71-529: 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
142-909: A cusp on the inner half of the upper molar grinds into a basin on the front half of the lower molar, like a mortar-and-pestle . This is a case of convergent evolution with the tribosphenic teeth of therian mammals. There is much uncertainty for how docodont teeth developed from their simpler ancestors. Their closest relatives may have been certain Triassic " symmetrodonts ", namely Woutersia , and Delsatia . The shuotheriids , another group of Jurassic mammaliaforms, also shared some dental characteristics with docodonts. One study has suggested that shuotheriids are closely related to docodonts, though others consider shuotheriids to be true mammals, perhaps related to monotremes . For much of their history of study, docodont fossils were represented by isolated teeth and jaws. The first docodont known from decent remains
213-411: A fairly consistent cusp pattern. A distinct concavity or basin is apparent in the front half of each lower molar, between cusps a, g, and b. This basin has been named the pseudotalonid. When the upper and lower teeth occlude (fit together), the pseudotalonid acts as a receptacle for cusp Y of the upper molar. Cusp Y is often termed the "pseudoprotocone" in this relationship. At the same time, cusp b of
284-406: 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
355-399: 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
426-535: A new clade, Docodontiformes, to encompass the two groups. Docodonts and other Mesozoic mammals were traditionally thought to have been primarily ground dwelling and insectivorous , but recent more complete fossils from China have shown this is not the case. Castorocauda from the Middle Jurassic of China, and possibly Haldanodon from the Upper Jurassic of Portugal, were specialised for
497-487: A pointed spur on its ankle, similar to defensive structures observed in male monotremes and several other early-branching mammals. Like other mammaliaforms, docodont teeth include peg-like incisors , fang-like canines , and numerous interlocking premolars and molars . Most mammaliaforms have fairly simple molars primarily suited for shearing and slicing food. Docodonts, on the other hand, have developed specialized molars with crushing surfaces. The shape of each molar
568-420: A saddle-shaped hyoid apparatus , and reduced postdentary jaw bones which are beginning to develop into middle ear ossicles . On the other hand, the postdentary bones are still attached to the jaw and skull, the nares (bony nostril rims) have yet to fuse, and in most species the spine's thoracic - lumbar transition is rather subdued. Docodonts have a long and low mandible (lower jaw), formed primarily by
639-506: A semi-aquatic lifestyle. Castorocauda had a flattened tail, similar to that of a beaver , and recurved molar cusps, which suggests a possible diet of fish or aquatic invertebrates. It was thought possible that docodonts had tendencies towards semi-aquatic habits, given their presence in wetland environments, although this could also be explained by the ease with which these environments preserve fossils compared with more terrestrial ones. Recent discoveries of other complete docodontans such as
710-497: 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
781-432: A slower metabolism and lower growth rates relative to modern mammals of the same size. The juvenile, which was 7 to 24 months old at the time of its death, was only 49% through the process of replacing its deciduous teeth with permanent (adult) teeth . Based on jaw length, the juvenile was 51-59% the weight of the 7-year-old adult. The closest comparisons among modern mammals were monotremes and hyraxes , though Krusatodon
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#1732772519894852-432: 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
923-576: 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
994-616: Is an order of extinct Mesozoic mammaliaforms (advanced cynodonts closely related to true crown-group mammals ). They were among the most common mammaliaforms of their time, persisting from the Middle Jurassic to the Early Cretaceous across the continent of Laurasia (modern-day North America, Europe, and Asia). They are distinguished from other early mammaliaforms by their relatively complex molar teeth. Docodont teeth have been described as "pseudotribosphenic":
1065-409: Is defined by a characteristic pattern of conical cusps , with sharp, concave crests connecting the center of each cusp to adjacent cusps. When seen from below, the upper molars have an overall subtriangular or figure-eight shape, wider (from side to side) than they are long (from front to back). The bulk of the tooth makes up four major cusps: cusps A, C, X, and Y. This overall structure is similar to
1136-475: Is not certain, and in recent analyses, Gondtherium falls outside the docodont family tree, albeit as a close relative to the group. Reigitherium , from the Late Cretaceous of Argentina , has previously been described as a docodont, though it is now considered a meridiolestidan mammal. Some authors have suggested splitting Docodonta into two families (Simpsonodontidae and Tegotheriidae), but
1207-474: 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
1278-481: 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
1349-405: Is positioned lingual to cusp B. The lower molars are longer than wide. On average, they have seven cusps arranged in two rows. The labial/outer row has the largest cusp, cusp a, which lies between two more cusps. The other major labial cusps are cusp b (a slightly smaller cusp in front of cusp a) and cusp d (a much smaller cusp behind cusp a). The lingual/inner row is shifted backwards (relative to
1420-472: Is potentially another case of convergent evolution, as shuotheriid are often considered true mammals related to modern monotremes . Docodont and shuotheriid teeth are so similar that some genera, namely Itatodon and Paritatodon , have been considered members of either group. A 2024 study, describing the new shuotheriid Feredocodon , even proposed that shuotheriids and docodonts were most closely related to each other among mammaliaforms. The study named
1491-539: Is prominent in Woutersia . Another proposed docodont relative, Tikitherium from India , was originally considered to have been a very early mammaliaform which lived during the Carnian stage of the Triassic. Later investigation found that Tikitherium was likely a misidentification of Neogene shrew teeth, completely unrelated to docodonts or any Mesozoic mammaliaforms. Unambiguous docodonts are restricted to
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#17327725198941562-399: Is variable between docodonts, as with many other mammaliaforms. The components of the atlas are unfused, attaching to the large and porous occipital condyles of the braincase. Vertebrae at the base of the tail often have expanded transverse processes (rib pedestals), supporting powerful tail musculature. Most docodonts have gradually shrinking ribs, forming a subdued transition between
1633-573: 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
1704-411: 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
1775-667: The Northern Hemisphere , abruptly appearing in the fossil record in the Middle Jurassic . Very few docodonts survived into the Cretaceous Period ; the youngest known members of the group are Sibirotherium and Khorotherium , from the Early Cretaceous of Siberia . One disputed docodont, Gondtherium , has been described from India, which was previously part of the Southern Hemisphere continent of Gondwana . However, this identification
1846-750: 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
1917-540: 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
1988-631: 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
2059-401: 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
2130-414: The nares (bony nostril holes) are small and paired, rather than fused into a single opening, and the rear edge of each naris is formed by a large septomaxilla , a bone which is no longer present in mammals. The nasal bones expand at the back and overlook thick lacrimals . The frontal and parietal bones of the skull roof are flat and broad, and there is no postorbital process forming the rear rim of
2201-472: The orbit (eye socket). Docodonts also see the first occurrence of a mammalian-style saddle-shaped complex of hyoids (throat bones). Microdocodon has a straight, sideways-oriented basihyal which connects to two pairs of bony structures: the anterior hyoid cornu (a jointed series of rods which snake up to the braincase), and the posterior thyrohyals (which link to the thyroid cartilage ). This hyoid system affords greater strength and flexibility than
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2272-405: The thoracic and lumbar regions of the spine. However, this developmental trait is not universal. For example, Agilodocodon lacks lumbar ribs, so it has an abrupt transition from the thoracic to lumbar vertebrae like many modern mammals. The forelimbs and hindlimbs generally have strong muscle attachments, and the olecranon process of the ulna is flexed inwards. All limb bones except
2343-431: The tibia lack epiphyses , plate-like ossified cartilage caps which terminate bone growth in adulthood. This suggests that docodont bones continued growing throughout their lifetime, like some other mammaliaforms and early mammals. The ankle is distinctive, with a downturned calcaneum and a stout astragalus which connects to the tibia via a trochlea (pulley-like joint). The only known specimen of Castorocauda has
2414-466: The tribosphenic teeth found in true therian mammals, like modern marsupials and placentals . However, there is little consensus for homologizing docodont cusps with those of modern mammals. Cusps A and C lie in a row along the labial edge of the tooth (i.e., on the outer side, facing the cheek). Cusp A is located in front of cusp C and is typically the largest cusp in the upper molars. Cusp X lies lingual to cusp A (i.e., positioned inwards, towards
2485-431: The 1880s, but their relationships and diversity have only recently been well-established. Monographs by George Gaylord Simpson in the 1920s argued that they were specialized " pantotheres ", part of a broad group ancestral to true therian mammals according to their complex molars. A 1956 paper by Bryan Patterson instead argued that docodont teeth were impossible to homologize with modern mammals. He drew comparisons to
2556-676: 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
2627-594: 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
2698-481: The closest relatives of Docodonta have been identified as certain Late Triassic "symmetrodonts", such as Delsatia and Woutersia (from the Norian - Rhaetian of France ). These "symmetrodonts" have three major cusps (c, a, and b) set in a triangular arrangement on their lower molars. These cusps would be homologous to cusps c, a, and g in docodonts, which have a similar size and position. The lingual cusp (cusp X)
2769-402: The dentary. The ectotympanic bone, also known as the angular , fits into a deep slot on the dentary which opens backwards, a characteristic unique to docodonts. The malleus (also known as the articular ) sends down a particularly well-developed prong known as the manubrium, which is sensitive to vibrations. The incus (also known as the quadrate ) is still relatively large and rests against
2840-496: 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
2911-478: 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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2982-493: 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
3053-438: 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
3124-471: 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 ),
3195-712: 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] Docodonts See text. Docodonta
3266-630: 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
3337-428: The labial row) and has two large cusps: cusp g (at the front) and cusp c (at the back). Two additional lingual cusps may be present: cusp e and cusp df. Cusp e lies in front of cusp g and is roughly lingual to cusp b. Cusp df (“docodont cuspule f”) lies behind cusp c and is lingual to cusp d. There is some variation in the relative sizes, position, or even presence of some of these cusps, though docodonts in general have
3408-504: The lower molar shears into an area labial to cusp Y. Occlusion is completed when the rest of the upper molar slides between adjacent lower molar teeth, letting the rear edge of the preceding lower molar scrape against cusp X. This shearing-and-grinding process is more specialized than in any other early mammaliaform. "Pseudotalonid" and "pseudoprotocone" are names which reference the talonid -and- protocone crushing complex which characterize tribosphenic teeth . Tribosphenic teeth show up in
3479-430: 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
3550-432: The midline of the skull). A distinct wear facet is found on the labial edge of cusp X, extending along the crest leading to cusp A. Cusp Y, a unique feature of docodonts, is positioned directly behind cusp X. Many docodonts have one or two additional cusps (cusps B and E) in front of cusp A. Cusp B is almost always present and is usually shifted slightly labial relative to cusp A. Cusp E, which may be absent in later docodonts,
3621-544: 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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#17327725198943692-561: The oldest fossils of therians , the mammalian subgroup containing marsupials and placentals . This is a case of convergent evolution , as therian talonids lie at the back of the lower molar rather than the front. The opposite is true for docodont teeth, which have been described as "pseudotribosphenic". Pseudotribosphenic teeth are also found in shuotheriids , an unusual collection of Jurassic mammals with tall pointed cusps. Relative to docodonts, shuotheriids have pseudotalonids which are positioned further forwards in their lower molars. This
3763-450: The origin of living mammals: monotremes , marsupials , and placentals . In other words, docodonts are outside of the mammalian crown group , which only includes animals descended from the last common ancestor of living mammals. Previously, docodonts were sometimes regarded as belonging to Mammalia , owing to the complexity of their molars and the fact that they possess a dentary-squamosal jaw joint . However, modern authors usually limit
3834-518: 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
3905-426: The petrosal bone of the braincase, a remnant of a pre-mammalian style jaw joint. In true mammals, the postdentary elements detach fully and shrink further, becoming the ossicles of the middle ear and embracing a circular eardrum . Docodont skulls are generally fairly low, and in general form are similar to other early mammaliforms such as morganucodonts . The snout is long and has several plesiomorphic traits:
3976-459: 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
4047-503: 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 :
4118-492: The same data is put through a Bayesian analysis . Cladogram based on a phylogenetic analysis of Zhou et al. (2019) focusing on a wide range of mammaliamorphs: † Morganucodonta Haldanodon Docofossor Castorocauda Agilodocodon Microdocodon † Hadrocodium † Haramiyida Yinotheria (including Monotremata ) † Fruitafossor Theriimorpha (including Metatheria and Eutheria ) Docodont fossils have been recognized since
4189-418: 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
4260-524: 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
4331-409: The simple, U-shaped hyoids of earlier cynodonts . It allows for a narrower and more muscular throat and tongue, which are correlated with uniquely mammalian behaviors such as suckling . The oldest unambiguous fossil evidence of hair is found in a well-preserved specimen of the docodont Castorocauda , though hair likely evolved much earlier in synapsids . The structure of the vertebral column
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#17327725198944402-439: 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
4473-399: The specialised digging species Docofossor , and specialised tree-dweller Agilodocodon suggest Docodonta were more ecologically diverse than previously suspected. Docofossor shows many of the same physical traits as the modern day golden mole , such as wide, shortened digits in the hands for digging. A 2024 study on adult and juvenile Krusatodon specimens found that docodonts had
4544-420: 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
4615-481: The teeth of Morganucodon and other " triconodont " mammaliaforms, which had fairly simple lower molars with a straight row of large cusps. However, re-evaluations of mammaliaform tooth homology in the late 1990s established that docodonts were not closely related to either morganucodonts or therians. Instead, they were found to be similar to certain early " symmetrodonts ", a broad and polyphyletic grouping of mammaliaforms with triangular upper molars. In particular,
4686-590: The term "Mammalia" to the crown group, excluding earlier mammaliaforms like the docodonts. Nevertheless, docodonts are still closely related to crown-Mammalia, to a greater extent than many other early mammaliaform groups such as Morganucodonta and Sinoconodon . Some authors also consider docodonts to lie crownward of the order Haramiyida , though most others consider haramiyidans to be closer to mammals than docodonts are. Docodonts may lie crownward of haramiyidans in phylogenetic analyses based on maximum parsimony , but shift stemward relative to haramiyidans when
4757-494: The tooth-bearing dentary bone. The dentary connects to the cranium via a joint with the squamosal , a connection which is strengthened relative to earlier mammaliaforms. The other bones in the jaw, known as postdentary elements, are still connected to the dentary and lie within a groove (the postdentary trough ) in the rear part of the dentary's inner edge. Nevertheless, they are very slender, hosting hooked prongs which start to converge towards an oval-shaped area immediately behind
4828-581: 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 ,
4899-638: Was Haldanodon , from the Late Jurassic Guimarota site of Portugal . Recently, exceptionally preserved skeletons have been discovered in the Jurassic Tiaojishan Formation of China . Chinese docodonts include otter -like, mole -like, and squirrel -like species, hinting at impressive ecological diversity within the group. Many docodonts have muscular limbs and broad tail vertebrae, adaptations for burrowing or swimming. Like true mammals, docodonts have hair,
4970-535: Was much smaller than either, at fewer than 156 g in adult body mass. The authors propose that the standard condition in modern small mammals (very high metabolism, rapid growth, and short lifespan) would not be adopted until true mammals in the Jurassic mammals. In addition, docodonts contradict earlier assumptions that high metabolism evolved in sync with ecological diversity, since their diversity far outpaces their metabolism. The lineage of Docodonta evolved prior to
5041-439: 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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