The trade-off that helped some trilobites survive mass extinctions

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Image of a grey rock containing a fossilized trilobite.

Hundreds of millions of years before anthropogenic climate change was ever a thing, life on Earth suffered mass extinctions due to climate change brought on by natural causes. Even so, there were still organisms that held on and adapted to survive an otherwise lethal lack of oxygen. Trilobites managed to make it through two devastating mass extinctions and still escape any surviving predators.

As paleobiologists Jorge Esteve of the University of Madrid and Nigel Hughes of UC Riverside found, one trilobite species, Auracopleura koninckii, was especially successful when oxygen in the ocean reached dangerously low levels. This creature persevered because of an unusual adaptation. Like most trilobites, A. koninckii curled its segmented body into a tight ball to avoid being eaten. But it also kept growing more segments with additional legs that doubled as gills.

Even though the additional segments could not roll up completely, more legs meant more breathing opportunities, which gave this species an advantage when oxygen levels were low.

Keep on rolling

Fossils of A. konickii have already been found in a curled position that is thought to have deterred predators from trying to take a bite. Trilobites could pull this off because their thoracic segments fit right above their heads when rolled up, much like modern pillbugs. The problem is that many of the A. knoickii fossils were also flattened by sediment that accumulated on the seafloor, making it difficult to interpret their anatomy. Still, there are a few specimens that kept their shape after some 250 million years. Esteve and Hughes studied a particular specimen, labeled NMP-L12807, that is impeccably preserved in its rolled posture.

When Hughes and Esteve compared their A. koninckii specimen to others in different developmental phases, they saw that younger individuals with up to 17 segments could roll into a perfect ball with their heads just under the tips of their tails. As they mature, however, the trilobites would add segments with each molt. By the time they had 18 to 22, they could no longer do this tuck. Simulations of longer trilobites rolling up showed that a part of the tail would have had to extend over the head.

“Our analysis suggests that in A. koninckii, a transition in enrollment style took place at the point at which the number and size of trunk segments exceeded the possibility for [rolling into a perfect ball],” the scientists said in a study recently published in Proceedings of the Royal Society B.

From the bottom up

Why would A. koninckii evolve to add more segments to its body if that could make it vulnerable to attack? What would otherwise be a disadvantage was actually an adaptation to diminishing oxygen levels near the seafloor it lived on. Every segment on a trilobite had two legs, which functioned as gills that helped them breathe by absorbing oxygen from the water and releasing carbon dioxide. More legs meant more incoming oxygen. Esteve and Hughes think that a dwindling oxygen supply could have been the reason that these trilobites started to add extra segments once they reached maturity.

There is still the issue of predators, but A. konickii probably had far fewer encounters with things with gaping mouths and too many teeth. The mass extinctions it survived by adding thoracic segments, which increased its breathing capabilities, meant a grim fate for creatures that could not adapt to anoxia, and predators were no exception. Another way of looking at its evolution, at least as the scientists suggested, is that fewer predators meant that this trilobite could tolerate developing more breathing equipment because it wasn’t at risk from snapping jaws.

“Above a certain size, threshold predatory pressure on A. koninckii [possibly] declined, mitigating the need for [tight rolling] and succeeded by evident variation in trunk segment numbers,” the researchers said in the same study.

Maybe this was a trade-off, but it apparently worked to the trilobite’s advantage. Organisms sometimes evolve in a way that protects them against the greatest of multiple dangers. A. koninckii must have been more threatened by a lack of oxygen than predators when it started adding extra segments, and the gills those segments added gave it an edge when it came to survival.

Proceedings of the Royal Society B, 2023. DOI: 10.1098/rspb.2023.0871 (About DOIs).

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