LIFE × EVOLUTION

The Mammals Fill the Gap

~66 to 56 million years ago

Phanerozoic · Cenozoic · Paleogene · Paleocene

Fewer than ten placental lineages are reckoned to have crossed the boundary at all. On the far side, animals the size of a rat reached the size of a bear within a few million years, and anatomy changed about three times faster than it had through the Cretaceous. Brains did not keep up: measured against the body, they got smaller before they got bigger.

Why it matters

An emptied world is rare, so there are few chances to watch what it does to survivors. The order here is not the one anyone would guess, and the reason is plain once seen: while there is space, size pays and nothing else does. Brains grew later, in the Eocene, when the space had filled and the competition was with each other.

Dating & uncertainty

Molecular clocks agree that the placental lineages split before the impact; the burst of body forms comes after it. Rates of anatomical change in the five million years following the extinction are about three times the Cretaceous background.

Sources

1. Bertrand et al. (2022), Science · Brawn before brains in placental mammals after the end-Cretaceous extinction
2. Halliday et al. (2016), Proceedings of the Royal Society B · Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous-Palaeogene mass extinction
3. Benevento et al. (2023), Palaeontology · Early Cenozoic increases in mammal diversity cannot be explained solely by expansion into larger body sizes

Earth history as one day

23:39:0320 min 56 s before midnight

if Earth’s 4.54 billion years were compressed into a single day