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{{Short description|Extinct order of mammals}}
{{automatic Taxobox
{{Automatic taxobox
| fossil_range = {{fossilrange|Early Cretaceous|Campanian}}
| fossil_range = {{fossilrange|Early Cretaceous|Campanian|latest=Paleocene}} <small>Possible [[Paleocene]] ghost lineage</small>
| image = Zhangheotherium quinquecuspidens.jpg
| image = Zhangheotherium quinquecuspidens.jpg
| image_width = 250px
| image_caption = Fossil specimen of ''[[Zhangheotherium quinquecuspidens]]''
| image_caption = Fossil specimen of ''[[Zhangheotherium quinquecuspidens]]''
| taxon = Symmetrodonta
| authority =
| authority =
| subdivision_ranks = Subgroups
| subdivision_ranks = Subgroups
| subdivision =
| subdivision =
*{{extinct}}[[Kuehneotheriidae]]
*{{extinct}}?''[[Chronoperates]]''
*{{extinct}}[[Spalacotheriidae]]
*{{extinct}}[[Spalacotheriidae]]
*{{extinct}}[[Zhangheotheriidae]]
*{{extinct}}[[Zhangheotheriidae]]
}}
}}
⚫
'''Symmetrodonta''' is a group of [[Mesozoic]] [[mammals]] and mammal-like synapsids characterized by the triangular aspect of the [[Molar (tooth)|molar]]s 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]]. One species, ''Spalacotheridium noblei'', is notable for its small size. It is one of the smallest known mammals. Each individual molar is little more than 0.25&nbsp;mm across. 'Symmetrodonts' are generally rare and poorly represented in the fossil record. Most research during the 21st century has concluded that they do not represent a discrete phylogenetic category. However, the name is still used informally by some researchers for convenience, or restricted to the spalacotheriids and zhangheotheriids.<ref name="mengetall2006">{{cite journal | doi = 10.1002/gj.1054 | last1 = Meng | first1 = J. | last2 = Hu | first2 = Y. | last3 = Li | first3 = C. | last4 = Wang | first4 = Y. | year = 2006 | title = The mammal fauna in the Early Cretaceous Jehol Biota: implications for diversity and biology of Mesozoic mammals | url = | journal = Geological Journal | volume = 41 | issue = 3–4| pages = 439–463 }}</ref>


⚫
'''Symmetrodonta''' is a group of [[Mesozoic]] [[mammals]] and mammal-like synapsids characterized by the triangular aspect of the [[Molar (tooth)|molar]]s 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]].<ref name ="Martin2018"> Martin, T., 2018. 6. Mesozoic mammals—early mammalian diversity and ecomorphological adaptations. In Mammalian evolution, diversity and systematics (pp. 199-300). De Gruyter.</ref><ref name="mengetall2006">{{cite journal | doi = 10.1002/gj.1054 | last1 = Meng | first1 = J. | last2 = Hu | first2 = Y. | last3 = Li | first3 = C. | last4 = Wang | first4 = Y. | year = 2006 | title = The mammal fauna in the Early Cretaceous Jehol Biota: implications for diversity and biology of Mesozoic mammals | journal = Geological Journal | volume = 41 | issue = 3–4| pages = 439–463 | s2cid = 84397126 }}</ref>
At least symmetrodonts with acutely-triangulated molar cusps (“acute-angled symmetrodonts”) seem to form a true [[monophyletic]] group, and lasted from the [[Early Cretaceous]] to the [[Campanian]].,<ref>Bi, S., Wang, Y., Guan, J., Sheng, X. & Meng, J. Three new Jurassic euharamiyidan species reinforce early divergence of mammals. Nature 514, 579–584 10.1038/nature13718 (2014).</ref><ref>Shundong Bi; Xiaoting Zheng; Jin Meng; Xiaoli Wang; Nicole Robinson; Brian Davis (2016). "A new symmetrodont mammal (Trechnotheria: Zhangheotheriidae) from the Early Cretaceous of China and trechnotherian character evolution". Scientific Reports. 6: Article number 26668. doi:10.1038/srep26668.</ref> though [[Zhangheotheriidae]] might be paraphyletic in relation to other forms.<ref>Shundong Bi; Xiaoting Zheng; Jin Meng; Xiaoli Wang; Nicole Robinson; Brian Davis (2016). "A new symmetrodont mammal (Trechnotheria: Zhangheotheriidae) from the Early Cretaceous of China and trechnotherian character evolution". Scientific Reports. 6: Article number 26668. doi:10.1038/srep26668.</ref>

There are some symmetrodonts with acutely-triangulated molar cusps (“acute-angled symmetrodonts”) that seem to form a true [[monophyletic]] group, and lasted from the [[Early Cretaceous]] to the [[Campanian]],<ref>{{cite journal|last1=Bi|first1=Shundong|last2=Wang|first2=Yuanqing|last3=Guan|first3=Jian|last4=Sheng|first4=Xia|last5=Meng|first5=Jin|title=Three new Jurassic euharamiyidan species reinforce early divergence of mammals|journal=Nature|date=2014|volume=514|issue=7524|pages=579–584|doi=10.1038/nature13718|pmid=25209669|bibcode=2014Natur.514..579B|s2cid=4471574}}</ref><ref name="Bi et al 2016">{{cite journal|last1=Bi|first1=Shundong|last2=Zheng|first2=Xiaoting|last3=Meng|first3=Jin|last4=Wang|first4=Xiaoli|last5=Robinson|first5=Nicole|last6=Davis|first6=Brian|title=A new symmetrodont mammal (Trechnotheria: Zhangheotheriidae) from the Early Cretaceous of China and trechnotherian character evolution|journal=Scientific Reports|date=2016|volume=6|pages=26668|doi=10.1038/srep26668|pmid=27215593|pmc=4877676|bibcode=2016NatSR...626668B}}</ref> although [[Zhangheotheriidae]] might be paraphyletic in relation to other forms.<ref name="Bi et al 2016"/> ''[[Chronoperates]]'' has been suggested to be a late surviving representative of this clade,<ref>{{cite journal | last1 = Meng | first1 = J. | last2 = Hu | first2 = Y. | last3 = Wang | first3 = Y. | last4 = Li | first4 = C. | year = 2003 | title = The ossified Meckel's cartilage and internal groove in Mesozoic mammaliaforms: implications to origin of the definitive mammalian middle ear | journal = Zoological Journal of the Linnean Society | volume = 138 | issue = 4 | pages = 431–448 | doi=10.1046/j.1096-3642.2003.00064.x| doi-access = free }}</ref> offering a ghost lineage extending to the late [[Paleocene]]; however, no recent phylogenetic studies have incorporated it.


Particular sub−groups of Symmetrodonta are better studied, e.g. [[Spalacotheriidae]], which has acute−angled molariform teeth, strongly reduced [[talonid]]s, and conspicuous anterior and posterior [[cingulid]]s.
Particular sub−groups of Symmetrodonta are better studied, e.g. [[Spalacotheriidae]], which has acute−angled molariform teeth, strongly reduced [[talonid]]s, and conspicuous anterior and posterior [[cingulid]]s.


==Biology==
==Biology==
Though some forms like ''[[Zhangheotherium]]'' retain a [[Meckelian groove]], at least [[Spalacotheriidae]] lost it, acquiring [[Evolution of mammalian auditory ossicles|modern ear anatomy]]. Their deciduous canines and premolars as well as long lower jaw indicate a [[carnivorous]]/[[insectivorous]] diet.<ref>Gang Han; Jin Meng (2016). "A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology, phylogeny, and palaeobiology of Laurasian 'symmetrodontans'". Zoological Journal of the Linnean Society. 178 (2): 343–380. doi:10.1111/zoj.12416.</ref>
Though some forms like ''[[Zhangheotherium]]'' retain a [[Meckelian groove]], at least [[Spalacotheriinae]] lost it, acquiring [[Evolution of mammalian auditory ossicles|modern ear anatomy]]. Their deciduous canines and premolars as well as long lower jaw indicate a [[carnivorous]]/[[insectivorous]] diet.<ref>{{cite journal|last1=Han|first1=Gang|last2=Meng|first2=Jin|title=A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology, phylogeny, and palaeobiology of Laurasian 'symmetrodontans'|journal=Zoological Journal of the Linnean Society|date=2016|volume=178|issue=2|pages=343–380|doi=10.1111/zoj.12416|doi-access=free}}</ref>


''[[Zhangheotherium]]'' was specialised to a [[scansorial]] lifestyle.<ref>Meng Chen, Gregory Philip Wilson, A multivariate approach to infer locomotor modes in Mesozoic mammals, Article in Paleobiology 41(02) · February 2015 DOI: 10.1017/pab.2014.14</ref> It shows evidence of tarsal spurs, indicating that, like most non-[[theria]]n [[Mammaliaformes]], at least some symmetrodonts were [[Venomous mammal|venomous]] like the modern [[platypus]].<ref>Hurum, Jørn H.; Luo, Zhe-Xi & Kielan-Jaworowska, Zofia (2006): Were mammals originally venomous? Acta Palaeontologica Polonica 51(1): 1–11.</ref>
''[[Zhangheotherium]]'' was specialised to a tree-dwelling lifestyle.<ref>{{cite journal|last1=Chen|first1=Meng|last2=Wilson|first2=Gregory P.|title=A multivariate approach to infer locomotor modes in Mesozoic mammals|journal=Paleobiology|date=2015|volume=41|issue=2|pages=280–312|doi=10.1017/pab.2014.14|s2cid=86087687}}</ref> It shows evidence of tarsal spurs, indicating that, like most non-[[theria]]n [[Mammaliaformes]], at least some symmetrodonts were [[Venomous mammal|venomous]] like the modern [[platypus]].<ref>{{cite journal|author1=Hurum, Jørn H. |author2=Luo, Zhe-Xi |author3=Kielan-Jaworowska, Zofia |year=2006|title= Were mammals originally venomous?|journal= Acta Palaeontologica Polonica|volume=51|number=1|pages= 1–11}}</ref>

One species, ''[[Spalacotheridium noblei]]'', is notable for its small size. It is one of the smallest known mammals. Each individual molar is little more than 0.25&nbsp;mm across.


==See also==
==See also==
* [[Evolution of mammals]]
* [[Evolution of mammals]]
* [[:pt:Spalacotheriidae|Spalacotheriidae]] (Full genus and species list on the Portuguese Wikipedia)


==References==
==References==
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==External links==
==External links==
* [http://www.carnegiemuseums.org/cmag/bk_issue/1998/marapr/feat3.htm Discovering a Missing Link]
* [https://web.archive.org/web/20060118104451/http://www.carnegiemuseums.org/cmag/bk_issue/1998/marapr/feat3.htm Discovering a Missing Link]

{{Mammalia|T.}}
{{Taxonbar|from=Q1938798}}


[[Category:Mammaliaformes]]
[[Category:Symmetrodonta| ]]

Latest revision as of 09:53, 15 June 2023

Symmetrodonta
Temporal range: Early Cretaceous–Campanian Possible Paleocene ghost lineage
Fossil specimen of Zhangheotherium quinquecuspidens
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Trechnotheria
Order: †Symmetrodonta
Subgroups

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.[1][2]

There are some symmetrodonts with acutely-triangulated molar cusps (“acute-angled symmetrodonts”) that seem to form a true monophyletic group, and lasted from the Early Cretaceous to the Campanian,[3][4] although Zhangheotheriidae might be paraphyletic in relation to other forms.[4] Chronoperates has been suggested to be a late surviving representative of this clade,[5] offering a ghost lineage extending to the late Paleocene; however, no recent phylogenetic studies have incorporated it.

Particular sub−groups of Symmetrodonta are better studied, e.g. Spalacotheriidae, which has acute−angled molariform teeth, strongly reduced talonids, and conspicuous anterior and posterior cingulids.

Biology

[edit]

Though some forms like Zhangheotherium retain a Meckelian groove, at least Spalacotheriinae lost it, acquiring modern ear anatomy. Their deciduous canines and premolars as well as long lower jaw indicate a carnivorous/insectivorous diet.[6]

Zhangheotherium was specialised to a tree-dwelling lifestyle.[7] It shows evidence of tarsal spurs, indicating that, like most non-therian Mammaliaformes, at least some symmetrodonts were venomous like the modern platypus.[8]

One species, Spalacotheridium noblei, is notable for its small size. It is one of the smallest known mammals. Each individual molar is little more than 0.25 mm across.

See also

[edit]

References

[edit]
  1. ^ Martin, T., 2018. 6. Mesozoic mammals—early mammalian diversity and ecomorphological adaptations. In Mammalian evolution, diversity and systematics (pp. 199-300). De Gruyter.
  2. ^ Meng, J.; Hu, Y.; Li, C.; Wang, Y. (2006). "The mammal fauna in the Early Cretaceous Jehol Biota: implications for diversity and biology of Mesozoic mammals". Geological Journal. 41 (3–4): 439–463. doi:10.1002/gj.1054. S2CID 84397126.
  3. ^ Bi, Shundong; Wang, Yuanqing; Guan, Jian; Sheng, Xia; Meng, Jin (2014). "Three new Jurassic euharamiyidan species reinforce early divergence of mammals". Nature. 514 (7524): 579–584. Bibcode:2014Natur.514..579B. doi:10.1038/nature13718. PMID 25209669. S2CID 4471574.
  4. ^ a b Bi, Shundong; Zheng, Xiaoting; Meng, Jin; Wang, Xiaoli; Robinson, Nicole; Davis, Brian (2016). "A new symmetrodont mammal (Trechnotheria: Zhangheotheriidae) from the Early Cretaceous of China and trechnotherian character evolution". Scientific Reports. 6: 26668. Bibcode:2016NatSR...626668B. doi:10.1038/srep26668. PMC 4877676. PMID 27215593.
  5. ^ Meng, J.; Hu, Y.; Wang, Y.; Li, C. (2003). "The ossified Meckel's cartilage and internal groove in Mesozoic mammaliaforms: implications to origin of the definitive mammalian middle ear". Zoological Journal of the Linnean Society. 138 (4): 431–448. doi:10.1046/j.1096-3642.2003.00064.x.
  6. ^ Han, Gang; Meng, Jin (2016). "A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology, phylogeny, and palaeobiology of Laurasian 'symmetrodontans'". Zoological Journal of the Linnean Society. 178 (2): 343–380. doi:10.1111/zoj.12416.
  7. ^ 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. S2CID 86087687.
  8. ^ Hurum, Jørn H.; Luo, Zhe-Xi; Kielan-Jaworowska, Zofia (2006). "Were mammals originally venomous?". Acta Palaeontologica Polonica. 51 (1): 1–11.
[edit]
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