Was world’s biggest ammonite’s size a defensive strategy?

The world’s largest ammonite, Parapuzosia seppenradensis, was a whopping 1.74 metres wide when it died near modern-day Seppenrade, Germany, and was preserved for around 80 million years.

The first uncovered specimen stunned the world when it was discovered at Seppenrade in 1895; there are now 10 known specimens from the same period from that part of Germany, one of which is mounted in a Seppenrade pub.

But the puzzle of this ancient marine colossus’ gigantic size relative to other known ammonites – some of which could be as small as an inch across – has long fascinated scientists.

In a new study in the journal PLoS One, scientists from Germany, Mexico and the UK say that while this evolutionary quirk remains as yet unsolved, the smoking gun may lie in the evolutionary trajectory of co-existent predators. Around the time P. seppenradensis was floating in the late-Campanian seas of Europe and, later, the Gulf of Mexico, mosasaurs, the ocean’s top predator at the time, were also growing dramatically.

Mosasaurs were fearsome, giant marine reptiles with powerful jaws that lived around the same time as ammonites; some of the largest specimens grew to around 15m in size. Ammonites are a common fossil, belonging to a type of shelled cephalopod that died out around 66 million years ago. In the past, the tightly wound ammonite fossils were thought to be the coiled-up bodies of dead snakes that had petrified.

Their coiled, chambered shells bore the story of their life history; ammonites were born with tiny shells and new chambers in the shells would grow as they grew. They would move their entire body into the new chamber, before sealing off their old, cramped living quarters with walls called septa.