Editing Prototheria

{{Short description|Subclass of mammalia}}{{Paraphyletic group
| auto = yes
| taxon = Prototheria
| fossil_range = [[Late Triassic]] - Recent
| image = Prototheria collage.png
| authority = [[Theodore Gill|Gill]], 1872
| name = Prototherians
| includes = * [[Monotreme|Monotremata]]
* †[[Morganucodonta]]
* †[[Docodonta]]
* †[[Triconodonta]]
* †[[Allotheria]]
| excludes = * [[Theriimorpha]]
}}

'''Prototheria''' ({{IPAc-en|ˌ|p|r|oʊ|t|ə|ˈ|θ|ɪər|i|ə|,_|-|t|oʊ|-}}, {{respell|PROH-toh-THEER-ee-ə}};{{refn|{{Cite dictionary |url=http://www.lexico.com/definition/Prototheria |archive-url=https://web.archive.org/web/20200322182634/https://www.lexico.com/definition/prototheria |url-status=dead |archive-date=2020-03-22 |title=Prototheria |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}} }} from [[Ancient Greek]] {{lang|grc|πρώτος}} {{lang|grc-Latn|prṓtos}} "first" and {{lang|grc|θήρ}} {{lang|grc-Latn|thḗr}} "wild animal") is an obsolete subclass of mammals which includes the living [[Monotreme|Monotremata]] and to which a variety of extinct groups, including [[Morganucodonta]], [[Docodonta]], [[Triconodonta]] and [[Multituberculata]], have also been assigned. It is today no longer considered a valid grouping, but rather a [[paraphyletic]] [[evolutionary grade]] of basal mammals and [[Mammaliaformes|mammaliaform]] [[cynodont]]s.<ref name=":0">{{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 |url=https://www.tandfonline.com/doi/full/10.1080/03115518.2022.2025900 |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 |s2cid=247542433|doi-access=free }}</ref>

Most of the animals in this group are extinct. The egg-laying monotremes are known from fossils of the [[Cretaceous]] and [[Cenozoic]] periods; they are represented today by the [[platypus]] and four species of [[echidna]].

The names Prototheria, [[Metatheria]], and [[Eutheria]] (loosely meaning "first beasts", "changed beasts", and "true beasts", respectively) refer to the three mammalian groupings of which we have living representatives. Each of the three may be defined as a total [[clade]] containing a living [[crown-group]] (respectively the Monotremata, [[Marsupialia]] and [[Placentalia]]) plus any fossil species which are more closely related to that crown-group than to any other living animals.

The threefold division of living mammals into monotremes, marsupials and placentals was already well established when [[Thomas Huxley]] proposed the names Metatheria and Eutheria to incorporate the two latter groups in 1880. Initially treated as subclasses, Metatheria and Eutheria are by convention now grouped as [[infraclass]]es of the subclass [[Theria]], and in more recent proposals have been demoted further (to cohorts or even [[magnorder]]s), as [[cladistic]] reappraisals of the relationships between living and fossil mammals have suggested that the Theria itself should be reduced in rank.<ref>Marsupialia and Eutheria/Placentalia appear as cohorts in McKenna & Bell 1997 and in Benton 2005, with Theria ranked as a [[Linnaean taxonomy#Taxonomic ranks|supercohort]] or an [[Linnaean taxonomy#Taxonomic ranks|infralegion]], respectively.</ref>

Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of a hypothesis which defined the group by two supposed [[synapomorphies]]:

# formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with the [[alisphenoid]] in therians; and
# a linear alignment of molar cusps, contrasting with a triangular arrangement in therians. These characters appeared to unite monotremes with a range of [[Mesozoic]] fossil orders ([[Morganucodonta]], [[Docodonta]], [[Triconodonta]] and [[Multituberculata]]) in a broader clade for which the name Prototheria was retained, and of which monotremes were thought to be only the last surviving branch.<ref>{{Cite book|title=Vertebrate Palaeontology|last=Benton|first=Michael J.|date=2005|publisher=Blackwell Science|isbn=978-0632056378|edition=3rd|location=Malden, Massachusetts|pages=300, 306|oclc=53970617}}</ref>

The evidence which was held to support this grouping is now universally discounted. In the first place, examination of embryos has revealed that the development of the braincase wall is essentially identical in therians and in 'prototherians': the anterior lamina simply fuses with the alisphenoid in therians, and therefore the 'prototherian' condition of the braincase wall is primitive for all mammals while the therian condition can be derived from it. Additionally, the linear alignment of molar cusps is also primitive for all mammals. Therefore, neither of these states can supply a uniquely shared derived character which would support a 'prototherian' grouping of orders in contradistinction to Theria.<ref>{{Cite journal|last=Kemp|first=T. S.|date=1983|title=The relationships of mammals|journal=Zoological Journal of the Linnean Society|language=en|volume=77|issue=4|pages=353–384|doi=10.1111/j.1096-3642.1983.tb00859.x|issn=0024-4082}}</ref>

In a further reappraisal, the molars of embryonic and fossil monotremes (living monotreme adults are toothless) appear to demonstrate an ancestral pattern of cusps which is similar to the triangular arrangement observed in therians. Some peculiarities of this dentition support an alternative grouping of monotremes with certain recently discovered fossil forms into a proposed new clade known as the [[Australosphenida]], and also suggest that the triangular array of cusps may have evolved independently in australosphenidans and therians.<ref>{{Cite journal|last1=Luo|first1=Zhe-Xi|last2=Cifelli|first2=Richard L.|last3=Kielan-Jaworowska|first3=Zofia|date=2001|title=Dual origin of tribosphenic mammals|url=http://luo-lab.uchicago.edu/pdfs/LuoEtAl%282001a%29.pdf|journal=Nature|language=En|volume=409|issue=6816|pages=53–57|doi=10.1038/35051023|pmid=11343108|issn=1476-4687|bibcode=2001Natur.409...53L|s2cid=4342585 }}</ref><ref>{{cite journal|last1=Luo|first1=Zhe-Xi|last2=Kielan-Jaworowska|first2=Zofia|last3=Cifelli|first3=Richard L|date=2002|title=In quest for a phylogeny of Mesozoic mammals|url=http://agro.icm.edu.pl/agro/element/bwmeta1.element.agro-article-a106466e-5e6b-4025-89a5-360d94a7eb83/c/app47-001.pdf|journal=Acta Palaeontologica Polonica|volume=47|issue=1|issn=0567-7920}}</ref>

The Australosphenida hypothesis remains controversial, and some taxonomists (e.g. McKenna & Bell 1997) prefer to maintain the name Prototheria as a fitting contrast to the other group of living mammals, the Theria.<ref>{{cite book|title=Classification of mammals above the species level|last1=McKenna|first1=Malcolm C|last2=Bell|first2=Susan K|last3=Simpson|first3=George Gaylord|date=2000|publisher=Columbia University Press|isbn=978-0-231-11013-6|location=New York|oclc=37345734}}</ref> In theory, the Prototheria is taxonomically redundant, since Monotremata is currently the only order which can still be confidently included, but its retention might be justified if new fossil evidence, or a re-examination of known fossils, enables extinct relatives of the monotremes to be identified and placed within a wider grouping.

== Notes ==
{{Commons category|Prototheria}}
{{Reflist}}

{{Mammals}}
{{Taxonbar|from=Q715778}}
{{Authority control}}

[[Category:Obsolete mammal taxa]]
[[Category:Taxa named by Theodore Gill]]