55-529: Therapsida is a clade comprising a major group of eupelycosaurian synapsids that includes mammals 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 many reptiles and amphibians . Therapsids evolved from earlier synapsids commonly called " pelycosaurs ", specifically within
110-595: A semi-sprawling gait . The forelimbs were even more robust than the hindlimbs, and the animal is thought to have been a powerful digger that nested in burrows. Lystrosaurus survived the Permian-Triassic extinction , 252 million years ago. In the Early Triassic , they were by far the most common terrestrial vertebrates, accounting for as many as 95% of the total individuals in some fossil beds. Researchers have offered various hypotheses for why Lystrosaurus survived
165-602: A lesser extent in parts of what are now India, China, Mongolia, European Russia, and Antarctica (which was not over the South Pole at the time). Most Lystrosaurus fossils have been found in the Balfour and Katberg Formations of the Karoo basin in South Africa ; these specimens offer the best prospects of identifying species because they are the most numerous and have been studied for
220-406: A lineage of the eucynodont suborder. Biarmosuchia Dinocephalia Anomodontia Gorgonopsia Therocephalia Cynodontia 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
275-399: 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. This suggests that hair may have first evolved in probainognathians, though it does not entirely rule out an earlier origin of fur. Whiskers probably evolved in probainognathian cynodonts. Some studies had inferred an earlier origin for whiskers based on
330-491: A single opening behind the eye. They are distinguished from the Caseasaurian synapsids by having a long, narrow supratemporal bone (instead of one that is as wide as it is long) and a frontal bone with a wider connection to the upper margin of the orbit . The only living descendants of basal eupelycosaurs are the mammals . The group was originally considered a suborder of pelycosaurs or "mammal like reptiles", but it
385-508: A while, 95% of land vertebrates were Lystrosaurus . This is the only time that a single species or genus of land animal dominated the Earth to such a degree. A few other Permian therapsid genera also survived the mass extinction and appear in Triassic rocks—the therocephalians Tetracynodon , Moschorhinus , Ictidosuchoides and Promoschorhynchus —but do not appear to have been abundant in
440-432: Is Edaphosaurus , a large [10–12-foot-long (3.0–3.7 m)] herbivore which had a sail on its back, probably used for thermoregulation and mating. Sphenacodontids , a family of carnivorous eupelycosaurs, included the famous Dimetrodon , which is sometimes mistaken for a dinosaur , and was the largest predator of the period. Like Edaphosaurus , Dimetrodon also had a distinctive sail on its back, and it probably served
495-415: Is λίστρον lístron ‘tool for leveling or smoothing, shovel, spade, hoe’) is an extinct genus of herbivorous dicynodont therapsids from the late Permian and Early Triassic epochs (around 248 million years ago). It lived in what is now Antarctica , India , China , Mongolia , European Russia and South Africa . Four to six species are currently recognized, although from the 1930s to 1970s
550-567: Is a large clade of animals characterized by the unique shape of their skull , encompassing all mammals and their closest extinct relatives. They first appeared 308 million years ago during the Early Pennsylvanian epoch, with the fossils of Echinerpeton and perhaps an even earlier genus, Protoclepsydrops , representing just one of the many stages in the evolution of mammals, in contrast to their earlier amniote ancestors. Eupelycosaurs are synapsids , animals whose skull has
605-473: Is notable for dominating southern Pangaea for millions of years during the Early Triassic . At least one unidentified species of this genus survived the end-Permian mass extinction and, in the absence of predators and herbivorous competitors, went on to thrive and re-radiate into a number of species within the genus, becoming the most common group of terrestrial vertebrates during the Early Triassic; for
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#1732851489103660-474: 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 are docodonts , which are mammaliaforms very closely related to crown-group mammals. Two "mummified" juvenile specimens of the dicynodont Lystrosaurus murrayi preserve skin impressions; the skin is hairless, leathery, and dimpled, somewhat comparable to elephant skin. Fossilized facial skin from
715-476: 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 . Near the end of the Permian, dicynodonts, therocephalians and cynodonts show parallel trends towards loss of
770-607: 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
825-821: The Damodar Valley and the Kamthi Formation of the Pranhita-Godavari Basin in India . Seven Lystrosaurus species have been described from the Early Triassic Jiucaiyuan , Guodikeng and Wutonggou formations of the Bogda Mountains in Xinjiang , China , although it is possible that only two ( L. youngi and L. hedini ) are valid; unusually, no Chinese Lystrosaurus specimens are known below
880-469: The Early Cretaceous . 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. Therapsid legs were positioned more vertically beneath their bodies than were
935-658: The Moscow Basin in Russia. It was probably closely related to the African Lystrosaurus curvatus , which is regarded as one of the least specialized species and has been found in very Late Permian and very Early Triassic sediments. L. murrayi , in addition to two undescribed species presently assigned to L. curvatus and L. declivis , is known from the Early Triassic Panchet Formation of
990-666: The Sphenacodontia , more than 279.5 million years ago. They replaced the pelycosaurs as the dominant large land animals in the Guadalupian 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
1045-527: The Theriodontia . 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
1100-581: The Transantarctic Mountains by Edwin H. Colbert and his team in 1969–1970 helped support the hypothesis of plate tectonics and strengthen the theory, since Lystrosaurus had already been found in the lower Triassic of southern Africa as well as in India and China. Lystrosaurus fossils have been found in many Late Permian and Early Triassic terrestrial bone beds , most abundantly in Africa, and to
1155-612: 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
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#17328514891031210-401: 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
1265-450: The tusk -like upper canines . Dicynodonts are generally thought to have had horny beaks like those of turtles, for shearing off pieces of vegetation, which were then ground on a horny secondary palate when the mouth was shut. The jaw joint was weak and moved backwards and forwards with a shearing action, instead of the more common sideways or up and down movements. It is thought that the jaw muscles were attached unusually far forward on
1320-587: The American Philosophical Society in 1870. Its name is derived from the Ancient Greek words listron , "shovel", and sauros , "lizard". Marsh belatedly purchased the skull in May ;1871, although his interest in an already-described specimen was unclear; he may have wanted to carefully scrutinize Cope's description and illustration. The discovery of Lystrosaurus fossils at Coalsack Bluff in
1375-599: 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 . 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
1430-580: The Early Permian of the United States has been hypothesized to be an even earlier-diverging therapsid, but more recent study has suggested it is more likely to be a non-therapsid sphenacodontian. 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
1485-586: The Karoo from an area in which Late Permian sediments have not been found. L. curvatus is found in a relatively narrow band of sediments from shortly before and after the extinction, and can be used as an approximate marker for the boundary between the Permian and Triassic periods. A skull identified as L. curvatus has been found in late Permian sediments from Zambia . For many years it had been thought that there were no Permian specimens of L. curvatus in
1540-474: The Karoo, which led to suggestions that L. curvatus immigrated from Zambia into the Karoo. However, a re-examination of Permian specimens in the Karoo has identified some as L. curvatus , and there is no need to assume immigration. L. murrayi and L. declivis are found only in Triassic sediments. Lystrosaurus georgi fossils have been found in the Earliest Triassic sediments of
1595-577: The Permian-Triassic boundary in this region. L. curvatus , L. murrayi , and L. maccaigi are known from the Fremouw Formation in the Transantarctic Mountains of Antarctica . Lystrosaurus was a dicynodont therapsid , between 0.6 to 2.5 m (2 to 8 ft) long with an average of about 0.9 m (3 ft) depending upon the species. Unlike other therapsids, dicynodonts had very short snouts and no teeth except for
1650-490: The dinocephalian Estemmenosuchus has been described as showing that the skin was glandular and lacked both scales and hair. Coprolites containing what appear to be hairs have been found from the Late Permian . 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
1705-472: The distinctive Burnetiamorpha , but support for the monophyly of Biarmosuchia is relatively low. Many biarmosuchians are known for extensive cranial ornamentation. Dinocephalia comprises two distinctive groups, the Anteosauria and Tapinocephalia . Historically, carnivorous dinocephalians, including both anteosaurs and titanosuchids, were called titanosuchians and classified as members of Theriodontia, while
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1760-486: The dominant land animals from the latest Carboniferous to the end of the Early Permian epoch. Ophiacodontids were common from their appearance in the late Carboniferous ( Pennsylvanian ) to the early Permian, but they became progressively smaller as the early Permian progressed. The edaphosaurids , along with the caseids , were the dominant herbivores in the early part of the Permian. The most renowned edaphosaurid
1815-438: 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
1870-465: The extinction event and thrived in the early Triassic. Dr. Elias Root Beadle, a Philadelphia missionary and avid fossil collector, discovered the first Lystrosaurus skull. Beadle wrote to the eminent paleontologist Othniel Charles Marsh , but received no reply. Marsh's rival, Edward Drinker Cope , was very interested in seeing the find, and described and named Lystrosaurus in the Proceedings of
1925-471: The first of the cynodonts . 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 ),
1980-797: The first true mammals . All non-therapsid synapsids, including all basal eupelycosaurs, as well as many other life forms, became extinct at the end of Permian period. The following cladogram is modified from Huttenlocker et al . (2021): Caseasauria Varanopidae Ophiacodontidae Edaphosauridae Haptodus Ianthodon Palaeohatteria Pantelosaurus Cutleria Secodontosaurus Cryptovenator Sphenacodon Ctenospondylus Dimetrodon Shashajaia bermani Raranimus Dinocephalia Anomodontia Biarmosuchia Gorgonopsia Therocephalia to Mammaliaformes [REDACTED] Lystrosaurus murrayi Lystrosaurus ( / ˌ l ɪ s t r oʊ ˈ s ɔːr ə s / ; 'shovel lizard'; proper Greek
2035-550: The fossils previously labeled as L. platyceps and L. oviceps as members of L. curvatus . L. maccaigi is the largest and apparently most specialized species, while L. curvatus was the least specialized. A Lystrosaurus -like fossil, Kwazulusaurus shakai , has also been found in South Africa. Although not assigned to the same genus , K. shakai is very similar to L. curvatus . Some paleontologists have therefore proposed that K. shakai
2090-630: The herbivorous Tapinocephalidae were classified as members of Anomodontia. Anomodontia includes the dicynodonts , a clade of tusked, beaked herbivores, and the most diverse and long-lived clade of non-cynodont therapsids. Other members of Anomodontia include Suminia , which is thought to have been a climbing form. Gorgonopsia is an abundant but morphologically homogeneous group of saber-toothed predators . It has been suggested that Therocephalia might not be monophyletic, with some species more closely related to cynodonts than others. However, most studies regard Therocephalia as monophyletic. Cynodonts are
2145-499: The longest time. As so often with fossils, there is debate in the paleontological community as to exactly how many species have been found in the Karoo basin. Studies from the 1930s to 1970s suggested a large number (23 in one case). However, by the 1980s and 1990s, only 6 species were recognized in the Karoo : L. curvatus , L. platyceps , L. oviceps , L. maccaigi , L. murrayi , and L. declivis . A study in 2011 reduced that number to four, treating
2200-430: The mammaliaform Morganucodon suggest that even early mammaliaforms had reptile-like metabolic rates. Evidence for respiratory turbinates, which have been hypothesized to be indicative of endothermy, was reported in the therocephalian Glanosuchus , but subsequent study showed that the apparent attachment sites for turbinates may simply be the result of distortion of the skull. The evolution of integument in therapsids
2255-531: The most diverse and longest-lived of the therapsid groups, as Cynodontia includes mammals . Cynodonts are the only major therapsid clade to lack a Middle Permian fossil record, with the earliest-known cynodont being Charassognathus from the Wuchiapingian age of the Late Permian. Non-mammalian cynodonts include both carnivorous and herbivorous forms. [REDACTED] Eupelycosauria Eupelycosauria
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2310-458: The number of species was thought to be much higher. They ranged in size from that of a small dog to 8 feet (2.5 meters) long. As a dicynodont, Lystrosaurus had only two teeth (a pair of tusk -like canines ), and is thought to have had a horny beak that was used for biting off pieces of vegetation. Lystrosaurus was a heavily built, herbivorous animal. The structure of its shoulders and hip joints suggests that Lystrosaurus moved with
2365-508: The other four clades are controversial. 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
2420-459: 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. In contrast, evidence from histology suggests that endothermy is shared across Therapsida, whereas estimates of blood flow rate and lifespan in
2475-496: The presence of foramina on the snout of therocephalians and early cynodonts, but the arrangement of foramina in these taxa actually closely resembles lizards, which would make the presence of mammal-like whiskers unlikely. Therapsids evolved from a group of pelycosaurs called sphenacodonts . Therapsids became the dominant land animals in the Middle Permian , displacing the pelycosaurs. Therapsida consists of four major clades :
2530-574: The sacral vertebrae are fused to each other and to the pelvis . A buttress above each acetabulum (hip socket) is thought to have prevented dislocation of the femur (thigh bone) while Lystrosaurus was walking with a semi-sprawling gait. The forelimbs of Lystrosaurus were massive, and Lystrosaurus is thought to have been a powerful burrower. Mummified specimens recovered from the Karoo Basin and described in 2022 revealed that Lystrosaurus had dimpled, leathery, and hairless skin. Lystrosaurus
2585-418: The same purpose - regulating heat. The varanopid family passingly resembled today's monitor lizards and may have had the same lifestyle. Therapsids descended from a clade closely related to the sphenacodontids. They became the succeeding dominant land animals for the rest of the Permian, and in the latter part of the Triassic , descendants of the cynodonts , an advanced group of therapsids, gave rise to
2640-512: 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
2695-439: 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. Tetraceratops from
2750-415: The skull and took up a lot of space on the top and back of the skull. As a result, the eyes were set high and well forward on the skull, and the face was short. Features of the skeleton indicate that Lystrosaurus moved with a semi-sprawling gait . The lower rear corner of the scapula (shoulder blade) was strongly ossified (built of strong bone), which suggests that movement of the scapula contributed to
2805-420: 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. The physiology of therapsids
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#17328514891032860-435: The stride length of the forelimbs and reduced the sideways flexing of the body. The five sacral vertebrae were massive but not fused to each other and to the pelvis , making the back more rigid and reducing sideways flexing while the animal was walking. Therapsids with fewer than five sacral vertebrae are thought to have had sprawling limbs, like those of modern lizards. In dinosaurs and mammals , which have erect limbs,
2915-568: 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, and 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 ,
2970-424: Was possibly an ancestor of or closely related to the ancestors of L. curvatus , while L. maccaigi arose from a different lineage. L. maccaigi is found only in sediments from the Permian period, and apparently did not survive the Permian–Triassic extinction event . Its specialized features and sudden appearance in the fossil record without an obvious ancestor may indicate that it immigrated into
3025-439: Was redefined in 1997, and the term pelycosaur itself has fallen into disfavor. We now know that the eupelycosaurs were not in fact reptiles nor of reptile lineage - the modern term stem mammal is used instead. Some recent studies suggested that one of its subgroups, Varanopidae , are really nested within sauropsids , leaving the other defined subgroup of it, Metopophora , as its synonym. Many non-therapsid eupelycosaurs were
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