- Prototheria
Taxobox
name = Prototherians
fossil_range = EarlyCretaceous - Recent
image_width = 200px
image_caption =Short-beaked Echidna
regnum =Animal ia
phylum = Chordata
classis =Mammal ia
subclassis = Prototheria
subclassis_authority = Gill, 1872
subdivision_ranks = Orders
subdivision =
*Monotremata Prototheria (pronEng|ˌproʊtəˈθɪərɪə, from Greek "prōtos", first, + "thēr", wild animal) is a
taxonomic group, ortaxon , to which the order Monotremata belongs. It is conventionally ranked as a subclass within themammal s.Most of the animals in this group are extinct. The egg-laying monotremes are known from fossils of the
Cretaceous andCenozoic periods; they are represented today by theplatypus and several species ofechidna .The names Prototheria,
Metatheria andEutheria (meaning "first beasts", "changed beasts", and "true beasts") refer to the three mammalian groupings which have living representatives. Each of the three may be defined as a putativeclade comprising a livingcrown-group (respectively the Monotremata,Marsupialia andPlacentalia ) plus any fossil species which are more closely related to that crown-group than to any other 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 asinfraclass es of the subclassTheria , and in more recent proposals have been demoted further (tocohort s or evenmagnorder s), ascladistic reappraisals of the relationships between living and fossil mammals have suggested that the Theria itself should be reduced in rank. [Marsupialia and Eutheria/Placentalia appear as cohorts in McKenna & Bell 1997 and in Benton 2005, with Theria ranked as a supercohort or an infralegion, respectively.]Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of an hypothesis which defined the group by two supposed
synapomorphies : (1) formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with the alisphenoid in therians; and (2) a linear alignment of molar cusps, contrasting with a triangular arrangement in therians. These characters appeared to unite monotremes with a range ofMesozoic fossil orders (Morganucodon ta,Triconodonta ,Docodonta andMultituberculata ) in a broader clade for which the name Prototheria was retained, and of which monotremes were thought to be only the last surviving branch (Benton 2005: 300, 306).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 (Kemp 1983).
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 (Luo "et al." 2001, 2002).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. 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
References
* Benton, Michael J. 2005. "Vertebrate Palaeontology". 3rd ed. Oxford: Blackwell Publishing. ISBN 0-632-05637-1
* Kemp, T.S. 1983. The relationships of mammals. "Zoological Journal of the Linnean Society" 77: 353-84.
* Luo, Z.-X., R.L. Cifelli and Z. Kielan-Jaworowska. 2001. Dual origin of tribosphenic mammals. "Nature" 409: 53-7.
* ———. 2002. In quest for a phylogeny of Mesozoic mammals. "Acta Palaeontologia Polonica" 47: 1-78.
* McKenna, Malcolm C., and Susan K. Bell. 1997. "Classification of Mammals Above the Species Level". New York: Columbia University Press. ISBN 0-231-11013-8
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