Researchers have unraveled the defense strategy of a peculiar trilobite species, a distant ancestor of spiders and lobsters, which managed to survive tumultuous shifts in Earth's oxygen levels. Trilobites thrived in the seas for a staggering 300 million years, from the Cambrian Period 520 million years ago, outlasting even the dinosaurs. Despite facing two mass extinctions, they dominated ocean ecosystems with their distinctive three-part armored bodies.
While most trilobite species exhibited a consistent number of segments in their middle sections upon maturity, a unique find was made with Aulacopleura koninckii. Unlike its counterparts, this species displayed an unusual range of 18 to 22 mid-section segments during adulthood. Nigel Hughes, a paleobiologist from the University of California Riverside, expressed bewilderment at this discovery, highlighting the puzzle it poses within the context of its contemporaneous trilobite counterparts.
Describing the trilobite's variable segments in its thorax, Hughes likened it to humans having differing numbers of vertebrae. In a study published in the Proceedings of the Royal Society B: Biological Sciences, the researchers sought to unravel this anomaly and its impact on the creature's defense mechanism.
Similar to modern-day pillbugs, trilobites adopted a protective ball shape to shield themselves from predators like squid-like creatures and fish. Rolling up enabled them to safeguard their soft tissues with their tough outer skeletons. Aulacopleura, however, displayed a unique behavior – 3D modeling revealed that rolling up provided effective protection for smaller, less-segmented forms.
Despite an increase in segment count, larger Aulacopleura couldn't tuck their tails securely under their heads while rolled up. Yet, intriguingly, it seems that when threatened, these trilobites would extend their tails past their heads, cleverly minimizing exposure.
As for the variation in mid-section segments, the researchers turned to their earlier findings. “Legs that serve as gills” were discovered beneath these segments, leading to a fascinating hypothesis. More segments meant more gill-covered surface area, which likely helped the trilobite adapt to fluctuating oxygen levels on the seafloor.
The growth of additional breathing structures likely bestowed upon them the ability to endure oxygen-depleted zones, deterring predators that couldn't withstand such conditions. This adaptation offers insights into the evolution of survival strategies, shedding light on the ancestral origins of modern arthropods like insects and arachnids.
Hughes explained that the study's aim was to delve into the evolution of development itself, demonstrating that flexibility, rather than meekness, could be a key factor in the path to dominance.