Appendages of arthropods serve various functions, including feeding, locomotion, and reproduction. Fossils dating back to the Cambrian period (539-487 million years ago) provide remarkable details of extinct arthropod appendages, enhancing our understanding of their anatomy and ecological roles. However, due to the limited number of fossils and often incomplete preservation, studies on appendage functions typically depend on idealized reconstructions. This new research focuses on the paleontological species Olenoides serratus, a prolific trilobite from the Cambrian Burgess Shale, noteworthy for its numerous well-preserved specimens featuring soft tissue that allow for a detailed analysis of appendage functionality.
Situated in British Columbia, Canada, the Burgess Shale is renowned for its exceptional fossil preservation, including soft tissues such as limbs and internal organs.
While trilobites are common in fossil records, their soft limbs are seldom preserved due to their hard exoskeleton, leading to a limited understanding of these structures.
The trilobite species Olenoides serratus offers a unique chance to investigate these appendages further.
Harvard paleontologist Sarah Ross and her team examined 156 limbs from 28 fossil specimens of Olenoides serratus to reconstruct the precise movements and functions of these ancient arthropod appendages, shedding light on one of the earliest successful aquatic animals.
“Understanding the behavior and movement of fossils poses challenges, as we cannot observe their activities like we do with living organisms,” stated Dr. Ross.
“Instead, we meticulously analyzed the morphology of numerous specimens while also utilizing modern analogues to infer how these ancient creatures lived.”
The researchers also assessed the range of motion of the legs of living horseshoe crab species Limulus polyphemus.
“Arthropods possess articulated legs composed of multiple segments that can flex upwards or downwards,” they noted.
“The range of motion is influenced by the specific directional capabilities of each joint.”
“This range, combined with the limbs’ shape and segment configuration, determines how the animal utilizes its appendages for walking, grasping, and burrowing.”
Horseshoe crabs, commonly found along the eastern coast of North America, are compared with trilobites due to their analogous behaviors.
“Despite their close relation to spiders and scorpions, horseshoe crabs are part of a different branch of the arthropod tree, whereas trilobite relationships remain ambiguous.”
The comparison arises from both animals’ adaptation of articulated limbs for navigating the seafloor.
However, the findings revealed that their similarities were minimal.
In contrast to horseshoe crabs, characterized by specialized limb joints for bending and expanding—facilitating feeding and protection—Olenoides serratus exhibited a simpler yet highly functional limb structure.
“We found that the limbs of Olenoides serratus had minimal extension, primarily far from the body,” Dr. Ross explained.
“Their limbs functioned differently than those of horseshoe crabs. Olenoides serratus could walk, dig, bring food to their mouths, and even elevate their bodies above the seafloor.”
To realize these findings, the scientists constructed advanced 3D digital models based on hundreds of fossil images captured from various angles.
Since trilobite limbs are often crushed and flattened, reconstructing them in three dimensions presents a significant challenge.
“We depend on exceptionally well-preserved specimens, comparing limb structures from multiple angles while leveraging related fossils to fill in any missing details,” said Professor Javier Ortega-Hernandez of Harvard University.
The team correlated the morphology of trace fossils to the movements of the limbs.
“The different movements of Olenoides serratus could create trace fossils with varying depths,” Dr. Ross elaborated.
“They were capable of raising their bodies on deposits, allowing them to traverse obstacles and navigate efficiently through swift currents.”
Remarkably, the researchers found that males possessed specialized appendages for mating, and each leg featured gills for respiration.
The findings were published in the journal BMC Biology on August 4th, 2025.
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Loss et al., 2025. Quantification of leg mobility in Burgess Shale Olenoides serratus reveals the functional differences between trilobite and Xiphosuran appendages. BMC Biol 23, 238; doi:10.1186/s12915-025-02335-3
Source: www.sci.news
