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The grouping embodies a hypothesis about the evolution of [[molar (tooth)|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 (dentistry)|cusps]] arranged in a triangle or V shape, which is known as the [[Tribosphenic molar|tribosphenic]] type of molar. Tribosphenic molars have long been held to characterize the subclass [[Theria]] ([[marsupial]]s, [[placental]]s and their extinct relatives), while monotremes were thought to be related to fossil groups with a linear alignment of cusps: [[morganucodon]]tids, [[docodont]]s, [[triconodont]]s and [[multituberculate]]s, 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. |
The grouping embodies a hypothesis about the evolution of [[molar (tooth)|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 (dentistry)|cusps]] arranged in a triangle or V shape, which is known as the [[Tribosphenic molar|tribosphenic]] type of molar. Tribosphenic molars have long been held to characterize the subclass [[Theria]] ([[marsupial]]s, [[placental]]s and their extinct relatives), while monotremes were thought to be related to fossil groups with a linear alignment of cusps: [[morganucodon]]tids, [[docodont]]s, [[triconodont]]s and [[multituberculate]]s, 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. |
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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 ausktribosphenids (two families of the Australian Cretaceous tribosphenids) in fact belong to the placentals and were therefore true tribosphenids, but unrelated to the ancestry of the monotremes.<ref>Benton 2005: 300, 306-308.</ref> Alternatively, Flannery and colleagues argue that the Australian Barremian-Aptian family Bishopidae are stem therians.<ref>{{Cite journal | |
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 ausktribosphenids (two families of the Australian Cretaceous tribosphenids) in fact belong to the placentals and were therefore true tribosphenids, but unrelated to the ancestry of the monotremes.<ref>Benton 2005: 300, 306-308.</ref> Alternatively, Flannery and colleagues argue that the Australian Barremian-Aptian family Bishopidae are stem therians.<ref>{{Cite journal |last1=Flannery |first1=Timothy F. |last2=Rich |first2=Thomas H. |last3=Vickers-Rich |first3=Patricia |last4=Veatch |first4=E. Grace |last5=Helgen |first5=Kristofer M. |date=2022-10-02 |title=The Gondwanan Origin of Tribosphenida (Mammalia) |url=https://www.tandfonline.com/doi/full/10.1080/03115518.2022.2132288 |journal=Alcheringa: An Australasian Journal of Palaeontology |language=en |volume=46 |issue=3–4 |pages=277–290 |doi=10.1080/03115518.2022.2132288 |s2cid=253323862 |issn=0311-5518}}</ref> If this were confirmed, it would entail abandoning the clade Australosphenida. |
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Most recent [[phylogenetics|phylogenetic]] studies, lump henosferids and aukstribosphenids alongside monotremes.<ref>Richard Stephen Thompson, Rachel O'Meara, Were There Miocene Meridiolestidans? Assessing the Phylogenetic Placement of Necrolestes patagonensis and the Presence of a 40 Million Year Meridiolestidan Ghost Lineage, Article in Journal of Mammalian Evolution · September 2014 DOI: 10.1007/s10914-013-9252-3</ref><ref name="Kollikodon ritchiei 2016">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.</ref> However in a 2022 review of montreme evolution noted that 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 molars, meaning that the two groups were likely unrelated.<ref>{{Cite journal | |
Most recent [[phylogenetics|phylogenetic]] studies, lump henosferids and aukstribosphenids alongside monotremes.<ref>Richard Stephen Thompson, Rachel O'Meara, Were There Miocene Meridiolestidans? Assessing the Phylogenetic Placement of Necrolestes patagonensis and the Presence of a 40 Million Year Meridiolestidan Ghost Lineage, Article in Journal of Mammalian Evolution · September 2014 DOI: 10.1007/s10914-013-9252-3</ref><ref name="Kollikodon ritchiei 2016">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.</ref> However in a 2022 review of montreme evolution noted that 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 molars, meaning that the two groups were likely unrelated.<ref>{{Cite journal |last1=Flannery |first1=Timothy F. |last2=Rich |first2=Thomas H. |last3=Vickers-Rich |first3=Patricia |last4=Ziegler |first4=Tim |last5=Veatch |first5=E. Grace |last6=Helgen |first6=Kristofer M. |date=2022-01-02 |title=A review of monotreme (Monotremata) evolution |journal=Alcheringa: An Australasian Journal of Palaeontology |language=en |volume=46 |issue=1 |pages=3–20 |doi=10.1080/03115518.2022.2025900 |issn=0311-5518|doi-access=free }}</ref> Later, Flannery and coauthors suggested that the core grouping of australosphenidans (excluding monotremes) were actually stem-[[Theria|therians]].<ref>{{Cite journal |last1=Flannery |first1=Timothy F. |last2=Rich |first2=Thomas H. |last3=Vickers-Rich |first3=Patricia |last4=Veatch |first4=E. Grace |last5=Helgen |first5=Kristofer M. |date=2022-11-01 |title=The Gondwanan Origin of Tribosphenida (Mammalia) |url=https://www.tandfonline.com/doi/full/10.1080/03115518.2022.2132288 |journal=Alcheringa: An Australasian Journal of Palaeontology |volume=46 |issue=3–4 |language=en |pages=277–290 |doi=10.1080/03115518.2022.2132288 |s2cid=253323862 |issn=0311-5518}}</ref> |
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==Notes== |
==Notes== |
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| last3 = Kielan-Jaworowska | first3 = Zofia |
| last3 = Kielan-Jaworowska | first3 = Zofia |
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| title = Dual origin of tribosphenic mammals |
| title = Dual origin of tribosphenic mammals |
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| journal = Nature | year = 2001 | volume = 409 | |
| journal = Nature | year = 2001 | volume = 409 | issue = 6816 |
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| pages = 53–57 |
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| doi = 10.1038/35051023 | pmid=11343108 |
| doi = 10.1038/35051023 | pmid=11343108 |
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| s2cid = 4342585 |
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}} |
}} |
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* {{Cite journal |
* {{Cite journal |
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| last1 = Luo | first1 = Zhe-Xi |
| last1 = Luo | first1 = Zhe-Xi |
Australosphenida
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Jaw fragment of Ambondro mahabo | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Mammalia |
Clade: | Australosphenida Luo, Cifelli, & Kielan-Jaworowska, 2001 |
Taxa | |
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The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. They are thought to have acquired their tribosphenic molars independently from those of Tribosphenida. Fossils of australosphenidans have been found from the JurassicofMadagascar 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.
This grouping includes the following taxa:
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 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 (marsupials, placentals and their extinct relatives), while monotremes were thought to be related to fossil groups with a linear alignment of cusps: morganucodontids, 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 ausktribosphenids (two families of the Australian Cretaceous tribosphenids) in fact belong to the placentals and were therefore true tribosphenids, but unrelated to the ancestry of the monotremes.[3] Alternatively, Flannery and colleagues argue that the Australian Barremian-Aptian family Bishopidae are stem therians.[4] If this were confirmed, it would entail abandoning the clade Australosphenida.
Most recent phylogenetic studies, lump henosferids and aukstribosphenids alongside monotremes.[5][6] However in a 2022 review of montreme evolution noted that 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 molars, meaning that the two groups were likely unrelated.[7] Later, Flannery and coauthors suggested that the core grouping of australosphenidans (excluding monotremes) were actually stem-therians.[8]
Extant mammal orders
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Yinotheria |
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Theria |
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Australosphenida |
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