Placental mammals split from marsupials
The two great live-bearing lineages part ways, and the fossils and the molecules disagree about when.
- When
- —
- Duration
- 34 million years
- Known to
- ± 15 million years
- Era
- Cretaceous, Mesozoic
Marsupials and placentals diverge in the Cretaceous, but the timing of the placental radiation is one of the sharper disputes in the field. Molecular clocks generally place the origins of the modern orders — primates, rodents, carnivorans, ungulates — well within the Cretaceous, implying they coexisted with dinosaurs for tens of millions of years. The fossil record shows almost nothing recognisable as a modern order until after the extinction. The likely resolution is a long fuse: lineages splitting early but staying small and generalised until the ecological ceiling was removed.