Timing and biogeography of the eutherian radiation: fossils and molecules compared

Abstract

Theria includes Eutheria and its sister taxon Metatheria. Placentalia includes extant eutherians plus their most recent common ancestor. The oldest eutherian is from 125 mya (million years ago). Molecular studies place this origin at about 130–185 mya. Older dates cannot be refuted based on fossil evidence as earliest eutherian remains are scarce. Earliest superordinal clades (hence Placentalia) range from 64–104 mya (median 84 mya) based on molecules, similar to 85–90 mya based on fossils. Superordinal clades Archonta, Ferungulata, Glires, and Paenungulata based on fossils are similar to molecularly based clades, except Afrotheria was not predicted by fossils. Both fossils and molecules recognize 16 of 18 extant placental orders. Fossils place the origins of orders around 65 mya as do some molecular studies, but others suggest ordinal diversification as old as 100 mya. Fossil evidence supports a Laurasian origin for Eutheria (and Metatheria) and Placentalia, although some molecular studies suggest a Gondwanan origin for both taxa.