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The Burgess Shale in British Columbia is renowned for its exceptional preservation of soft tissues in fossils, including limbs and guts. While trilobites are abundant in the fossil record thanks to their hard exoskeleton, their soft limbs are rarely preserved and poorly understood. However, Olenoides serratus, a particularly abundant and well-preserved Burgess Shale trilobite, offers a unique opportunity to study these appendages.

In a new study in BMC Biology, researchers led by Sarah Losso, postdoctoral fellow in the Department of Organismic and Evolutionary Biology (OEB) at Harvard, analyzed 156 limbs from 28 O. serratus fossil specimens to reconstruct the precise movement and function of these ancient arthropod appendages—shedding light on one of the planet's earliest and most successful animals.

"Understanding behavior and movement of fossils is challenging, because you cannot observe this activity like in living animals," said Losso. "Instead, we had to rely on carefully examining the morphology in as many specimens as possible, as well as using modern analogs to understand how these ancient animals lived."

Arthropods have jointed legs composed of multiple segments that can reach upwards (extend) or downwards (flex). The range of motion depends on the difference between how far each joint can reach in either direction. This range, along with the leg and shape of each segment, determines how the animal uses the limb for walking, grabbing, and burrowing.

Horseshoe crabs, common arthropods found along the eastern shore of North America, are frequently compared to trilobites even though they are not closely related. Horseshoe crabs belong to a different branch of the arthropod tree, more closely related to spiders and scorpions, whereas trilobites' family ties remain uncertain. The comparison is due to the similarity in that both animals patrol the ocean floor on jointed legs. The results, however, showed less similarity between the two animals.

Unlike horseshoe crabs, whose limb joints alternate in their specialization for flexing and extending—a pattern that facilitates both feeding and protection—O. serratus displayed a simpler, but highly functional limb design.

"We found that the limbs of O. serratus had a smaller range of extension and only in the part of the limb farther from the body," explained Losso. Although their limbs were not used in exactly the same way as horseshoe crabs, Olenoides could walk, burrow, bring food towards its mouth, and even raise its body above the seafloor.

To bring their findings to life, the team created sophisticated 3D digital models based on hundreds of fossil images preserved at different angles. Because fossilized trilobite limbs are usually squashed flat, reconstructing them in three-dimensions posed a challenge.

"We relied on exceptionally well-preserved specimens, comparing limb preservation across many angles and filling in missing details using related fossils," said senior author Professor Javier Ortega-Hernández, also in OEB.

The team compared the shape of trace fossils with the movement of the limbs.

"Olenoides serratus could create trace fossils of different depths using different movements," Losso explained. "They could raise their body above the sediment in order to walk over obstacles or to move more efficiently in fast-flowing water."

Surprisingly, the researchers discovered that the male species also had specialized appendages used for mating, and that each leg also had a gill used for breathing.

While more than 22,000 species of trilobites have been described, less than 0.2% show any trace of legs at all. Nevertheless, lack of preservation does not imply these ancient arthropods went legless—rather, their soft limbs simply seldom survived the fossilization process. The rare conditions of the Burgess Shale—a fast burial by underwater landslides cutting off oxygen—were key to capturing such fleeting biological details.

The study provides a rare window into a more dynamic picture of life more than half a billion years ago, as trilobites like Olenoides serratus scuttled across the seabed with sophisticated limbs that could burrow and foraged through prehistoric seas, revealing not just how they survived, but how they thrived.