''Taoheodon'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typic ...

therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...

from the Sunjiagou Formation in the

Shanxi province Shanxi (; ; formerly romanised as Shansi) is a landlocked province of the People's Republic of China and is part of the North China region. The capital and largest city of the province is Taiyuan, while its next most populated prefecture-level ...

of China, dated to the

Wuchiapingian In the geologic timescale, the Wuchiapingian or Wujiapingian (from in the Liangshan area of Hanzhong, Shaanxi Province) is an age or stage of the Permian. It is also the lower or earlier of two subdivisions of the Lopingian Epoch or Series. Th ...

age Age or AGE may refer to: Time and its effects * Age, the amount of time someone or something has been alive or has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

of the

Late Permian Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...

. Its type and only known species is ''T. baizhijuni''. ''Taoheodon'' was a close relative of the well known ''

Dicynodon ''Dicynodon'' ("two dog-teeth") is a genus of dicynodont therapsid that flourished during the Upper Permian period. Like all dicynodonts, it was herbivorous animal. This reptile was toothless, except for prominent tusks, hence the name. It proba ...

'', and may represent a biogeographical link between the

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...

and similar dicynodonts found in Laos.

Description

''Taoheodon'' was a medium-sized dicynodont (basal

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

length over long) currently only known from an incomplete skull and lower jaw. Based on the anatomy of other closely related dicynodonts such as ''Dicynodon'', ''Taoheodon'' was probably a squat, sprawling quadruped with a short tail and a proportionally large head. Like other dicynodonts, ''Taoheodon'' was almost entirely toothless, sporting only a pair of

tusks Tusks are elongated, continuously growing front teeth that protrude well beyond the mouth of certain mammal species. They are most commonly canine teeth, as with pigs and walruses, or, in the case of elephants, elongated incisors. Tusks share ...

and a

tortoise Tortoises () are reptiles of the family Testudinidae of the order Testudines (Latin: ''tortoise''). Like other turtles, tortoises have a shell to protect from predation and other threats. The shell in tortoises is generally hard, and like oth ...

-like

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for eating, preening, manipulating objects, killing prey, fighting, probing for foo ...

Skull

Like other dicynodonts, ''Taoheodon'' had a short skull with large

temporal fenestra An infratemporal fenestra, also called the lateral temporal fenestra or simply temporal fenestra, is an opening in the skull behind the orbit in some animals. It is ventrally bordered by a zygomatic arch. An opening in front of the eye sockets ...

at the back, large

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...

s and a short snout, which in ''Taoheodon'' was proportionately short even for a dicynodont. Its skull is slightly longer than wide, with elongated temporal fenestra. The external

nostril A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbi ...

is rounded and not especially large for a dicynodont, but the area of bone behind and beneath it is hollowed out and concave compared to the rest of the snout. The nasal bones along the roof of the snout are relatively flat, but are nonetheless rugosely textured and bore a single weakly developed boss of tough skin or

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...

on top of the snout. Likewise, the lacrimal and

prefrontal bone The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern a ...

form a distinct boss that bulges out to the side in front of each eye. By comparison, the

postorbital bar The postorbital bar (or postorbital bone) is a bony arched structure that connects the frontal bone of the skull to the zygomatic arch, which runs laterally around the eye socket. It is a trait that only occurs in mammalian taxa, such as most strep ...

behind the eyes is smooth and unornamented. The caniniform process housing the tusk is directed downwards from the snout, and sits entirely in front of the eyes. The

pineal foramen A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...

(or "third eye") on the roof of the skull is large and positioned relatively far back.

Lower jaw

The

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

of ''Taoheodon'' is mostly known from part of the

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

, with portions of the angular,

surangular The suprangular or surangular is a jaw bone found in most land vertebrates, except mammals. Usually in the back of the jaw, on the upper edge, it is connected to all other jaw bones: dentary, angular, splenial and articular. It is often a mu ...

and

splenial The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side (closest to the tongue) between the angular and surangular The suprangular or surangular is a jaw bone found in most land ver ...

bones. The dentary is large and robust, with a rough, pitted surface texture at its front and along the top surface, corresponding to the horny beak typical of dicynodonts. The tip of the lower jaw is missing, so the exact shape of the beak is unknown; however, a low and wide curved ridge defines a clear edge between the side and front faces of the beak. Like other dicynodonts, the angular supports a prominent reflected lamina, which may have supported the eardrum in non- mammalian

s. In ''Taoheodon'', the reflected lamina is large and rounded, facing down and back from the

mandibular fenestra The skull is a bone protective Cranial cavity, cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible ...

History of discovery

The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

and only known specimen of ''Taoheodon'', IVPP V 25335, was discovered in the valley of a tributary of the Tao He river, running through the lower part of the Sunjiagou Formation. The Sunjiagou Formation has been dated to the Late Permian (

Lopingian The Lopingian is the uppermost series/last epoch of the Permian. It is the last epoch of the Paleozoic. The Lopingian was preceded by the Guadalupian and followed by the Early Triassic. The Lopingian is often synonymous with the informal terms l ...

epoch), although the exact age of the lower beds has been debated; either representing the late Wuchiapingian age or early

Changhsingian In the geologic time scale, the Changhsingian or Changxingian is the latest age or uppermost stage of the Permian. It is also the upper or latest of two subdivisions of the Lopingian Epoch or Series. The Changhsingian lasted from to 251.902 ...

. The lower Sunjiagou Formation is composed of grey to greenish-grey

mudstones Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' ...

and fine grained

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s, although the fossil of ''Taoheodon'' itself was found contained within an eroded rock nodule. This erosion resulted in the loss of the

zygomatic arch In anatomy, the zygomatic arch, or cheek bone, is a part of the skull formed by the zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the opening of the ear) and the temporal process of the zygo ...

es and the tip of the snout from the specimen which had been exposed prior to collection, and the specimen has also been slightly compressed from top to bottom during fossilisation. The specimen was described in 2020 by Jun Liu as a new genus and species, ''Taoheodon baizhijuni''. ''Taoheodon'' was named for the nearby Tao He river where it was discovered, combined with the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

for 'tooth', a common suffix in dicynodont generic names. The specific name is in honour of the fossil collector Bai Zhijun who discovered the specimen.

Classification

''Taoheodon'' was a member of the dicynodont

infraorder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

Dicynodontoidea Dicynodontoidea is an infraorder of dicynodont therapsids that includes the famous dicynodont ''Dicynodon'', ''Lystrosaurus'' and the Triassic Kannemeyeriiformes, as well as numerous other closely related species. The name was coined by American ...

, and is distinguished from all other dicynodonts by three unique

autapomorphies In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to t ...

: the top of postorbital bars behind the eyes have a shallow depression ( fossa) where they meet the rest of the skull, the basisphenoid (a part on the underside of the braincase) slopes anterodorsally at a shallow angle in the basisphenoid-

basioccipital The basilar part of the occipital bone (also basioccipital) extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline. In the young skull this area is rough and uneven, and is joined t ...

tubera, and unlike other dicynodonts it lacks a keel on the

pterygoid bone The pterygoid is a paired bone forming part of the palate of many vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fis ...

s of the palate. ''Taoheodon'' was included in an updated

phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

of dicynodonts using the combined datasets of Olivier ''et al.'' (2019) and Kammerer (2019). Within Dicynodontoidea, ''Taoheodon'' was found to group within a clade containing ''Dicynodon'' and very similar

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

that Liu identified as the "core-''Dicynodon''" clade. The

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

produced by Liu (2020), simplified and focused on the relationships of dicynodontoids, is shown below: The results of the analysis are almost identical to the cladograms produced from the previous studies, however, the position of the two Laotian dicynodonts ''

Counillonia ''Counillonia'' is an extinct genus of dicynodont therapsid from the area of Luang Prabang in Laos, Southeast Asia that lived at around the time of the Permian-Triassic boundary and possibly dates to the earliest Early Triassic. Its type and ...

'' and ''

Repelinosaurus ''Repelinosaurus'' is an extinct genus of dicynodont from the Purple Claystone Formation of Luang Prabang in Laos, Southeast Asia that lived at around the time of the Permian-Triassic boundary and possibly dates to the earliest Early Triassic. I ...

'' differs from their original descriptions. The two Laotian genera were found to clade together with ''Taoheodon'' in the "core-''Dicynodon''" clade, contrasting with the analysis of Olivier and colleagues which originally found ''Repelinosaurus'' to be the basalmost

kannemeyeriiform Kannemeyeriiformes is a group of large-bodied Triassic dicynodonts. As a clade, Kannemeyeriiformes has been defined to include the species ''Kannemeyeria simocephalus'' and all dicynodonts more closely related to it than to the species '' Lystro ...

. Liu found ''Taoheodon'' and the Laotian dicynodonts to share a number of features, including notably short snouts, pineal foramens placed further back on the roof of the skull, anteriorly inclined

occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...

s, a fairly straight suture between the nasals and frontals, lacking the

postfrontal The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, t ...

bones, fairly flat

postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ...

s in the temporal area, and a large fossa on the ventral surface of the intertemporal bar.

Palaeobiogeography

The close relationship of ''Taoheodon'' to the Laotian dicynodonts suggests that there was a direct link between the North China Block,

South China Block The Yangtze Plate, also called the South China Block or the South China Subplate, comprises the bulk of southern China. It is separated on the east from the Okinawa Plate by a rift that forms the Okinawa Trough which is a back-arc basin, on the so ...

and

Indochina Block The Sunda Plate is a minor tectonic plate straddling the Equator in the Eastern Hemisphere on which the majority of Southeast Asia is located. The Sunda Plate was formerly considered a part of the Eurasian Plate, but the GPS measurements have ...

that created a corridor of land for dicynodonts in Northern China to disperse into Laos on the Indochina Block and speciate. This paleobiogeographic inference has implications for the timing of the collisions between these landmasses, which although uncertainly dated, have typically been inferred to have occurred later during the

Triassic period The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

. The presence of a clade of closely related dicynodonts between these landmasses suggests that they were connected in some way by the end of the Permian. Furthermore, their position within a "core-''Dicynodon''" clade indicates that ''Taoheodon'' was part of a lineage of dicynodonts that could freely migrate from North China through

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...

to South Africa.

Palaeoecology

In the lower part of the Sunjiagou Formation, ''Taoheodon'' coexisted with the diminutive pareiasaurian

parareptile Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near th ...

'' Sanchuansaurus'' (a possible a relative of the better known ''

Elginia ''Elginia'' is an extinct genus of pareiasaurid known from the Late Permian of Scotland and China. It was named for the area around Elgin in Scotland, which has yielded many fossils referred to as the Elgin Reptiles. Discovery The type sp ...

''), a probable large cryptodont dicynodont, and the late surviving semi-aquatic predatory embolomere '' Seroherpeton''. The lower part of the Sunjiagou Formation is largely composed of fluvial and

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

sediments with infrequent

coal seam Coal mining is the process of extracting coal from the ground. Coal is valued for its energy content and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron fro ...

s that indicate a wet, swampy environment with abundant lake-shore habitats that preserve vertical burrows and rich, intensive bioturbation by aquatic organisms. Plant remains include '' Ningxiaites specialis'', a partially

deciduous In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, ...

conifer with an

evergreen In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, whic ...

canopy that would shed only some of its leaves annually.

References

{{Taxonbar, from=Q97580103 Dicynodonts Anomodont genera Lopingian synapsids of Asia Lopingian genus extinctions Paleontology in Shanxi Fossil taxa described in 2020