Life › Seas, forests & the move to land › Great Ordovician Biodiversification Event
Ammonites
c. 410–66 million years ago· Organism or group· Animals
Well establishedStrong fossil, genetic or geological evidence.
Squid-like sea animals with coiled, chambered shells that thrived from about 410 to 66 million years ago.
See it on the Life map →What happened
Ammonites (strictly, ammonoids) were cephalopods, relatives of squid and octopus, that evolved in the Devonian from straight-shelled nautiloids, a group that had flourished in the Ordovician radiation. Gas-filled chambers kept their shells buoyant, and the walls between chambers grew ever more frilly over time. Most were a few centimeters to tens of centimeters across; the largest known shell is about 1.8 m. They nearly died out in the Late Devonian, end-Permian and end-Triassic crises before the K–Pg extinction ended them.
Why it matters
Because species changed fast and spread widely, ammonites are prime index fossils, used to date Mesozoic rocks to within a million years or less.
The evidence (fossils, genes, rocks)
Abundant shells in marine rocks worldwide, found in orderly sequences, plus rare fossils preserving jaws and stomach contents. Medieval English legend called them snakes turned to stone.
Still debated
Why ammonites died at the K–Pg while their nautilus cousins survived; one idea is that their tiny floating young were hit hardest by the collapse of plankton.
Connections (2)
▸The Great Dying (end-Permian extinction) nearly wiped out AmmonitesWell established
The connection
Ammonites and their relatives, shelled cousins of squid and octopus, were common in Permian seas. The Great Dying cut them down to very few surviving lineages. Those survivors rebounded fast, diversifying again within a million or two years, and the group thrived until the asteroid impact 66 million years ago.
When and how
Seas worldwide about 252 million years ago; boundary sections in South China and Pakistan's Salt Range record the losses and the rebound.
The evidence
Fossil shells counted bed by bed show most groups disappearing at the boundary, followed by a burst of new Triassic forms.
Debates and open questions
How many lineages survived, perhaps only one or two, and why ammonoids recovered so much faster than most other groups are debated.
▸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.