Scientists have discovered how trilobites survived two mass extinctions while everything else died. The key is flexible adaptation to the environment.

Scientists have discovered how a unique trilobite species could survive and defend itself against predators during Earth’s oxygen level fluctuations. Trilobites, ancient sea-dwelling creatures related to spiders and lobsters, were abundant in the seas for nearly 300 million years, starting in the Cambrian Period. They survived two mass extinctions and dominated ocean floor ecosystems during their long existence.

These odd little creatures have three main body sections; a head, a middle section, and a rigid tail. There are over 20,000 known trilobite species, and most mature ones have a specific number of segments in their mid-sections. However, scientists found that Aulacopleura koninckii was different. Although each early growth stage was similar in size and shape, mature Aulacopleura had anywhere between 18 and 22 mid-section segments, which is a unique characteristic of this species.

“Seeing trilobites with variable numbers of segments in the thorax is like seeing humans born with different numbers of vertebrae in their backs,” Hughes said.

In a recent study published in the Proceedings of the Royal Society B: Biological Sciences, researchers explored an anomaly that affected the ability of animals to protect themselves and why it developed in this way. The study provides answers to these questions.

Similar to modern pillbugs or “rollie pollies”, trilobites would curl up into a ball shape to protect themselves from predators such as large squid-like creatures or fish. While in this position, they could protect their soft tissues with their hard exterior skeletons by tucking their tails under their heads. The study found that Aulacopleura could only protect itself during rolling up when it was in a smaller, immature form with less than 18 segments in the middle, as shown through 3D modeling.

“As the number of segments increased, the body proportions did not allow them to tuck their posteriors neatly under their heads and still be completely shielded,” Hughes said. “So, why did this species keep adding segments anyway, and how could it survive the nasty predators?”

Based on their virtual reconstructions, it seems highly likely that when Aulacopleura with a large number of mid-segments felt threatened, they would roll up like their relatives and simply let their tails extend past their heads, minimizing the exposed gap.

“Other possible defense maneuvers would have left gaps on the sides that exposed critical organs — highly unlikely,” Hughes said.

As to the question of why this trilobite varied in the number of mid-section segments, the researchers turned to their earlier work. “What is underneath these segments? Legs that serve as gills!” Hughes said. “The more segments, the more surface area for respiration.”

Additional breathing apparatus likely allowed these animals to survive drops in oxygen levels on the seafloor, which other species could not tolerate. This forced predators to retreat to areas with enough oxygen to survive. However, larger Aulacopleura, with their extra gills, could remain in place without any predators.

Studying how this species adapted to biological and physical pressures can provide insight into the evolution of survival strategies. How trilobites developed can help scientists understand how the common ancestor of modern arthropod groups, such as insects and arachnids, first evolved.

“One of the reasons to study these animals is to study how development itself has evolved,” Hughes said. “It’s not so much that the meek will inherit the Earth, but the flexible.”