LIFE × EVOLUTION

The Mammals Fill the Gap

~66 to 56 million years ago

Phanerozoic · Cenozoic · Paleogene · Paleocene

Fewer than ten lineages of placental mammals are reckoned to have come through the end-Cretaceous extinction 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 sequence is not the one anyone would guess, and the reason is plain once seen: while there is space, size pays first. Brains grew later, in the Eocene, once ecosystems had filled and competition intensified.

Dating & uncertainty

Dating the mammal family tree places the origin of the placentals in the Late Cretaceous, 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