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K–Pg extinction: end of the non-bird dinosaurs
66 million years ago· Extinction or crisis· Earth, climate & extinctions
Well establishedStrong fossil, genetic or geological evidence.
About 66 million years ago, roughly three-quarters of species vanished, including every dinosaur except the birds.
See it on the Life map →What happened
The victims included all non-bird dinosaurs, the pterosaurs, the last big marine reptiles, the coiled-shell ammonites and much of the ocean's plankton. Many mammals, birds and plants died too. Survivors tended to be small, able to shelter in burrows or water, and able to live on seeds, roots or dead material rather than fresh plant growth. Ferns briefly carpeted devastated land before forests returned. 'K–Pg' stands for Cretaceous–Paleogene, the periods on either side.
Why it matters
It ended the age of dinosaurs and cleared the way for mammals, including our primate ancestors, and for the birds that survived.
Think of it like
Like a severe market crash: the big, heavily leveraged players fail, while small tenants with low overhead and cash reserves ride it out.
The evidence (fossils, genes, rocks)
A thin worldwide clay layer marks the boundary: dinosaur and plankton fossils stop below it, and it holds iridium, shocked quartz and a 'fern spike' of spores. 'K' is geologists' symbol for the Cretaceous.
Still debated
How the asteroid and Deccan volcanism shared the blame, and whether dinosaurs were already declining beforehand, are debated; most recent evidence says they were not in steep decline.
Inside this (2)
Connections (10)
▸K–Pg extinction: end of the non-bird dinosaurs is one of the Big Five in Extinction & mass extinctionsWell established
The connection
The K–Pg event is the most recent of the 'Big Five' mass extinctions that David Raup and Jack Sepkoski picked out in 1982 from counts of marine fossil families. It is also the only one with a confirmed asteroid cause, which made it the model case when scientists began asking what triggers mass extinctions.
When and how
University of Chicago, 1982; K–Pg ranks alongside the end-Ordovician, late Devonian, end-Permian and end-Triassic events.
The evidence
Databases of fossil marine families and genera show five sharp spikes in extinction rates above background; K–Pg is one of the clearest, though smaller than the end-Permian.
Debates and open questions
Whether the Big Five share common causes, and whether today's species losses amount to a sixth mass extinction, are actively debated.
▸Pterosaurs were ended by K–Pg extinction: end of the non-bird dinosaursWell established
The connection
Pterosaurs flew for more than 150 million years, but none are known after the K–Pg boundary. Most late Cretaceous species were large, like Quetzalcoatlus, needing lots of food and slow to breed, a poor fit for years of collapsed food chains. Small birds, by contrast, made it through.
When and how
Worldwide, 66 million years ago. Late Cretaceous pterosaur fossils come from places such as Texas, Morocco, Romania and Jordan.
The evidence
No pterosaur fossils occur above the boundary anywhere, while latest Cretaceous rocks in Morocco show several families still living right up to it (2018 study).
Debates and open questions
Whether pterosaurs were already declining, with few small species left, or were cut down in their prime is debated; their fragile bones fossilize poorly.
▸Dinosaurs were ended, except birds, by K–Pg extinction: end of the non-bird dinosaursWell established
The connection
After more than 160 million years, every non-bird dinosaur, from long-necked sauropods to Triceratops and T. rex, vanished at the K–Pg boundary. Large bodies, big food needs and slow-growing young left them exposed when the impact cut off plant growth, while some small, seed-eating ground birds survived.
When and how
Worldwide, 66 million years ago; the best record of the final dinosaurs comes from the Hell Creek Formation of Montana and the Dakotas.
The evidence
Dinosaur fossils occur close below the boundary clay in Montana and elsewhere, and none above it, apart from a few disputed reworked bones.
Debates and open questions
Whether dinosaur diversity was already falling in the last few million years is debated; recent studies suggest any apparent decline reflects gaps in the fossil record.
▸Tyrannosaurus rex was among the last victims of K–Pg extinction: end of the non-bird dinosaursWell established
The connection
T. rex lived in the final few million years before the impact and was still a top predator in western North America when the asteroid hit. As a giant with large food needs, it depended on big plant-eaters such as Triceratops and Edmontosaurus, whose food supply collapsed in the darkness after impact.
When and how
Hell Creek Formation of Montana and the Dakotas, and similar rocks in Wyoming, Alberta and Saskatchewan, up to 66 million years ago.
The evidence
T. rex bones occur in the upper Hell Creek rocks, close below the boundary clay, and in no younger rocks anywhere.
Debates and open questions
Bones are scarce in the last few meters below the boundary, once read as a sign of decline before the impact; later finds have largely filled that 'three-meter gap'.
▸Chicxulub asteroid impact caused K–Pg extinction: end of the non-bird dinosaursWell established
The connection
The impact threw dust, soot and sulfur droplets high into the atmosphere. Sunlight dimmed for years, temperatures fell sharply in an 'impact winter', and photosynthesis on land and in the sea nearly stopped. With plants and plankton failing, food chains collapsed, killing roughly three-quarters of species, especially large animals and those reliant on fresh plant growth.
When and how
Yucatán Peninsula, Mexico, 66 million years ago, with effects worldwide within hours (fires, tsunamis) and lasting years (darkness and cold).
The evidence
The extinction and the impact debris lie in the same thin layer worldwide, dated as effectively simultaneous; climate models reproduce a lethal winter.
Debates and open questions
Which killing mechanism mattered most, soot from fires, sulfur droplets or fine rock dust, is debated; a 2023 study argued fine dust was the main sunlight blocker.
▸Deccan Traps eruptions may have contributed to K–Pg extinction: end of the non-bird dinosaursDebated
The connection
Deccan eruptions released vast amounts of carbon dioxide and sulfur gases before and after the impact. Some researchers, notably Gerta Keller of Princeton, argue the volcanism stressed ecosystems through warming, acidification and cold snaps, weakening life before the asteroid or even causing most of the extinction. Most now see it as a secondary factor at most.
When and how
Western India, about 66.3–65.5 million years ago, with eruption pulses on both sides of the impact; key dating studies came from Princeton and Berkeley teams, 2015–2019.
How sure are we?
Ocean sediments show warming of roughly 2–4 °C a few hundred thousand years before the impact, along with mercury spikes linked to the eruptions.
Debates and open questions
Studies in 2020 found the extinction pattern fits the impact far better than volcanism, but whether Deccan gases slowed recovery afterward is still debated.
▸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.
▸K–Pg extinction: end of the non-bird dinosaurs was survived by Birds evolve from dinosaursWell established
The connection
Most Cretaceous bird lineages, including toothed birds and the once-common enantiornithines, died out. A few small, ground-dwelling, beaked lineages related to the ancestors of ducks, chickens and ostriches made it through, perhaps because they could eat seeds. Birds then diversified rapidly into today's roughly 11,000 species.
When and how
Worldwide, 66 million years ago; Asteriornis, a fowl-like bird from Belgium about 66.7 million years old, described in 2020, is among the oldest modern birds.
The evidence
Modern-type bird fossils from just before the boundary, and DNA family trees showing a burst of bird branching just after it; a 2018 study linked survival to ground living.
Debates and open questions
How many modern bird lineages crossed the boundary, and whether beaks and seed-eating explain their survival, are debated.
▸K–Pg extinction: end of the non-bird dinosaurs ended the last of the Marine reptiles (ichthyosaurs, plesiosaurs, mosasaurs)Well established
The connection
Ichthyosaurs had already disappeared about 90 million years ago, but plesiosaurs and giant mosasaurs were still top ocean predators. When plankton collapsed after the impact, the food chains feeding these hunters failed, and both groups vanished. Sea turtles and crocodile relatives, also marine reptiles, survived.
When and how
Oceans worldwide, 66 million years ago; latest Cretaceous mosasaur and plesiosaur fossils come from Morocco, the Netherlands (Maastricht) and the United States.
The evidence
Abundant mosasaur and plesiosaur fossils occur in rocks right up to the boundary, and none above it.
Debates and open questions
Why sea turtles and crocodilians survived when other marine reptiles did not is unclear; slower metabolisms and varied diets are leading ideas.
▸K–Pg extinction: end of the non-bird dinosaurs ended the last AmmonitesWell established
The connection
Ammonites had survived the Devonian, Permian and Triassic extinctions, each time rebounding from a few surviving lineages. The asteroid impact 66 million years ago, with its darkness, plankton collapse and acidified surface waters, killed the remaining ammonites. Their nautilus relatives, with larger, slower-growing young living in deeper water, survived.
When and how
Oceans worldwide, 66 million years ago; the last ammonite fossils lie just below the impact layer in Denmark, the United States and elsewhere.
The evidence
Ammonite fossils occur right up to the impact layer and essentially vanish above it, at sites around the world.
Debates and open questions
A few ammonites just above the boundary in Denmark suggest some lingered briefly; whether they lived then or were washed in from older rock is debated.