Fractal structures in the ammonoid shells millions of years ago gave the animals special protection from predators when they bit them

A Kosmoceras ammonoid fossil. A CT scan render. Source: Robert Lemanis

Ammonoids are a group of extinct marine mollusk animals that are now an iconic fossil group often collected by amateurs. Over 350 million years of evolution, ammonoids developed increasingly elaborate shells with fractal-like geometry. For about 200 years, scientists have debated why these animals show a trend of increasing complexity in their shell structures. Now, researchers from the Technical University Dresden have found evidence that it probably was a defense strategy against a wide array of predators.
For a long time, it had been puzzled whether perhaps the reason for the detailed structures in the animals‘ chambers was the water pressure in the depth of the seas, the muscle attachments in the body, the conditions of the respiration, or the possibility to dive up and down.

The finding of the team around Dr. Robert Lemanis and Dr. Igor Zlotnikov from the B CUBE – Center for Molecular Bioengineering at TU Dresden indicate a fascinating correlation between the evolving complexity of the ammonoid shell and its resilience against external forces.

As these ancient creatures roamed the oceans, their shells shielded them against predators and other environmental factors. The researchers‘ simulations now show that the fractal patterns in the septa of the chambers give particular resistance to punctual stress. Such stresses are typical when predators bite.

Ammonoid shells of the different species are common fossils. Photo: KHeitland / <a href=""target="_blank">Wikimedia Commons</a>, <a href=""target="_blank">Creative Commons License</a>

At the same time, the shells are comparatively thin, at least in the present-day relatives of the animals. The material, for example, in the Nautilus cephalopod, is aragonite, a form of calcium carbonate, which gives the 3 layers stability and strength. On the very inside lies a mother-of-pearl coating.

B CUBE – Center for Molecular Bioengineering was founded as a Center for Innovation Competence within the initiative “Unternehmen Region” of the German Federal Ministry of Education and Research. It is part of the Center for Molecular and Cellular Bioengineering (CMCB). B CUBE research focuses on the investigation of living structures on a molecular level, translating the ensuing knowledge into innovative methods, materials and technologies.

Science Advances

B CUBE – Center for Molecular Bioengineering

(15.08.2023; 08.15.2023)