Why Ammonoids Were Lost During the Last Extinction

A class of marine cephalopods called Ammonoids, or ammonites, was destroyed during the last mass extinction event, after previously surviving three other events of even greater magnitude. Experts recently set out to discover what actually killed off these animals.

Ammonoids died out alongside dinosaurs about 65.5 million years ago, during the Cretaceous-Paleogene (K-T) extinction event, the last great mass dying in history. These creatures were related to modern-day squid and nautiloids.

What is especially puzzling about their demise is that it occurred even though they had proven themselves to be the ultimate survivors for the 300 million years leading up to the K-T event.

According to an international collaboration of investigators led by University of Zürich paleontologists, the reproductive strategy changes that made Ammonoids so successful when they first appeared may be what doomed them during the last extinction.

In the latest issue of the esteemed scientific journal Evolution, the team describes these changes as having a boomerang-like effect on this class of animals. Studies demonstrated some time ago that these creatures had straighter shells when they first evolved.

At the beginning of the Devonian Period, they joined other shelled animals in starting to coil their shells, a change that also affected their embryos. Scientists cannot yet say for sure why these changes occurred in the first place, but suspect that Ammonoids' natural predators may have influenced it.

“In the oldest Ammonoids, the embryonic shells were considerably bigger and coiled less tightly than in later forms,” University of Zürich paleontologist and study team member, Kenneth De Baets, explains.

Since the size of the shell increased constantly, the hatchlings themselves became smaller, leading to a much higher average weight for the Ammonoids. This suggests that the animals were able to multiply very fast after each extinction. Apparently, this did not happen after the K-T event.

"The large number of offspring could have been the key to the rapid proliferation of the ammonoids in the aftermath of each mass extinction,” De Baets concludes, quoted by Astrobiology Magazine.