Australosphenida
Encyclopedia
The Australosphenida are a clade
of mammals. Today, living specimens exist only in Australia
and New Guinea
with only five surviving species, but fossils have been found in Madagascar
and Argentina
. The surviving species consist of the platypus
and 4 species of echidna
.
This grouping includes the following animals:
'†' denotes an extinct taxon
.
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.
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 cusp
s 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
(marsupial
s, placentals and their extinct relatives) while monotremes were thought to be related to fossil groups with a linear alignment of cusps: morganucodon
tids, docodonts, 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
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 (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.
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of mammals. Today, living specimens exist only in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and New Guinea
New Guinea
New Guinea is the world's second largest island, after Greenland, covering a land area of 786,000 km2. Located in the southwest Pacific Ocean, it lies geographically to the east of the Malay Archipelago, with which it is sometimes included as part of a greater Indo-Australian Archipelago...
with only five surviving species, but fossils have been found in Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
and Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
. The surviving species consist of the platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
and 4 species of echidna
Echidna
Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
.
This grouping includes the following animals:
- Monotremata, divided into families Kollikodontidae†, OrnithorhynchidaeOrnithorhynchidaeOrnithorhynchidae is one of the two extant families in the order Monotremata, and contains the Platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas...
(platypus), SteropodontidaeSteropodontidaeThe Steropodontidae was a family of monotremes that are known from fossils from the Early Cretaceous in Australia.There are two genera placed in this family; Steropodon, and Teinolophos which has been tentatively placed in the family due to the similarity of the lower molars in these two...
† and Tachyglossidae (echidnas) and the genus Kryoryctes; - Ausktribosphenida†, including the generaGenusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
AmbondroAmbondro (genus)Ambondro is a genus of mammal from the middle Jurassic of Madagascar. The single species, A. mahabo, is known from a fragmentary lower jaw with three teeth, interpreted as the last premolar and the first two molars...
, AsfaltomylosAsfaltomylosAsfaltomylos is an extinct genus of Australosphenida from the middle Jurassic of Argentina. Only one species is recorded, Asfaltomylos patagonicus, from the Cañadon Asfalto Formation, Chubut Province, Patagonia.-References:...
, AusktribosphenosAusktribosphenosAusktribosphenos is an extinct genus of Australosphenida from Early Cretaceous of Australia. The only recorded species, Ausktribosphenos nyktos, was found on Flat Rocks, Victoria.-References:...
, BishopsBishops (genus)Bishops is an extinct genus of Australosphenida or Theria from Early Cretaceous of Australia. The only recorded species, Bishops whitmorei , was found on Flat Rocks, Victoria. The genus was named in honour of the Dr Barry Bishop, the former Chairman of the Committee for Research and Exploration,...
, and HenosferusHenosferusHenosferus is an extinct genus of Australosphenida from Middle Jurassic of Argentina. The only recorded species, Henosferus molus, was found on Cañadón Asfalto Formation, Chubut Province, Patagonia.-References:...
from the Middle JurassicJurassicThe Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
to Lower CretaceousCretaceousThe Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
of Argentina, Madagascar and Australia.
'†' denotes an extinct taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
.
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
Linnaean taxonomy
Linnaean taxonomy can mean either of two related concepts:# the particular form of biological classification set up by Carl Linnaeus, as set forth in his Systema Naturæ and subsequent works...
. In Benton (2005) it is ranked as a 'superdivision', i.e. one or two levels below the infraclass.
The grouping embodies a hypothesis about the evolution of molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
teeth in mammals. Living monotremes are toothless as adults, but the juvenile platypus, fossil monotremes and Ausktribosphenida all share a pattern of three molar cusp
Cusp (dentistry)
A cusp is an occlusal or incisal eminence on a tooth.Canine teeth, otherwise known as cuspids, each possess a single cusp, while premolars, otherwise known as bicuspids, possess two each. Molars normally possess either four or five cusps...
s 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
Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
(marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s, placentals and their extinct relatives) while monotremes were thought to be related to fossil groups with a linear alignment of cusps: morganucodon
Morganucodon
Morganucodon is an early mammalian genus which lived during the Late Triassic. It first appeared about 205 million years ago. This has also been identified with Eozostrodon. Unlike many other early mammals, Morganucodon is well represented by abundant and well preserved, though in the vast...
tids, docodonts, triconodonts and multituberculates, all of which were united with the monotremes into the 'subclass Prototheria
Prototheria
Prototheria is a taxonomic group, or taxon, to which the order Monotremata belongs. It is conventionally ranked as a subclass within the mammals.Most of the animals in this group are extinct...
'. 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
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
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
Tribosphenida
Tribosphenida is a group of mammals that is defined as including the ancestor of Hypomylos, Necrolestidae, Aegialodontia and supercohort Theria .-External links:* * * -Further reading:...
, or Boreosphenida (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.