Published online by Cambridge University Press: 05 June 2012
The phylogenetic and geographic origins of most extant mammalian orders are still poorly documented. Many first appear in the fossil record during the Paleocene-Eocene Thermal Maximum (PETM) at the beginning of the Eocene epoch about 55.5 million years ago (Smith et al., 2006). However, three prominent orders are exceptions to this pattern. Rodents first appeared in North America about 0.5–1.0 million years before the PETM, but probably had an Asian origin like other Glires (Meng et al., 2003). Bats and whales are not known with any certainty before Middle Ypresian, about 54 mya.
The earliest known bats are small, insectivorous forms that are preserved in both terrestrial and lacustrine fossil faunas. Their phylogenetic and geographic origins are still unknown, although the absence of clear transitional forms in the fossil record suggests that bat origins are potentially either quite ancient or their evolution from non-volant mammals was quite rapid. Although morphological evidence has generally supported an origin from within Euarchontoglires, sequence data from multiple genes strongly supports an origin of bats from within Laurasiatheria (Springer et al., 2003; Gunnell and Simmons, 2005).
The oldest known fossil bats are early-middle Early Eocene taxa, and the first members of modern bat families and superfamilies seem to appear in the fossil record in the Middle Eocene (Gunnell and Simmons, 2005). We thus here restrict the term “early bats” to the species known from the Early and early-middle Middle Eocene (Ypresian and Lutetian, and global equivalents, encompassing European mammalian reference levels MP7 through MP13). These early bats mainly include “eochiropterans” (Eochiroptera Van Valen 1979 is a controversial paraphyletic group composed of primitive taxa; see Simmons and Geisler, 1998 for an overview) and a few taxa belonging to the first members of modern families.