Laurasiatheria


Laurasiatheria is a clade of placental mammals that includes shrews, even-toed ungulates, whales, bats, odd-toed ungulates, pangolins, and carnivorans, among others. The clade originated on the northern supercontinent of Laurasia. Its last common ancestor is supposed to have diversified ca. 76 to 91 million years ago.

Classification and phylogeny

Laurasiatheria was discovered on the basis of the similar gene sequences shared by the mammals belonging to it; no anatomical features have yet been found that unite the group. The Laurasiatheria clade is based on DNA sequence analyses and retrotransposon presence/absence data. The name comes from the theory that these mammals evolved on the supercontinent of Laurasia, after it split from Gondwana when Pangaea broke up. It is a sister group to Euarchontoglires with which it forms the clade Boreoeutheria. Laurasiatheria includes the following extant taxa:
Uncertainty still exists regarding the phylogenetic tree for extant laurasiatherians, primarily due to disagreement about the placement of Chiroptera and Perissodactyla. Based on morphological grounds, Chiroptera had long been classified in the superorder Archonta until genetic research instead showed their kinship with the other laurasiatherians. The studies conflicted in terms of the exact placement of Chiroptera, however, with it being linked most closely to groups such as Eulipotyphla, Ferae or with Perissodactyla and Ferae in the Pegasoferae proposal. A 2011 study found that "trees reconstructed for the 1,608-gene data set fully support a basal position for Eulipotyphla and a more apical position for Chiroptera" and concluded that "Pegasoferae does not appear to be a natural group." A 2012 study supports the previous conclusions using a large genomic dataset, and places Eulipotyphla as a basal order and Chiroptera as sister to Cetartiodactyla, with maximal support for all nodes of their phylogenetic tree. The exact position of Perissodactyla remains less certain, with some studies linking it with Ferae into a proposed clade Zooamata while others unite it with Cetartiodactyla into Euungulata, a clade of 'true ungulates', yet some authors found better support for the latter, while others found Perissodactyla to be sister to Carnivora.
Two 2013 studies retrieve that chiropterans, carnivores, and euungulates form a clade, therefore involving that Eulipotyphla might be the sister group to all other Laurasiatheria taxa.
Laurasiatheria is also posited to include several extinct orders and superorders. At least some of these are considered wastebasket taxa, historically lumping together several lineages based on superficial attributes and assumed relations to modern mammals. In some cases, these orders have turned out to either be paraphyletic assemblages, or to be composed of mammals now understood not to be laurasiatheres at all.