The story
Ammonites are shelled cephalopods, relatives of squid and octopus, with a coiled shell split into chambers. The Great Dying left only a handful of lineages, yet within a few million years they radiate again in the Early Triassic and become one of the great success stories of Mesozoic seas. Their shells record each growth stage, with wavy internal lines called sutures, and species evolved so fast that geologists use them as index fossils for dating rock layers.
A tube called the siphuncle let the animal pump gas and fluid between chambers to control buoyancy. They ate small prey and were eaten by ichthyosaurs, plesiosaurs and mosasaurs. The largest, Parapuzosia seppenradensis from Germany, has a shell about 1.8 m across. Ammonites survived the end-Triassic extinction by a narrow margin and then thrived until the asteroid at the end of the Cretaceous finished them, along with all the rest of their group.
Why it mattered
- Ammonite species changed so quickly that they became the clock for Jurassic and Cretaceous geology.
- They were a main food source for marine reptiles, linking to the rise of ichthyosaurs and mosasaurs.
- Their loss at the end of the Cretaceous is one of the sharpest markers of that extinction in the marine record.
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