Zhangheotherium

Last updated

Zhangheotherium
Temporal range: Early Cretaceous, 125  Ma
O
S
D
C
P
T
J
K
Pg
N
Zhangheotherium quinquecuspidens.jpg
Fossil specimen, Hong Kong Science Museum
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Symmetrodonta
Family: Zhangheotheriidae
Genus: Zhangheotherium
Hu, Y.Q. Wang, Luo & C.K. Li, 1997
Type species
Zhangheotherium quinquecuspidens
Hu, Wang, Luo & Li, 1997

Zhangheotherium is an extinct genus of "symmetrodont" mammal from the Early Cretaceous of China. A single species is known, Zhangheotherium quinquecuspidens from Jianshangou Beds of the Yixian Formation. 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 fossil from Liaoning Province prior to its 1997 description. The specific name is Latin for "five-cusped teeth". [1]

Contents

"Symmetrodonts" and other archaic mammals such as multituberculates and monotremes 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 (such as placentals and marsupials) as well as more "primitive" mammalians. Most likely, "symmetrodonts" are a grade of stem-group therians, with representatives incrementally closer to true therians than to monotremes. [1]

Description

Holotype fossil displayed at the Paleozoological Museum of China Zhangheotherium-Paleozoological Museum of China.jpg
Holotype fossil displayed at the Paleozoological Museum of China

The promontorium of the inner ear is straight and slender, suggesting that the cochlea is uncoiled, in contrast to living mammals. The chest of Zhangheotherium retains a small V-shaped interclavicle, which links the clavicles (collarbones) to the manubrium (upper component of the sternum 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 (shoulder blade). On the other hand, the humerus (upper arm bone) is more reptilian in form, with a small greater tubercle and underdeveloped trochlea. [1] As a result, Zhangheotherium probably walked in a sprawling manner, [1] like monotremes and some other Early Cretaceous mammals such as Jeholodens and Repenomamus .

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 "heel" of true therian molars. [1]

Paleoecology

Reconstructed skeleton Zhangheotherium quinquecuspidens 3.JPG
Reconstructed skeleton

Zhangheotherium is one of several extinct mammals reported to bear a spur-like bone in the ankle. This may have had venomous capabilities, similar to the spur seen in males of the modern platypus. [2] 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. [3] Specimen GMV 2124 of the feathered dinosaur Sinosauropteryx? sp. contained two jaws of Zhangheotherium in its stomach region. Thus, it seems to have preyed on this primitive mammal, possibly on a regular basis. [2]

Related Research Articles

<span class="mw-page-title-main">Molar (tooth)</span> Large tooth at the back of the mouth

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", from mola, millstone and dens, tooth. Molars show a great deal of diversity in size and shape across mammal groups. The third molar of humans is sometimes vestigial.

<i>Catopsbaatar</i> Extinct species of mammal

Catopsbaatar is a genus of multituberculate, an extinct order of rodent-like mammals. It lived in what is now Mongolia during the late Campanian age of the Late Cretaceous epoch, about 72 million years ago. The first fossils were collected in the early 1970s, and the animal was named as a new species of the genus Djadochtatherium in 1974, D. catopsaloides. The specific name refers to the animal's similarity to the genus Catopsalis. The species was moved to the genus Catopsalis in 1979, and received its own genus in 1994. Five skulls, one molar, and one skeleton with a skull are known; the last is the genus' most complete specimen. Catopsbaatar was a member of the family Djadochtatheriidae.

<span class="mw-page-title-main">Prototheria</span> Subclass of mammalia

Prototheria is an obsolete subclass of mammals which includes the living Monotremata and to which variety of extinct groups, including Morganucodonta, Docodonta, Triconodonta and Multituberculata have also been assigned. It is today no longer considered a valid grouping, but a paraphyletic evolutionary grade of basal mammals and mammaliaform cynodonts.

<i>Maotherium</i> Extinct family of mammals

Maotherium is a genus extinct symmetrodont mammal that was discovered in Early Cretaceous rocks in Liaoning Province, China, in 2003. Its scientific name directly translates to "fur beast", in reference to the impressions of fur around the fossil. Maotherium belongs to an extinct group of Mesozoic mammals called symmetrodonts. Though little is known about this group, the symmetrodonts have several similarities - specifically their teeth. They have tall pointed, but simple molars in a triangular arrangement. Originally symmetrodonts were known since the 1920s. Now a vast majority have been restored, such as Zhangheotherium and Akidolestes, during the early 21st century. One of the fossils of Maotherium preserved the imprints of fur, like the mammals Eomaia and Sinodelphys.

<span class="mw-page-title-main">Symmetrodonta</span> Extinct order of mammals

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 age from the latest Triassic to the Late Cretaceous, but most research in the last 20-30 years has concluded that they are not a true taxonomic group, but include several unrelated branches of the mammal tree. Despite this, the name is still used informally by some researchers for convenience, usually restricted to the spalacotheriids and zhangheotheriids.

<i>Akidolestes</i> Extinct genus of mammals

Akidolestes is an extinct genus of mammals of the family Spalacotheriidae, a group of mammals related to therians.

<span class="mw-page-title-main">Docodonta</span> Extinct order of mammaliaforms

Docodonta is an order of extinct Mesozoic mammaliaforms. They were among the most common mammaliaforms of their time, persisting from the Middle Jurassic to the Early Cretaceous across the continent of Laurasia. They are distinguished from other early mammaliaforms by their relatively complex molar teeth. Docodont teeth have been described as "pseudotribosphenic": 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, Delsatia, and Tikitherium.

<span class="mw-page-title-main">Mammaliaformes</span> Clade of mammals and extinct relatives

Mammaliaformes is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia. Besides Morganucodonta and the crown group mammals, Mammaliaformes includes Docodonta and Hadrocodium as well as the Triassic Tikitherium, the earliest known member of the group.

<span class="mw-page-title-main">Australosphenida</span> Subclass of mammals

The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. They are generally thought to have acquired their tribosphenic molars independently from those of Tribosphenida. Fossils of australosphenidans have been found from the Jurassic of Madagascar and Argentina, and Cretaceous of Australia and Argentina. Monotremes have also been considered a part of this group in some studies, but this is disputed.

<span class="mw-page-title-main">Cusp (anatomy)</span> Pointed, projecting, or elevated anatomical feature

A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two cusps, is also known as the bicuspid valve, and the tricuspid valve has three cusps.

<span class="mw-page-title-main">Eutriconodonta</span> Extinct order of mammals

Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack et al. in 1973 as a replacement name for the paraphyletic Triconodonta.

<i>Gobiconodon</i> Extinct genus of mammals

Gobiconodon is an extinct genus of carnivorous mammals belonging to the family Gobiconodontidae. Undisputed records of Gobiconodon are restricted to the Early Cretaceous of Asia and North America, but isolated teeth attributed to the genus have also been described from formations in England and Morocco dating as far back as the Middle Jurassic. Species of Gobiconodon varied considerably in size, with G. ostromi, one of the larger species, being around the size of a modern Virginia opossum. Like other gobiconodontids, it possessed several speciations towards carnivory, such as shearing molariform teeth, large canine-like incisors and powerful jaw and forelimb musculature, indicating that it probably fed on vertebrate prey. Unusually among predatory mammals and other eutriconodonts, the lower canines were vestigial, with the first lower incisor pair having become massive and canine-like. Like the larger Repenomamus there might be some evidence of scavenging.

<span class="mw-page-title-main">Cladotheria</span> Clade of mammals

Cladotheria is a clade of mammals. It contains modern therian mammals and several extinct groups, such as the "dryolestoids", amphitheriids and peramurids. The clade was named in 1975 by Malcolm McKenna. In 2002, it was defined as a node-based taxon containing "the common ancestor of dryolestids and living therians, plus all its descendants". A different, stem-based definition was given in 2013, in which Cladotheria contains all taxa that are closer to Mus musculus than to the "symmetrodont" Spalacotherium tricuspidens.

<span class="mw-page-title-main">Monotreme</span> Order of egg-laying mammals

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 by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts, compared to the more common mammalian types. Although they are different from almost all mammals in that they lay eggs, like all mammals, the female monotremes nurse their young with milk.

<i>Ambondro mahabo</i> Species of small mammal from the middle Jurassic of Madagascar

Ambondro mahabo is a mammal from the Middle Jurassic (Bathonian) Isalo III Formation of Madagascar. The only described species of the genus Ambondro, it is known from a fragmentary lower jaw with three teeth, interpreted as the last premolar and the first two molars. The premolar consists of a central cusp with one or two smaller cusps and a cingulum (shelf) on the inner, or lingual, side of the tooth. The molars also have such a lingual cingulum. They consist of two groups of cusps: a trigonid of three cusps at the front and a talonid with a main cusp, a smaller cusp, and a crest at the back. Features of the talonid suggest that Ambondro had tribosphenic molars, the basic arrangement of molar features also present in marsupial and placental mammals. It is the oldest known mammal with putatively tribosphenic teeth; at the time of its discovery it antedated the second oldest example by about 25 million years.

Several mammals are known from the Mesozoic of Madagascar. The Bathonian Ambondro, known from a piece of jaw with three teeth, is the earliest known mammal with molars showing the modern, tribosphenic pattern that is characteristic of marsupial and placental mammals. Interpretations of its affinities have differed; one proposal places it in a group known as Australosphenida with other Mesozoic tribosphenic mammals from the southern continents (Gondwana) as well as the monotremes, while others favor closer affinities with northern (Laurasian) tribosphenic mammals or specifically with placentals. At least five species are known from the Maastrichtian, including a yet undescribed species known from a nearly complete skeleton that may represent a completely new group of mammals. The gondwanathere Lavanify, known from two teeth, is most closely related to other gondwanatheres found in India and Argentina. Two other teeth may represent another gondwanathere or a different kind of mammal. One molar fragment is one of the few known remains of a multituberculate mammal from Gondwana and another has been interpreted as either a marsupial or a placental.

<i>Megaconus</i> Extinct genus of mammaliaforms

Megaconus is an extinct genus of allotherian mammal from the Middle Jurassic Tiaojishan Formation of Inner Mongolia, China. The type and only species, Megaconus mammaliaformis was first described in the journal Nature in 2013. Megaconus is thought to have been a herbivore that lived on the ground, having a similar posture to modern-day armadillos and rock hyraxes. Megaconus was in its initial description found to be member of a group called Haramiyida. A phylogenetic analysis published along its description suggested that haramiyidans originated before the appearance of true mammals, but in contrast, the later description of the haramiyidan Arboroharamiya in the same issue of Nature indicated that haramyidans were true mammals. If haramiyidans are not mammals, Megaconus would be one of the most basal ("primitive") mammaliaforms to possess fur, and an indicator that fur evolved in the ancestors of mammals and not the mammals themselves. However, later studies cast doubt on the euharamiyidan intrepretation, instead finding it to be a basal allotherian mammal.

<span class="mw-page-title-main">Yinotheria</span> Subclass of mammals

Yinotheria is a proposed basal subclass clade of crown mammals uniting the Shuotheriidae, an extinct group of mammals from the Jurassic of Eurasia, with Australosphenida, a group of mammals known from the Jurassic to Cretaceous of Gondwana, which possibly include living monotremes. Today, there are only five surviving species of monotremes which live in Australia and New Guinea, consisting of the platypus and four species of echidna. Fossils of yinotheres have been found in Britain, China, Russia, Madagascar and Argentina. Contrary to other known crown mammals, they retained postdentary bones as shown by the presence of a postdentary trough. The extant members (monotremes) developed the mammalian middle ear independently.

Vilevolodon is an extinct, monotypic genus of volant, arboreal euharamiyids from the Oxfordian age of the Late Jurassic of China. The type species is Vilevolodon diplomylos. The genus name Vilevolodon references its gliding capabilities, Vilevol, while don is a common suffix for mammalian taxon titles. The species name diplomylos refers to the dual mortar-and-pestle occlusion of upper and lower molars observed in the holotype; diplo, mylos.

<i>Holoclemensia</i> Extinct family of mammals

Holoclemensia is an extinct genus of mammal of uncertain phylogenetic placement. It lived during the Early Cretaceous and its fossil remains were discovered in Texas.

References

  1. 1 2 3 4 5 Hu, Yaoming; Wang, Yuanqing; Luo, Zhexi; Li, Chuankui (1997). "A new symmetrodont mammal from China and its implications for mammalian evolution". Nature. 390 (6656): 137–142. doi:10.1038/36505. ISSN   1476-4687.
  2. 1 2 Hurum, Jørn H.; Luo, Zhe-Xi; Kielan-Jaworowska, Zofia (2006). "Were mammals originally venomous?" (PDF). Acta Palaeontologica Polonica. 51 (1): 1–11.
  3. Chen, Meng; Wilson, Gregory P. (2015). "A multivariate approach to infer locomotor modes in Mesozoic mammals". Paleobiology. 41 (2): 280–312. doi:10.1017/pab.2014.14. ISSN   0094-8373.