Over 500 million years ago, long before dinosaurs roamed the Earth, the oceans were home to trilobites—one of the planet’s earliest and most successful groups of animals.
Thanks to their hard outer shells, trilobites are common in the fossil record, but their soft body parts, like legs and gills, are rarely preserved.
That has made it difficult for scientists to know exactly how these ancient creatures moved or lived.
Now, a new study of a Burgess Shale trilobite species called Olenoides serratus has provided an unusually clear view of these hidden features.
The Burgess Shale in British Columbia, Canada, is famous for its remarkable preservation of soft tissues in fossils.
This unique site has kept not only the exoskeletons of trilobites but also delicate structures like limbs, allowing researchers to study them in detail.
Led by Sarah Losso, a postdoctoral fellow in Harvard University’s Department of Organismic and Evolutionary Biology, the research team examined 156 limbs from 28 fossil specimens of O. serratus.
By carefully studying the shape and range of motion of these limbs, they were able to reconstruct how this ancient animal walked, burrowed, and fed.
“Understanding the behavior of fossils is tricky—you can’t watch them move like living animals,” Losso explained. “We had to rely on the shapes of their limbs and compare them to modern arthropods to figure out how they worked.”
Modern arthropods, such as crabs, insects, and spiders, have jointed legs with multiple segments. Each joint can bend upward (extend) or downward (flex), and the range of movement depends on the shape of the segments.
This determines whether a limb is better suited for walking, digging, grabbing food, or other tasks.
Although trilobites are often compared to today’s horseshoe crabs, the study found that O. serratus had a simpler but still highly functional leg design. Horseshoe crabs have legs that alternate in their ability to flex or extend, which helps them both move and protect themselves.
In contrast, O. serratus had a smaller range of movement, mainly in the parts of the limb farthest from the body. Still, this was enough for the trilobite to walk, dig into the seabed, pull food toward its mouth, and even lift its body off the seafloor.
To understand these movements more clearly, the team created 3D digital models of the trilobite’s limbs based on hundreds of fossil images. Because fossilized limbs are usually flattened, building these models required comparing specimens preserved from different angles and filling in missing details using related fossils.
The research also linked limb movements to certain types of fossilized tracks, or trace fossils. Depending on how it moved, O. serratus could leave shallow marks when walking lightly or deeper ones when digging into sediment.
The ability to raise its body would have helped it cross obstacles or move more easily in strong currents. Surprisingly, the team also discovered that males had specialized limbs for mating, and that each leg carried a gill for breathing.
While more than 22,000 trilobite species are known, fewer than 0.2% show fossil evidence of legs. The exceptional preservation in the Burgess Shale—caused by rapid underwater landslides that cut off oxygen—was crucial for capturing such rare details.
This research offers a vivid glimpse into life in ancient seas, revealing how Olenoides serratus used its versatile limbs to thrive.
Far from being simple bottom-dwellers, these trilobites were skilled walkers, diggers, and foragers, perfectly adapted to the prehistoric world they inhabited.
