Arthropod appendages are specialized for diverse roles including feeding, walking, and mating. Fossils from the Cambrian period (539 to 487 million years ago) preserve exceptional details of extinct arthropod appendages that can illuminate their anatomy and ecology. However, fossils are typically limited by small sample sizes or incomplete preservation, and thus functional studies of the appendages usually rely on idealized reconstructions. In new research, paleontologists focused on Olenoides serratus, a particularly abundant trilobite species in the Cambrian Burgess Shale that is unique among trilobites owing to the availability of numerous specimens with soft tissue preservation that allow us to quantify its appendages’ functional morphology.
Olenoides serratus from the Burgess Shale. Image credit: Losso et al., doi: 10.1186/s12915-025-02335-3.
The Burgess Shale in British Columbia, Canada, 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.
The trilobite species Olenoides serratus offers a unique opportunity to study these appendages.
Harvard University paleontologist Sarah Losso and her colleagues analyzed 156 limbs from 28 fossil specimens of Olenoides serratus 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,” Dr. Losso said.
“Instead, we had to rely on carefully examining the morphology in as many specimens as possible, as well as using modern analogues to understand how these ancient animals lived.”
The researchers also measured the range of motion of the legs in the living horseshoe crab species Limulus polyphemus.
“Arthropods have jointed legs composed of multiple segments that can reach upwards (extend) or downwards (flex),” they said.
“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 — Olenoides serratus displayed a simpler, but highly functional limb design.
“We found that the limbs of Olenoides serratus had a smaller range of extension and only in the part of the limb farther from the body,” Dr. Losso said.
“Although their limbs were not used in exactly the same way as horseshoe crabs, Olenoides serratus could walk, burrow, bring food towards its mouth, and even raise its body above the seafloor.”
To bring their findings to life, the scientists 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 Harvard University’s Professor Javier Ortega-Hernández.
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,” Dr. 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, we discovered that the male species also had specialized appendages used for mating, and that each leg also had a gill used for breathing.”
The results were published on August 4, 2025 in the journal BMC Biology.
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S.R. Losso et al. 2025. Quantification of leg mobility in the Burgess Shale Olenoides serratus indicates functional differences between trilobite and xiphosuran appendages. BMC Biol 23, 238; doi: 10.1186/s12915-025-02335-3