The Australosphenida are a
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final sta ...
. They are thought to have acquired their tribosphenic molars independently from those of
Tribosphenida. Fossils of australosphenidans have been found from the
Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
of
Madagascar
Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Afric ...
and
Argentina
Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the List of South American countries by area, second-largest ...
, and
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
of Australia and Argentina.
Monotremes have also been considered a part of this group in some studies, but this is disputed.
Taxonomy
This grouping includes the following taxa:
*†
Henosferidae, including the
genera
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 nomenclat ...
''
Ambondro'', ''
Asfaltomylos'', and ''
Henosferus'' from the
Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
of Argentina and Madagascar.
*†
Ausktribosphenidae, including the genera ''
Ausktribosphenos'', ''
Bishops
A bishop is an ordained clergy member who is entrusted with a position of authority and oversight in a religious institution.
In Christianity, bishops are normally responsible for the governance of dioceses. The role or office of bishop is ca ...
'' from the Lower
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
of Australia and Mid-Cretaceous of Argentina
*†''
Vincelestes
''Vincelestes'' ("Vince's thief") is an extinct genus of actively mobile mammal, that lived in what would be South America during the Early Cretaceous from 130 to 112 mya, existing for approximately .
Description
''Vincelestes neuquenianus'' ...
'', sometimes recovered as an australosphenidan (when not inversely considered a
cladotherian).
[Nicholas Chimento, Frederico Agnolin, Agustin Martinelli, Mesozoic Mammals from South America: Implications for understanding early mammalian faunas from Gondwana, May 2016]
*†''
Tendagurutherium'', also recently recovered as an australosphenidan.
The clade Australosphenida was proposed by Luo ''et al.'' (2001, 2002) and was initially left unranked, as the authors do not apply the
Linnaean hierarchy. In Benton (2005), it is ranked as a '
superdivision', i.e. one or two levels below the
infraclass
In biological classification, class ( la, classis) is a taxonomic rank, as well as a taxonomic unit, a taxon, in that rank. It is a group of related taxonomic orders. Other well-known ranks in descending order of size are life, domain, king ...
.
Australosphenid teeth
The grouping embodies a hypothesis about the evolution of
molar teeth in mammals. Living monotremes are toothless as adults, but the juvenile platypus, fossil monotremes and Ausktribosphenida all share a pattern of three molar
cusps arranged in a triangle or V shape, which is known as the
tribosphenic type of molar. Tribosphenic molars have long been held to characterize the subclass
Theria
Theria (; Greek: , wild beast) 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.
...
(
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in ...
s,
placental
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 distinguishe ...
s and their extinct relatives), while monotremes were thought to be related to fossil groups with a linear alignment of cusps:
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 ...
tids,
docodont
Docodonta is an order of extinct mammaliaforms that lived during the Mesozoic, from the Middle Jurassic to Early Cretaceous. They are distinguished from other early mammaliaforms by their relatively complex molar teeth, from which the order ge ...
s,
triconodonts and
multituberculates, all of which were united with the monotremes into the 'subclass
Prototheria'. Defined in this way, the 'Prototheria' is no longer recognised as a valid clade, since the linear cusp pattern is a primitive condition within Mammalia and cannot supply the shared derived character, which is required to establish a subgroup. Instead, the available evidence suggests that the monotremes descend from a
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
radiation of tribosphenic mammals in the southern continents (hence the name Australosphenida, meaning 'southern wedges'), but this interpretation is highly controversial.
According to Luo ''et al.'', tribosphenic molars were evolved by the Australosphenida independently of the true
Tribosphenida, or
Boreosphenida
Tribosphenida is a group (infralegion) of mammals that includes the ancestor of ''Hypomylos'', Aegialodontia and Theria (the last common ancestor of marsupials and placentals plus all of its descendants). Its current definition is more or less s ...
(that is, the therians and their relatives) in the northern continents. Others contend that the Ausktribosphenida in fact belong to the placentals and were therefore true tribosphenids, but unrelated to the ancestry of the monotremes. If this were confirmed, it would entail abandoning the clade Australosphenida.
Most recent
phylogenetic
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 ...
studies validate this clade, lumping henosferids and aukstribosphenids alongside monotremes.
[Rebecca Pian; Michael Archer; Suzanne J. Hand; Robin M.D. Beck; Andrew Cody (2016). "The upper dentition and relationships of the enigmatic Australian Cretaceous mammal Kollikodon ritchiei". Memoirs of Museum Victoria. 74: 97–105.] However in a 2022 review of montreme evolution, it was noted that the most primitive monotreme ''
Teinolophos'' differed substantially from other non-monotreme Australosphenidans'','' having five molars as opposed to three in all other non-monotreme australosphenidans, and having non-tribosphenic teeth, which appears to have only evolved in more derived monotremes, meaning that the two groups were likely to be unrelated. Later, the same authors suggested that the core grouping of australosphenidans (excluding monotremes) were actually stem-
therians.
Notes
References
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{{Taxonbar, from=Q783432
Mammal taxonomy