Yinotheria is a proposed basal subclass clade of crown

mammals A mammal () is a vertebrate animal of the 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 middle e ...

uniting the Shuotheriidae, an extinct group of mammals from the Jurassic of Eurasia, with

Australosphenida The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. Although they have often been suggested to have acquired tribosphenic molars independently from those ...

, a group of mammals known from the Jurassic to Cretaceous of

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

, which possibly include living monotremes. Today, there are only five surviving

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

of monotremes which live in

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

and

New Guinea New Guinea (; Hiri Motu: ''Niu Gini''; , fossilized , also known as Papua or historically ) is the List of islands by area, world's second-largest island, with an area of . Located in Melanesia in the southwestern Pacific Ocean, the island is ...

, consisting of the

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 ...

and four species of

echidna Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the Family (biology), family Tachyglossidae , living in Australia and New Guinea. The four Extant taxon, extant species of echidnas ...

. Fossils of yinotheres have been found in

Britain Britain most often refers to: * Great Britain, a large island comprising the countries of England, Scotland and Wales * The United Kingdom of Great Britain and Northern Ireland, a sovereign state in Europe comprising Great Britain and the north-eas ...

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 ...

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...

and

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

. 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 The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...

independently. Other studies have rejected Yinotheria, finding Shuotheriidae to be unrelated to Australosphenida and outside crown Mammalia.

Evolutionary history

According to genetic studies, Yinotheria diverged from other

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 ...

s around 220 to 210 million years ago, at some point in the

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic� ...

. The oldest-known fossils are a bit younger, dating around 168 to 163 million years in the

Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...

. These fossils are the genera '' Pseudotribos'' of China, ''

Shuotherium ''Shuotherium'' is a fossil mammaliaform known from Middle Jurassic, Middle-Late Jurassic of the Forest Marble Formation of England,Sigogneau−Russell, D. 1998. Discovery of a Late Jurassic Chinese mammal in the upper Bathonian of England. Compt ...

'' of both China and the United Kingdom, '' Itatodon'' of

Siberia Siberia ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...

and '' Paritatodon'' of

Kyrgyzstan Kyrgyzstan, officially the Kyrgyz Republic, is a landlocked country in Central Asia lying in the Tian Shan and Pamir Mountains, Pamir mountain ranges. Bishkek is the Capital city, capital and List of cities in Kyrgyzstan, largest city. Kyrgyz ...

and the UK. These, which belong to the family Shuotheriidae, are the only known northern hemisphere group of yinotherians. The infraclass

appeared around the same time as Shuotheriidae. The family Henosferidae, comprising the genera '' Henosferus'', '' Ambondro'', and '' Asfaltomylos'', has been found in the southern hemisphere at locations in Argentina and Madagascar. This suggests that this family could have been more widespread and diverse in

during that time; however, due to their fragile state, some fossils might have been destroyed by geological events. The family Ausktribosphenidae and the first

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 appeared in 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 ...

, in the region that is now known as

Australasia Australasia is a subregion of Oceania, comprising Australia, New Zealand (overlapping with Polynesia), and sometimes including New Guinea and surrounding islands (overlapping with Melanesia). The term is used in a number of different context ...

. Despite being found in the same region of the world and in the same time period, recent work has found that the older Henosferidae is the sister taxon to Monotremata, with Ausktribosphenidae being the next sister taxa in Australosphenida.Averianov ''et al.'', 2014 Ausktribosphenidae includes the genera ''

Bishops A bishop is an ordained member of the clergy who is entrusted with a position of Episcopal polity, authority and oversight in a religious institution. In Christianity, bishops are normally responsible for the governance and administration of di ...

'' and '' Ausktribosphenos''. Some 110-million-year-old monotreme fossil jaw fragments were found at Lightning Ridge,

New South Wales New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...

. These fragments, from the species '' Steropodon galmani'', are the oldest known monotreme fossils. Fossils from the genera '' Kollikodon'', ''

Teinolophos ''Teinolophos'', from Ancient Greek τείνω (''teínō''), meaning "extend", and λόφος (''lóphos''), meaning "crest", is a genus of Teinolophidae, teinolophid monotreme that lived during the late Barremian Age (geology), age of the Ear ...

'', and '' Obdurodon'' have also been discovered. In 1991, a fossil tooth of a 61-million-year-old platypus was found in southern

(since named ''Monotrematum'', though it is now considered to be an ''Obdurodon'' species). (See fossil monotremes below.)

Molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleot ...

and fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19 to 48 million years ago, but another putting it at 17 to 89 million years ago. All these dates are more recent than the oldest known platypus fossils, suggesting that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.

Systematics

History of classification

Prototheria

Originally, monotremes were classified as a subclass of mammals known as Prototheria. The names Prototheria,

Metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as wel ...

and

Eutheria Eutheria (from Greek , 'good, right' and , 'beast'; ), also called Pan-Placentalia, is the clade consisting of Placentalia, placental mammals and all therian mammals that are more closely related to placentals than to marsupials. Eutherians ...

(meaning "first beasts", "changed beasts", and "true beasts", respectively) refer to the three mammalian groupings that have living representatives. Each of the three may be defined as a total

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

containing a living crown-group (respectively, the Monotremata,

Marsupialia 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 ...

and

Placentalia Placental mammals ( infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distingui ...

) plus any fossil species that are more closely related to that crown-group than to any other living animals. The threefold division of living mammals into monotremes, marsupials and placentals was already well established when

Thomas Huxley Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist who specialized in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution. The stor ...

proposed the names Metatheria and Eutheria to incorporate the two latter groups in 1880. Initially treated as subclasses, Metatheria and Eutheria are by convention now grouped as infraclasses of the subclass

Theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

, and in more recent proposals have been demoted further (to cohorts or even

magnorder Order () 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 recognized ...

s), as

cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...

reappraisals of the relationships between living and fossil mammals have suggested that the Theria itself should be reduced in rank. Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of a hypothesis that defined the group by two supposed

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ...

: (1) formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with the alisphenoid in therians; and (2) a linear alignment of molar cusps, contrasting with a triangular arrangement in therians. These characters appeared to unite monotremes with a range of

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

fossil orders (

Morganucodon ''Morganucodon'' ("Glamorgan tooth") is an early mammaliaform genus that lived from the Late Triassic to the Middle Jurassic. It first appeared about 205 million years ago. Unlike many other early mammaliaforms, ''Morganucodon'' is well represent ...

ta, Triconodonta,

Docodonta Docodonta is an Order (biology), order of extinct Mesozoic Mammaliaformes, mammaliaforms (advanced cynodonts closely related to true Crown group, crown-group mammals). They were among the most common mammaliaforms of their time, persisting from t ...

and

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

) in a broader clade for which the name Prototheria was retained, and of which monotremes were thought to be only the last surviving branch (Benton 2005: 300, 306).

Australosphenida hypothesis and Yinotheria

The evidence that was held to support Prototheria is now universally discounted. In the first place, the examination of embryos has revealed that the development of the braincase wall is essentially identical in therians and in 'prototherians': the anterior lamina simply fuses with the alisphenoid in therians, and therefore the 'prototherian' condition of the braincase wall is primitive for all mammals, while the therian condition can be derived from it. Additionally, the linear alignment of molar cusps is also primitive for all mammals. Therefore, neither of these states can supply a uniquely shared derived character that would support a 'prototherian' grouping of orders in contradistinction to Theria (Kemp 1983). In a further reappraisal, the molars of embryonic and fossil monotremes (living monotreme adults are toothless) appear to demonstrate an ancestral pattern of cusps that is similar to the triangular arrangement observed in therians. Some peculiarities of this dentition support an alternative grouping of monotremes with certain recently discovered fossil forms into a proposed new clade known as the

, and also suggest that the triangular array of cusps may have evolved independently in australosphenidans and therians (Luo ''et al.'' 2001, 2002). Australosphenida is characterized by the shared presence of a cingulum on the outer front corner of the lower molars, a short and broad talonid, a relatively low trigonid, and a triangulated last lower premolar. The Australosphenida hypothesis remains controversial; for example, lingual cingula seem to be a presence in various non-australosphenidan mammalsSigogneau-Russell et al., 2001, p. 146 and some work has shown the possibility of Eutheria being the sister group to Australosphenida, without monotremes. As a result, some taxonomists (e.g. McKenna & Bell 1997) prefer to maintain the name Prototheria as a fitting contrast to the other group of living mammals, the Theria. In theory, the Prototheria is taxonomically redundant, since Monotremata is currently the only order that can still be confidently included, but its retention might be justified if new fossil evidence, or a re-examination of known fossils, enables extinct relatives of the monotremes to be identified and placed within a wider grouping. When systematic work was performed, it was also found that Australosphenida is the sister taxon to Shuotheriidae, an obscure group of Mesozoic mammals that were found in what is now

. had this to say regarding the Shuotheriidae, particularly ''Shuotherium'': Yinotheria is named for this grouping. Other scholars have rejected Yinotheria, finding instead that Shuotheriidae is closely related to

outside crown Mammalia.

Taxonomy

In comparison to

and

, where there seems to be a better understanding on the relationships among taxa with substantial fossil evidence, Yinotheria has few fossils; mostly consisting of (with few exceptions) the jawbones and teeth. In addition, the group seems not to have been as diverse in their evolutionary history, in comparison to members of both Metatheria and Eutheria. Future analysis and better fossil remains could affect the membership of Yinotheria as well as rearranging and revising the relationships of stem-monotremes and crowned monotremes. * Subclass Yinotheria Chow & Rich 1982 sensu Kielan-Jaworowska, Cifelli & Luo 2004 rototheria Gill 1872** Order Shuotherida Chow & Rich 1982 huotheridia; Shuotheria*** Family Shuotheriidae Chow & Rich 1982 **** Genus '' Pseudotribos'' Luo, Ji & Yuan 2007 ***** '' Pseudotribos robustus'' Luo, Ji & Yuan 2007 **** Genus ''

'' Chow & Rich 1982 ***** '' Shuotherium dongi'' Chow & Rich 1982 ***** '' Shuotherium shilongi'' Wang et al. 1998 ** Infraclass

Luo, Cifelli & Kielan-Jaworowska 2001 sensu Kielan-Jaworowska, Cifelli & Luo 2004 *** Order Ausktribosphenida Luo, Cifelli & Kielan-Jaworowska 2001 **** Family Ausktribosphenidae Rich et al. 1997 ***** Genus ''

'' Rich et al. 2001 ****** '' Bishops whitmorei'' Rich et al. 2001 ***** Genus '' Ausktribosphenos'' Rich et al. 1997 ****** '' Ausktribosphenos nyktos'' Rich et al. 1997 *** Order Henosferida Averianov & Lopatin 2011 **** Family Henosferidae Rougier et al. 2007 ***** Genus '' Ambondro''Woodburne, 2003 ****** ''

Ambondro mahabo ''Ambondro mahabo'' is a mammal from the Middle Jurassic (Bathonian) Isalo III Formation (about 167 million years ago) of Madagascar. The only described species of the genus ''Ambondro'', it is known from a fragmentary mandible, lower jaw with ...

'' Flynn et al. 1999 ***** Genus '' Asfaltomylos'' Rauhut et al. 2002 ****** '' Asfaltomylos patagonicus'' Rauhut et al. 2002 ***** Genus '' Henosferus'' Rougier et al. 2007 ****** '' Henosferus molus'' Rougier et al. 2007 *** Order Monotremata Bonaparte 1837 sensu Luo, Cifelli & Kielan-Jaworowska 2001 **** Family

Incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

***** Genus '' Kryoryctes'' Pridmore et al. 2005 (either a stem-monotreme or junior synonym of '' Steropodon'') ****** '' Kryoryctes cadburyi'' Pridmore et al. 2005 **** Family Tachyglossidae Gill 1872 Echidnidae Burnett 1830">Echidnidae.html" ;"title="Echidnidae">Echidnidae Burnett 1830(echidnas, spiny anteaters) ***** Genus ''Megalibgwilia'' Griffiths, Wells & Barrie 1991 (subjective synonym or subgenus of ''Zaglossus'') ****** ''Megalibgwilia robusta'' (Dun 1895) (subjective synonym of ''Zaglossus robustus'') ****** '' Megalibgwilia ramsayi'' (Owen 1884) (senior synonym of '' Megalibgwilia owenii''Ashwell, 2013) ***** Genus '' Zaglossus'' Gill 1877 'Proechidna'' Gervais 1877; ''Acanthoglossus">Proechidna.html" ;"title="'Proechidna">'Proechidna'' Gervais 1877; ''Acanthoglossus'' Gervais 1877; ''Bruynia'' Dubois 1882; ''Bruijia'' Thomas 1883; ''Prozaglossus'' Kerbert 1913] (Long-beaked Echidnas) ****** ''Murrayglossus'' Glauert 1914 (Hackett's Giant Echidna) (might belong to a new genus) ****** '' Zaglossus bruijni'' (Peters & Doria 1876) (Western Long-beaked/Bruinj's/Brown False Echidna) ****** '' Zaglossus attenboroughii'' Flannery & Groves 1998 (Attenborough's/Sir David's/Cyclops Long-beaked Echidna) ****** '' Zaglossus bartoni'' Thomas 1907 (Eastern/Barton's Long-beaked/Grey False Echidna) ***** Genus '' Tachyglossus'' Illiger 1811 'Acanthonotus'' Goldfuss 1809; ''Echidna">Acanthonotus.html" ;"title="'Acanthonotus">'Acanthonotus'' Goldfuss 1809; ''Echidna'' Cuvier 1798; ''Echinopus'' Fischer de Waldheim 1814; ''Syphomia'' Rafinesque 1815] ****** ''Tachyglossus aculeatus'' (Shaw 1792) Illiger 1811 (short-beaked Echidna) **** Family Kollikodontidae Flannery et al. 1995 ***** Genus '' Kollikodon'' Flannery et al. 1995 (might be a mammaliform of uncertain placement) ****** '' Kollikodon ritchiei'' Flannery et al. 1995 **** Family Steropodontidae Archer et al. 1985 ***** Genus ''

'' Rich et al. 1999 (might be a basal monotreme or a stem-monotreme) ****** '' Teinolophos trusleri'' Rich et al. 1999 ***** Genus '' Steropodon'' Archer et al. 1985 ****** '' Steropodon galmani'' Archer et al. 1985 **** Family Ornithorhynchidae Gray 1825 ***** Genus ''

Monotrematum ''Monotrematum sudamericanum'' is an extinct monotreme species from the Paleocene (Peligran) Salamanca Formation in Patagonia, Argentina. It is one of only two monotremes found outside Oceania. Taxonomy A species described in 1992 and assigned t ...

'' Pascual et al. 1992 (here considered to be a basal ornithorhynchid; others subjective synonym of ''Obdurodon'') ****** '' Monotrematum sudamericanum'' Pascual et al. 1992 ***** Genus '' Obdurodon'' Woodburne & Tedford 1975 ****** '' Obdurodon insignis'' Woodburne & Tedford 1975 ****** '' Obdurodon tharalkooschild'' Pian, Archer & Hand 2013 ****** '' Obdurodon dicksoni'' Archer et al. 1992 (Riversleigh platypus) ***** Genus '' Ornithorhynchus'' Blumenbach 1800 'Dermipus'' Wiedermann 1800; ''Platypus">Dermipus.html" ;"title="'Dermipus">'Dermipus'' Wiedermann 1800; ''Platypus'' Shaw 1799 non Herbst 1793] ****** ''Ornithorhynchus anatinus'' (Shaw 1799) Blumenbach 1800 (platypus)

Phylogeny

Below is a simplified tree on Averianov ''et al.'', 2014 after Woodburn, 2003 and Ashwell, 2013

References

Bibliography

* * * * * * * McKenna, Malcolm C., and Susan K. Bell. 1997. ''Classification of Mammals Above the Species Level''. New York: Columbia University Press. *

External links

* * {{Taxonbar, from=Q21448022 Mammal taxonomy Vertebrate subclasses Extant Late Triassic first appearances Taxa named by Zhou Mingzhen Taxa named by Tom Rich