Though now extinct, ammonoids (a group of marine mollusks with shells) evolved for 350 million years, always with a tendency toward increasingly elaborate shell structures. Scientists have studied their shells — fractal structures of increasing complexity — to try to decipher the driving force behind the shells’ shape and its evolution over time.
"Over the course of 350 million years of evolution, ammonoids repeatedly evolved shells with increasingly complex inner walls. The persistence and repetitiveness of this trend imply some driving force; the question that has long remained unanswered is: what driving force? Opposition to water pressure, muscle attachments, respiration, Cartesian devils. All of these have been proposed as explanations for this trend but evidence for them is scarce. So we decided to explore a neglected idea," explains Dr. Robert Lemanis, one of the two authors of a recent article with a hypothesis to explain why the shells evolved the way they did.
Dr. Lemanis and Dr. Igor Zlotnikov used linear and nonlinear mechanical simulations to demonstrate that the increase in the complexity of the shell’s geometry leads to an increase in the shell’s stability and strength. Their models showed a correlation between the fractal geometry shaping a shell’s internal walls, and its structural stability against “buckling” forces.
They theorize that ammonoids evolved this shape to protect them from predators and other forces that might crack their shells, affecting their well-being.
"Consider that the ammonoid shell was a relatively thin structure and once it was fractured, the animal could not repair it. A robust shell -- one that can resist the damage -- provided higher chances of survival," says Dr. Lemanis.
The implications of their findings are vast. "Our work bridges biology and engineering, underscoring how animals harness the power of fractal morphology to design more robust biomaterials. It can provide inspiration for resilient structural designs," explains Dr. Zlotnikov.
Sources: ScienceDaily, Science Advances