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'''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]]. 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]]. 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> |
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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 |
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>{{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}}</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}}</ref> although [[Zhangheotheriidae]] might be paraphyletic in relation to other forms.<ref name="Bi et al 2016"/> |
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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. |
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==Biology== |
==Biology== |
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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> |
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>{{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}}</ref> |
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''[[Zhangheotherium]]'' was specialised to a [[scansorial]] lifestyle.<ref> |
''[[Zhangheotherium]]'' was specialised to a [[scansorial]] 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=02|pages=280–312|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>{{cite journal|authors=Hurum, Jørn H.; Luo, Zhe-Xi & Kielan-Jaworowska, Zofia |year=2006|title= Were mammals originally venomous?|journal= Acta Palaeontologica Polonica|volume=51|number=1|pages= 1–11}}</ref> |
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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. |
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. |
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Revision as of 04:44, 14 November 2016
| Symmetrodonta Temporal range:
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|---|---|
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| Fossil specimen of Zhangheotherium quinquecuspidens | |
Scientific 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. 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.[1]
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.,[2][3] although Zhangheotheriidae might be paraphyletic in relation to other forms.[3]
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
Though some forms like Zhangheotherium retain a Meckelian groove, at least Spalacotheriidae lost it, acquiring modern ear anatomy. Their deciduous canines and premolars as well as long lower jaw indicate a carnivorous/insectivorous diet.[4]
Zhangheotherium was specialised to a scansorial lifestyle.[5] It shows evidence of tarsal spurs, indicating that, like most non-therian Mammaliaformes, at least some symmetrodonts were venomous like the modern platypus.[6]
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
References
- ^ 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.
- ^ 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. doi:10.1038/nature13718.
- ^ 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. doi:10.1038/srep26668.
- ^ 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.
- ^ Chen, Meng; Wilson, Gregory P. (2015). "A multivariate approach to infer locomotor modes in Mesozoic mammals". Paleobiology. 41 (02): 280–312. doi:10.1017/pab.2014.14.
- ^ "Were mammals originally venomous?". Acta Palaeontologica Polonica. 51 (1): 1–11. 2006.
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