Afrotheria - Evolutionary Relationships

Evolutionary Relationships

Afrotheria were originally proposed in 1998 based on analyses of DNA sequence data. However, previous studies hinting at the close interrelationships among subsets of endemic African mammals date to the 1920s, and much later sporadic papers in the 1980s and 1990s. The core of Afrotheria consists of the Paenungulata, i.e., elephants, sea cows, and hyraxes, a group with a long history among comparative anatomists. Hence, while DNA sequence data have proven essential to infer the existence of Afrotheria as a whole, and while the insectivoran-grade afrotheres (tenrecs, golden moles, sengis) would probably not have been recognized as part of Afrotheria without DNA data, there is some precedent in the comparative anatomical literature for the idea that at least part of this group forms a clade.

Since the 1990s, increasing amounts of molecular and anatomical data have been applied that support the idea that afrotherian mammals are descended from a single common ancestor to the exclusion of other mammals. On the anatomical side, features shared by most or all afrotherians include a high vertebral count, features of placentation, shape of the ankle bones, and relatively late eruption of the permanent dentition. Studies of genomic data, including millions of aligned nucleotides sampled for a growing number of placental mammals, also support Afrotheria as a clade.

Afrotheria are now recognized as one of four major groups within Eutheria (containing placental mammals). Relations within the four cohorts, Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires, and the identity of the placental root, remain somewhat controversial. Afrotheria as a clade is usually discussed without a Linnaean rank, but has been assigned the rank of cohort or magnorder, and superorder. One reconstruction that applies the molecular clock proposes that the oldest split occurred between Afrotheria and the other three some 105 million years ago when the African continent was separated from other major land masses. This idea is consistent with the fossil record of Xenarthra, which is restricted to South America (following recent consensus that Eurotamandua is not a xenarthran). However, Afrotheria itself does not have a fossil record restricted to Africa, although this does seem to be true for the oldest, undisputed afrotherians. Furthermore, the correspondence of Afrotherian origins with the Africa-South America tectonic split is not consistent with other applications of the molecular clock or with the mammalian fossil record. More recent, genomic-scale phylogenies favor the hypothesis that Afrotheria and Xenarthra comprise sister taxa at the base of the placental mammal radiation.

Relations between the orders of the Afrotheria are still being studied. On the basis of molecular studies, it appears that elephants and manatees may be related, and likewise elephant shrews and aardvarks. These findings are compatible with the work of earlier anatomists.