''Zhangheotherium'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of "

symmetrodont Symmetrodonta is a group of Mesozoic mammals and mammal-like synapsids characterized by the triangular aspect of the molars when viewed from above, and the absence of a well-developed talonid. The traditional group of 'symmetrodonts' ranges in ...

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

from the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. A single species is known, ''Zhangheotherium quinquecuspidens'' from Jianshangou Beds of the

Yixian Formation The Yixian Formation (; formerly Romanization of Chinese, transcribed as Yihsien Formation or Yixiang Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the Barremian stage of the Early Cretaceous. I ...

. ''Zhangheotherium'' was the first "symmetrodont" known from a nearly complete skeleton, expanding knowledge of the group beyond isolated teeth and jaws. The genus name honors Zhang He, who collected the

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

fossil from

Liaoning Province ) , image_skyline = , image_alt = , image_caption = Clockwise: Mukden Palace in Shenyang, Xinghai Square in Dalian, Dalian coast, Yalu River at Dandong , image_map = Liaoning in China (+all claims hatched).svg , ...

prior to its 1997 description. The specific name is

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

for "five-cusped teeth". "Symmetrodonts" and other archaic mammals such as

multituberculate Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, a ...

s and

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

s are still being debated on their taxonomical relationships. ''Zhangheotherium'' provided insight into the evolution of "symmetrodonts", revealing a combination of traits similar to modern

therians Theria ( or ; ) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes and various extinct mamm ...

(such as placentals and

marsupials Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a ...

) as well as more "primitive" mammalians. Most likely, "symmetrodonts" are a grade of

stem-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

therians, with representatives incrementally closer to true therians than to monotremes.

Description

Zhangheotherium-Paleozoological Museum of China

The promontorium of the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

is straight and slender, suggesting that the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...

is uncoiled, in contrast to living mammals. The chest of ''Zhangheotherium'' retains a small V-shaped

interclavicle An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals ( marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In ...

, which links the

clavicles The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavicle is the ...

(collarbones) to the

manubrium The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, human lung, lungs, and ma ...

(upper component of the

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

or breastbone). Monotremes possess a large hourglass-shaped interclavicle while in therians the bone shrinks and fuses with the manubrium during embryonic development. In ''Zhangheotherium'', the clavicles have flexible connections to their neighboring bones, allowing for a greater potential for movement in the

scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...

(shoulder blade). On the other hand, the

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

(upper arm bone) is more reptilian in form, with a small

greater tubercle The greater tubercle of the humerus is the outward part the upper end of that bone, adjacent to the large rounded prominence of the humerus head. It provides attachment points for the supraspinatus, infraspinatus, and teres minor muscles, th ...

and underdeveloped trochlea. As a result, ''Zhangheotherium'' probably walked in a sprawling manner, like monotremes and some other Early Cretaceous mammals such as '' Jeholodens'' and ''

Repenomamus ''Repenomamus'' (Latin: "reptile" (reptilis), "mammal" (mammalis)) is a genus of opossum- to badger-sized Gobiconodontidae, gobiconodontid mammal containing two species, ''Repenomamus robustus'' and ''Repenomamus giganticus''. Both species are kn ...

''. Like other "symmetrodonts", each molar tooth of ''Zhangheotherium'' consists chiefly of three large pointed cusps set into an acute triangular arrangement. In ''Zhangheotherium'', each molar is supplemented with a pair of minor cusps, bringing the number of cusps up to five. All of the molar cusps are characteristically robust and conical, and lack cristae (ridges bridging between cusps). In addition, the cingulae (horizontal crests at the base of the crown) are underdeveloped in the upper molars and absent in the lower molars. By emphasizing strong interlocking cusps over shearing crests and ridges, the teeth of ''Zhangheotherium'' were more specialized than other "symmetrodonts" for the purpose of crushing and grinding food. Nevertheless, ''Zhangheotherium'' lacks the grinding

talonid The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone tooth ...

"heel" of true therian molars.

Paleoecology

''Zhangheotherium'' is one of several extinct mammals reported to bear a spur-like bone in the ankle. This may have had

venomous Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a sti ...

capabilities, similar to the spur seen in males of the modern

platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or monotypi ...

. A 2015 study argued that ''Zhangheotherium'' led a possibly scansorial lifestyle, possessing long hindlimbs and a large plantar area on the foot, both optimal for climbing. A probable tyrannosauroid specimen GMV 2124 (also known as NGMC 2124), previously attributed to ''

Sinosauropteryx ''Sinosauropteryx'' (meaning "Chinese reptilian wing") is an extinct genus of coelurosaurian theropod dinosaurs. Described in 1996, it was the first dinosaur taxon outside of Avialae (birds and their immediate relatives) to be found with eviden ...

'', contained two jaws of ''Zhangheotherium'' in its stomach region. Thus, it seems to have preyed on this primitive mammal, possibly on a regular basis.

References

* {{Taxonbar, from=Q15298265 Symmetrodonta Early Cretaceous mammals of Asia Fossil taxa described in 1997 Prehistoric mammal genera