2020 Volume 60 Issue 2 Pages 251-267
The topic of macroevolution of mammals has recently been disputed by paleontologists and molecular biologists, specifically in regards to the dispersal timing of crown Placentalia (crown Eutheria) and placental orders around the Cretaceous/Paleogene (K/Pg) boundary (ca. 66 Ma). Except Monotremata, there is no positive fossil evidence that Marsupialia (crown Metatheria) and Placentalia existed in the Mesozoic. Current fossil records from Cenozoic localities indicate that the oldest species included in the placental orders appeared at the beginning of the Paleocene (ca. 60–50 Ma). This strongly supports the explosive divergence model, which claims that crown placentals evolved immediately after the K/Pg boundary. However, this hypothesis is criticized primarily because the interval of diversification after the K/Pg boundary is too short in terms of molecular evolution of mammals. A plausible model that agrees with both paleontological and molecular phylogenetic studies explains that major crown clades of placentals (e.g., Xenarthra and Laurasiatheria) originated in the Late Cretaceous and then ordinal groups dispersed in the early Paleocene.
