Molecular Classification For The Living Orders Of Placental Mammals

The well-resolved molecular tree that we present provides a basis for classifying the living orders of placental mammals (Table 5). Following McKenna and Bell (1997), we used mirorder, grandorder, superorder, magnorder, cohort, supercohort, and infralegion as successively more inclusive taxonomic ranks above the rank of order. Our classification includes clades that have been recognized in previous classifications, although in many cases these have not been recognized with formal Linnaean ranks. Our classification also includes newly named clades. All names above the ordinal level are suggested for crown-clades with node-based definitions.

We recognize Placentalia as a clade of crown-group eutherians at the tax-onomic rank of infralegion. Placentalia is divided into the supercohorts Afrotheria and Notolegia. The name Notolegia is new and is suggested for a clade with southern (Gondwanan) origins that subsequently gave rise to legions of placental taxa. Our basal split between Afrotheria and Notolegia contrasts with two competing hypotheses for the base of the placental tree, both of which are associated with likelihood scores that are only slightly lower than when the root is placed between Afrotheria and Notolegia. First, Atlantogenata (Waddell et al., 1999) versus Boreoeutheria (Springer and de Jong, 2001), which places the root between Gondwanan- and Laurasian-origin clades. Second, between Xenarthra and Epitheria; Epitheria is compatible with some morphological analyses (McKenna and Bell, 1997).

Within Afrotheria, we recognize a fundamental split between the grandorders Fossoromorpha and Paenungulata. Fossoromorpha is a newly named clade that includes aardvarks, elephant shrews, and afrosoricidans. The name Fossoromorpha is suggested based on the occurrence of fossorial adaptations in many taxa within this clade (e.g., golden moles, aardvarks). Among fossoromorphs, as well as other afrotherians that have been investigated, Macroscelidea and Afrosoricida share a haemochorial placenta (Carter, 2001). In recognition of this feature, we suggest the new name Haemochorialia for this clade. Paenungulata (= Uranotheria of McKenna and Bell, 1997), although not previously recognized at the rank of grandorder, is a feature of some morphological classifications (e.g., Simpson, 1945). Because relationships within Paenungulata have proved difficult to resolve with molecular data, we do not recognize additional classificatory structure within this group.

Notolegia includes Xenarthra and Boreoeutheria as its constituent cohorts. Xenarthra includes the orders Cingulata (armadillos) and Pilosa (sloths, anteaters). Boreoeutheria is divided into the magnorders Laurasiatheria and Euarchontoglires. Within Laurasiatheria, the superorder Variamana includes all taxa excepting the order Eulipotyphla. We suggest the name Variamana for the clade that includes Chiroptera, Perissodactyla, Cetartiodactyla, Pholidota, and Carnivora in recognition of the variable hand that occurs in different members of this group, e.g., flippers in cetaceans, wings in bats, hooves in

Table 5. Classification of living orders of placental mammals1

Infralegion Placentalia Owen, 1837, new rank

Supercohort Afrotheria Stanhope, Waddell, Madsen, de Jong, Hedges, Cleven, Kao & Springer, 1998, new rank

Grandorder Fossoromorpha, new2 Order Tubulidentata Huxley, 1872 Mirorder Haemochorialia, new Order Macroscelidea Butler, 1956

Order Afrosoricida Stanhope, Waddell, Madsen, de Jong, Hedges, Cleven, Kao

& Springer, 1998 Grandorder Paenungulata Simpson 1945 Order Hyracoidea, Huxley, 1869 Order Sirenia Illiger, 1811 Order Proboscidea Illiger 1811

Table 5. (Continued)

Supercohort Notolegia, new3 Cohort Xenarthra Cope, 1889, new rank Order Cingulata Illiger, 1811 Order Pilosa Flower, 1883 Cohort Boreoeutheria Springer & de Jong 2001, new rank Magnorder Laurasiatheria Waddell, Okada & Hasegawa, 1999, new rank Order Eulipotyphla Waddell, Okada & Hasegawa, 1999 Superorder Variamana, new4

Order Chiroptera Blumenbach, 1779 Grandorder Fereuungulata Waddell, Okada & Hasegawa, 1999, new rank Order Cetartiodactyla Montgelard, Catzeflis & Douzery, 1997 Order Perissodactyla Owen, 1848 Mirorder Ostentoria, new5

Order Carnivora Bowdich, 1821 Order Pholidota Weber, 1904 Magnorder Euarchontoglires Murphy, Stanyon & O'Brien, 2001 Grandorder Glires Linnaeus, 1758, new rank Order Lagomorpha Brandt, 1855 Order Rodentia Bowdich, 1821 Grandorder Euarchonta Waddell, Okada & Hasegawa, 1999, new rank Order Primates Linnaeus, 1758 Mirorder Paraprimates, new6 Order Dermoptera Illiger, 1811 Order Scandentia Wagner, 1855

1To maintain stability, and subject to the requirement of monophyly, traditional mammalian orders have been retained at the Linnaean rank of order. All taxa above the rank of order are intended as crown clades with node-based definitions (i.e., the most recent common ancestor of all living members of a group, plus all of the descendants, living or extinct, of this common ancestor; see examples below for newly defined groups). Our classification explicitly avoids redundant taxonomic names that fail to convey additional phylogenetic information. For example, the grandorder Fossoromorpha includes the orders Tubulidentata, Afrosoricida, and Macroscelidea. Of these, Afrosoricida and Macroscelidea are hypothesized as sister-taxa in the mirorder Haemochorialia. Tubulidentata is the sister-taxon to Haemochorialia, but we have not erected a redundant mirorder for Tubulidentata. This does not imply that Tubulidentata is incertae sedis in Fossoromorpha. We appreciate that categorical subordination is a convenient approach to tag sister-groups in Linnaean classifications, but agree with Wiley (1981) that redundant or empty categories should not be introduced unnecessarily. Also, categorical subordination is less relevant for sister-groups with node-based definitions than for sister-groups with stem-based definitions. Only in the latter case can we expect sister-groups that have precisely equivalent origination times that result from the same cladogenic event. Our classification retains flexibility for adding names for taxa with stem-based definitions.

2Definition: for the most recent common ancestor of Fossoromorpha and Tubulidentata and all of its descendants. The name Fossoromorpha was suggested by Kris Helgen.

3 Definition: for the most recent common ancestor of Xenarthra, Eulipotyphla, Chiroptera, Cetartiodactyla, Perissodactyla, Carnivora, Pholidota, Rodentia, Lagomorpha, Primates, Scandentia, and Dermoptera and all of its descendants.

4Definition: for the most recent common ancestor of Chiroptera, Cetartiodactyla, Perissodactyla, Carnivora, and Pholidota and all of its descendants. The name Variamana was suggested by Kris Helgen.

5Definition: for the most recent common ancestor of Carnivora and Pholidota and all of its descendants.

6 Definition: for the most recent common ancestor of Dermoptera and Scandentia and all of its descendants.

ungulates. Variamana is suggested as an alternative to Scrotifera (Waddell et al., 1999), which we find less appropriate for this clade. Within Variamana, our analyses support Fereuungulata (Waddell et al., 1999). Interordinal relationships within Fereuungulata are not resolved except for Carnivora + Pholidota. Numerous morphological studies unite pholidotans with xenarthrans, but there is also support for a carnivore-pholidotan alliance (Shoshani and McKenna, 1998). We suggest that Ostentoria is an appropriate name for this hypothesis given the osseous tentorium that occurs in carnivores and pangolins (Shoshani and McKenna, 1998). Waddell et al. (1999) suggested the name Ferae for this clade, but Gregory's (1910) monograph reveals that this name has a long and complicated history that begins in 1758 with Linnaeus, who included carnivores in Ferae and pangolins in Bruta. Simpson (1945) included carnivores and creodonts in the superorder Ferae and pangolins in the cohort Unguiculata.

Within Euarchontoglires, we recognize the grandorders Glires and Euarchonta. Glires includes lagomorphs and rodents and is recognized in most morphological classifications. The name Euarchonta was suggested by Waddell et al. (1999) for an emended archontan clade that includes primates, scandentians, and dermopterans, but not chiropterans. Peters (1864) included tree shrews, flying lemurs, and elephant shrews in one of his two great groups of insectivores (reviewed in Gregory, 1910). Although Haeckel (1866) removed flying lemurs from this group when he erected Menotyphla, it is now evident that Scandentia and Dermoptera are more closely related to each other than either is to elephant shrews. We suggest the new name Paraprimates for the Scandentia + Dermoptera hypothesis in recognition of the phylogenetic proximity of this clade to Primates.

Was this article helpful?

+1 0

Post a comment