathians, though it does not entirely rule out an earlier origin of fur.[13]
Whiskers probably evolved in probainognathian cynodonts.[13][14] 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,[15] which would make the presence of mammal-like whiskers unlikely.[14]
Therapsidsevolved from a group of pelycosaurs calledsphenacodonts.[17][18] Therapsids became the dominant land animals in the MiddlePermian, displacing the pelycosaurs. Therapsida consists of four majorclades: thedinocephalians, the herbivorousanomodonts, the carnivorousbiarmosuchians, and the mostly carnivoroustheriodonts. After a brief burst of evolutionary diversity, the dinocephalians died out in the later Middle Permian (Guadalupian) but the anomodontdicynodonts as well as the theriodontgorgonopsians andtherocephalians flourished, being joined at the very end of the Permian by the first of thecynodonts.
Like all land animals, the therapsids were seriously affected by thePermian–Triassic extinction event, with the very successful gorgonopsians and the biarmosuchians dying out altogether and the remaining groups—dicynodonts,therocephalians andcynodonts—reduced to a handful of species each by the earliestTriassic. Surviving dicynodonts were represented by two families ofdisaster taxa (Lystrosauridae andMyosauridae), the scarcely knownKombuisia, and a single group of large stockyherbivores, theKannemeyeriiformes, which were the only dicynodont lineage to thrive during the Triassic.[20] 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 ofPangea at the end of theCarnian (Late Triassic), although they continued for some time longer in the wet equatorial band and the south.[citation needed]
Some exceptions were the still further derivedeucynodonts.[21] At least three groups of them survived. They all appeared in theLate Triassic period. The extremelymammal-like family,Tritylodontidae, survived into the EarlyCretaceous. Another extremely mammal-like family,Tritheledontidae, are unknown later than the EarlyJurassic.Mammaliaformes was the third group, includingMorganucodon and similar animals. Some taxonomists refer to these animals as "mammals", though most limit the term to the mammaliancrown group.
The non-eucynodont cynodonts survived the Permian–Triassic extinction;Thrinaxodon,Galesaurus andPlatycraniellus are known from theEarly Triassic. By theMiddle Triassic, however, only the eucynodonts remained.
Thetherocephalians, 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 ofdiapsids, 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 mammaliancrown group, which evolved in theEarly Jurassic period, radiated from a group of mammaliaforms that included thedocodonts. The mammaliaforms themselves evolved fromprobainognathians, a lineage of the eucynodont suborder.
| The Hopson and Barghausen paradigm for therapsid relationships |
Six major groups of therapsids are generally recognized:Biarmosuchia,Dinocephalia,Anomodontia,Gorgonopsia,Therocephalia andCynodontia. A clade uniting therocephalians and cynodonts, calledEutheriodontia, is well supported, but relationships among the other four clades are controversial.[23] 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 theTheriodontia. 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.
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.[24]Tetraceratops from the Early Permian of the United States has been hypothesized to be an even earlier-diverging therapsid,[25][26] but more recent study has suggested it is more likely to be a non-therapsid sphenacodontian.[27]
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 distinctiveBurnetiamorpha, but support for the monophyly of Biarmosuchia is relatively low. Many biarmosuchians are known for extensive cranial ornamentation.
Dinocephalia comprises two distinctive groups, theAnteosauria andTapinocephalia.
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.
Anomodontia includes thedicynodonts, a clade of tusked, beaked herbivores, and the most diverse and long-lived clade of non-cynodont therapsids. Other members of Anomodontia includeSuminia, which is thought to have been a climbing form.
Gorgonopsia is an abundant but morphologically homogeneous group ofsaber-toothed predators.
It has been suggested that Therocephalia might not be monophyletic, with some species more closely related to cynodonts than others.[28] However, most studies regard Therocephalia as monophyletic.
Cynodonts are the most diverse and longest-lived of the therapsid groups, as Cynodontia includesmammals. Cynodonts are the only major therapsid clade to lack a Middle Permian fossil record, with the earliest-known cynodont beingCharassognathus from theWuchiapingian age of the Late Permian. Non-mammalian cynodonts include both carnivorous and herbivorous forms.
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