History of classification Prototheria Originally, monotremes were classified as a subclass of mammals known as
Prototheria. The names Prototheria,
Metatheria and
Eutheria (meaning "first beasts", "changed beasts", and "true beasts", respectively) refer to the three mammalian groupings that 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 that 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
infraclasses of the subclass
Theria, and in more recent proposals have been demoted further (to
cohorts or even
magnorders), as
cladistic reappraisals of the relationships between living and fossil mammals have suggested that the Theria itself should be reduced in rank. Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of a hypothesis that 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 of
Mesozoic fossil orders (
Morganucodonta,
Triconodonta,
Docodonta 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 (Benton 2005: 300, 306).
Australosphenida hypothesis and Yinotheria The evidence that was held to support Prototheria is now universally discounted. In the first place, the 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 that 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 that 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). Australosphenida is characterized by the shared presence of a cingulum on the outer front corner of the lower molars, a short and broad talonid, a relatively low trigonid, and a triangulated last lower premolar. The Australosphenida hypothesis remains controversial; for example, lingual cingula seem to be a presence in various non-australosphenidan mammals and some work has shown the possibility of Eutheria being the sister group to Australosphenida, without monotremes. As a result, 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 that 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. When systematic work was performed, it was also found that Australosphenida is the sister taxon to
Shuotheriidae, an obscure group of Mesozoic mammals that were found in what is now
China. had this to say regarding the Shuotheriidae, particularly
Shuotherium: Yinotheria is named for this grouping. Other scholars have rejected Yinotheria, finding instead that Shuotheriidae is closely related to
Docodonta outside crown Mammalia. •
Ambondro mahabo Flynn et al. 1999 • Genus
Asfaltomylos Rauhut et al. 2002 •
Asfaltomylos patagonicus Rauhut et al. 2002 • Genus
Henosferus Rougier et al. 2007 •
Henosferus molus Rougier et al. 2007 • Order
Monotremata Bonaparte 1837 sensu Luo, Cifelli & Kielan-Jaworowska 2001 • Family
Incertae sedis • Genus
Kryoryctes Pridmore et al. 2005 (either a stem-monotreme or junior synonym of
Steropodon) •
Kryoryctes cadburyi Pridmore et al. 2005 • Family
Tachyglossidae Gill 1872 [
Echidnidae Burnett 1830] (echidnas, spiny anteaters) • Genus
Megalibgwilia Griffiths, Wells & Barrie 1991 (subjective synonym or subgenus of
Zaglossus) •
Megalibgwilia robusta (Dun 1895) (subjective synonym of
Zaglossus robustus) •
Megalibgwilia ramsayi (Owen 1884) (senior synonym of
Megalibgwilia owenii) • Genus
Zaglossus Gill 1877 [
Proechidna Gervais 1877;
Acanthoglossus Gervais 1877;
Bruynia Dubois 1882;
Bruijia Thomas 1883;
Prozaglossus Kerbert 1913] (Long-beaked Echidnas) •
Murrayglossus Glauert 1914 (Hackett's Giant Echidna) (might belong to a new genus) •
Zaglossus bruijni (Peters & Doria 1876) (Western Long-beaked/Bruinj's/Brown False Echidna) •
Zaglossus attenboroughii Flannery & Groves 1998 (Attenborough's/Sir David's/Cyclops Long-beaked Echidna) •
Zaglossus bartoni Thomas 1907 (Eastern/Barton's Long-beaked/Grey False Echidna) • Genus
Tachyglossus Illiger 1811 [
Acanthonotus Goldfuss 1809;
Echidna Cuvier 1798;
Echinopus Fischer de Waldheim 1814;
Syphomia Rafinesque 1815] •
Tachyglossus aculeatus (Shaw 1792) Illiger 1811 (short-beaked Echidna) • Family
Kollikodontidae Flannery et al. 1995 • Genus
Kollikodon Flannery et al. 1995 (might be a
mammaliform of uncertain placement) •
Kollikodon ritchiei Flannery et al. 1995 • Family
Steropodontidae Archer et al. 1985 • Genus
Teinolophos Rich et al. 1999 (might be a basal monotreme or a stem-monotreme) •
Teinolophos trusleri Rich et al. 1999 • Genus
Steropodon Archer et al. 1985 •
Steropodon galmani Archer et al. 1985 • Family
Ornithorhynchidae Gray 1825 • Genus
Monotrematum Pascual et al. 1992 (here considered to be a basal ornithorhynchid; others subjective synonym of
Obdurodon) •
Monotrematum sudamericanum Pascual et al. 1992 • Genus
Obdurodon Woodburne & Tedford 1975 •
Obdurodon insignis Woodburne & Tedford 1975 •
Obdurodon tharalkooschild Pian, Archer & Hand 2013 •
Obdurodon dicksoni Archer et al. 1992 (Riversleigh platypus) • Genus
Ornithorhynchus Blumenbach 1800 [
Dermipus Wiedermann 1800;
Platypus Shaw 1799 non Herbst 1793] •
Ornithorhynchus anatinus (Shaw 1799) Blumenbach 1800 (platypus)
Phylogeny Below is a simplified tree on Averianov
et al., 2014 after Woodburn, 2003 and Ashwell, 2013 }} ==References==