- The Paleontological Society
Where did the modern lineages of placental mammals originate? Recent molecular data seemingly have overturned not only schemata of placental relationships based on morphological data, but also hypotheses about the time and place of origin of the modern lineages. The original hypothesis of Northern Hemisphere origin, based on the fossil record, has been replaced by a “Garden of Eden” hypothesis of origins on a southern continent, based on molecular phylogenies. But, do the molecular data really support this new view of placental mammal origins?
Recent molecular phylogenies have converged on the finding of two basal placental clades (Springer et al. 1997; Stanhope et al. 1998; Madsen et al. 2001; Murphy et al. 2001), Afrotheria and Xenarthra, each endemic to separate southern continents, and a third clade, Boreoeutheria, of apparent northern continent affinity (Murphy et al. 2001). This finding, in association with molecular divergence estimates starting in the mid Cretaceous (see Springer et al. 2003 for a recent synthesis), has been used to support a mid Cretaceous origin of placentals in Gondwana (Eizirik et al. 2001; Murphy et al. 2001). These divergence time estimates imply a radiation of the ordinal lineages starting ∼105 million years ago (Springer et al. 2003), millions of years prior to the appearance of undoubted crown placentals in the early Cenozoic (Foote et al. 1999; Novacek 1999; Meng et al. 2003; Wible et al. 2004; Asher et al. 2005).
A biogeographic scenario proposed to account for Gondwanan origins calls for vicariance, associated with mid-Cretaceous separation of Africa from South America, followed by later dispersal into the northern continents in the Late Cretaceous. This biogeographic scenario has been criticized on varied paleontological grounds (Ji et al. 2002; Archibald 2003; Luo et al. 2003; Hunter and Janis …