Placental Evolution in Therian Mammals

One of the great transformations in the history of vertebrates involves the origin and early diversification of the mammals. Mammals are distinguished from other vertebrates by almost countless aspects of their anatomy, physiology, behavior, reproduction, and life history. Many of these features appear to have arisen as an integrated complex during the early evolution of mammals. An exceedingly detailed fossil record has aided our understanding of this transition, and decades of work have led to an understanding of many of the unique adaptations of this group. In particular, the intimate interdependence of reproduction, anatomy, and physiology in mammals and their roles during the transition to the mammalian condition has presented a fascinating puzzle to vertebrate biologists (e.g., Crompton 1980; Crompton and Jenkins 1973; Crompton et al. 1978; Farmer 2000; Guillette and Hotton 1986; Hopson 1973; Jenkins 1984; Kemp 2006, 2007; Koteja 2012; McNab 1978; Pond 1977; Ruben 1995). While the origin of mammals has provided one set of puzzles, the diversification of mammals is also ripe with questions. Three extant major groups of mammals exist. Monotremes retain many primitive features and are generally considered to have diverged from the therians in the Triassic to Jurassic periods (e.g., O’Leary