Life › The age of mammals
Mammals radiate after the dinosaurs
from 66 million years ago· Event· Animals
Well establishedStrong fossil, genetic or geological evidence.
After the dinosaurs vanished, mammals rapidly diversified into the empty roles, producing most of today's major mammal groups.
See it on the Life map →What happened
For about 160 million years, mammals lived in the dinosaurs' shadow, most no bigger than a cat. After the extinction, they spread into the vacant niches. Within about 15 million years there were early bats, whales, primates, rodents, hoofed mammals and meat-eaters, and some plant-eaters weighed hundreds of kilograms. Placental mammals, which nourish their young in the womb through a placenta, led the boom, while marsupials thrived in South America and Australia.
Why it matters
It produced the mammal groups alive today, from mice to blue whales, and ultimately the primates that led to humans.
Think of it like
Like a downtown after the anchor tenants leave: small, nimble businesses expand into the vacant space and grow into new anchors.
The evidence (fossils, genes, rocks)
Fossil beds such as Colorado's Corral Bluffs (published 2019) show mammals growing larger within a million years of the impact; DNA family trees of living mammals give branching dates.
Still debated
Molecular clocks suggest many mammal groups split before the impact, while fossils show their boom after. How much was a 'long fuse' versus a sudden explosion is debated.
Inside this (4)
Connections (7)
▸First mammals survived to launch Mammals radiate after the dinosaursWell established
The connection
Mammals had already existed for about 160 million years, mostly small but varied: swimmers, gliders, burrowers. Small size, burrowing and broad diets helped some lineages survive the K–Pg extinction, though most species died. Those survivors, especially early placentals, quickly filled the roles the dinosaurs left empty.
When and how
Survivors crossed the boundary 66 million years ago; good records come from Montana's Hell Creek area and Colorado's Corral Bluffs.
The evidence
Fossil teeth and jaws above and below the boundary show which groups persisted; a 2016 study estimated about 93% of mammal species in western North America died.
Debates and open questions
Which placental lineages already existed before the impact is debated, since molecular clocks and fossils give different answers.
▸K–Pg extinction: end of the non-bird dinosaurs opened the world to Mammals radiate after the dinosaursWell established
The connection
With non-bird dinosaurs gone, roles like large plant-eater and top predator stood empty on every continent. Surviving mammals grew bigger and more varied fast: in Colorado, the largest reached about 50 kilograms within roughly 700,000 years, and new groups appeared within a few million years. Recovering forests supplied fruit and seeds.
When and how
Worldwide from 66 million years ago; Corral Bluffs, near Colorado Springs, described in 2019 by Tyler Lyson and Ian Miller of the Denver Museum of Nature & Science.
The evidence
Fossil sequences across the boundary show mammal size, diet and diversity climbing quickly afterward, alongside fossil leaves and pollen recording forest recovery.
Debates and open questions
Whether dinosaurs, or climate and plants, kept mammals small before the impact is debated; some mammal groups were already diversifying in the late Cretaceous.
▸Mammals radiate after the dinosaurs gave rise to Whales return to the seaWell established
The connection
Among the hoofed mammals that spread after the dinosaurs, one even-toed group living near water in South Asia began feeding in rivers and along coasts. Within about 10 million years its descendants went from wading to fully marine, taking over large ocean-predator roles left open since the marine reptiles vanished.
When and how
Shores of the ancient Tethys Sea, in present-day Pakistan and India, about 50–40 million years ago.
The evidence
Dated fossil series from Pakistan and India, the shared ankle bone with hoofed mammals, and DNA placing whales next to hippos among even-toed hoofed mammals.
Debates and open questions
Why these mammals took to water is debated: new food sources, escape from predators, or open ocean niches after marine reptiles vanished.
▸Mammals radiate after the dinosaurs produced early PrimatesWell established
The connection
Primates are one of the placental lineages that diversified after the impact. Squirrel-like plesiadapiforms such as Purgatorius, possible early primates, appear within a few hundred thousand years of the extinction, feeding on fruit and insects in recovering forests. Clear primates follow about 10 million years later.
When and how
Montana, about 66–65 million years ago (Purgatorius); first clear primates in Asia, Europe and North America about 56 million years ago.
The evidence
Teeth and ankle bones of Purgatorius, described in detail in 2015, show tree-climbing traits shared with primates.
Debates and open questions
Molecular clocks suggest the primate line split off before the impact, perhaps 80–70 million years ago, but no confirmed Cretaceous primate fossils are known.
▸Mammals radiate after the dinosaurs gave rise to HorsesWell established
The connection
Horses come from the odd-toed hoofed mammals, the group that also includes rhinos and tapirs, which appear suddenly around 56 million years ago. The earliest horses were small leaf-eaters in warm forests. From there, the horse family branched repeatedly, mainly in North America, over the next 50 million years.
When and how
North America and Europe, about 56–55 million years ago; Sifrhippus, from Wyoming's Bighorn Basin, is among the oldest known horses.
The evidence
Fossil teeth and limbs from Wyoming and Europe, and DNA and protein family trees placing horses with rhinos and tapirs.
Debates and open questions
Where odd-toed hoofed mammals came from is debated; a 2014 study pointed to India, with the 54.5-million-year-old Cambaytherium as a close relative.
▸PETM: a sudden hothouse spurred the spread of Mammals radiate after the dinosaursStrong evidence
The connection
During the PETM's warmth, forests extended toward the poles, opening routes across Arctic land bridges between Asia, Europe and North America. Early primates, even-toed hoofed mammals (ancestors of deer and pigs) and odd-toed ones (ancestors of horses) appeared across the northern continents geologically almost at once. Some mammals also shrank as they adapted to the heat.
When and how
Northern continents, about 56 million years ago; Wyoming's Bighorn Basin holds the clearest record, studied by Philip Gingerich and others.
How sure are we?
In the Bighorn Basin these groups first appear exactly at the carbon-isotope shift marking the PETM; early horse Sifrhippus shrank by about 30%, then grew back.
Debates and open questions
Where these groups came from, probably Asia, and whether the heat itself drove their spread or simply opened migration routes is debated.
▸Messel Pit (Germany) preserves a snapshot of Mammals radiate after the dinosaursWell established
The connection
About 19 million years after the extinction, Messel captures a mature mammal community: early horses, primates, bats, rodents, and anteater-like and pangolin-like mammals in a warm subtropical forest. Fur outlines, stomach contents and even unborn young reveal diets, behavior and biology partway through the mammal boom.
When and how
Near Darmstadt, Hesse, Germany, about 47 million years ago; excavated systematically since the 1970s, largely by the Senckenberg research institute.
The evidence
Thousands of articulated skeletons in dated lake sediments; stomach contents and preserved soft tissue allow direct study of diet and anatomy.
Debates and open questions
How representative Messel is of wider Europe is debated, because a lake mostly preserves animals living near water or falling in.