A recent research investigation reveals that a trilobite species, boasting remarkably well-preserved fossils found in upstate New York, possesses an extra set of legs located under its head. This study indicates that the existence of a fifth pair of appendages may be more common among trilobites than previously realized, enhancing our understanding of how trilobite heads are structured.
A recent research investigation reveals that a trilobite species, boasting remarkably well-preserved fossils found in upstate New York, possesses an extra set of legs located under its head. This research, conducted by the American Museum of Natural History and Nanjing University in China, suggests that the presence of a fifth pair of appendages might be more prevalent among trilobites than previously believed. The findings, published in the journal Palaeontology, contribute to a deeper insight into the segmentation of trilobite heads.
Trilobites are a category of extinct arthropods, with present-day relatives such as lobsters and spiders. Similar to other arthropods, trilobite bodies comprise multiple segments, with the head consisting of various fused segments. Each segment of the trilobite’s body— the thorax and tail— is associated with appendages that serve functions ranging from sensory perception to movement and feeding.
“The count of these segments and their correlation with significant features, like eyes and legs, is crucial for understanding the relationships among arthropods and their evolutionary development,” explained Melanie Hopkins, curator and chair of the Museum’s Division of Paleontology.
Scientists can count the segments in a trilobite’s head using two methods: by inspecting the grooves (referred to as furrows) on the upper portion of the fossilized exoskeleton or by tallying the pairs of preserved antennae and legs found underneath the fossil. However, the soft appendages of trilobites hardly survive fossilization, leading to frequent discrepancies between these two counting methods when assessing the segments in the trilobite head.
In this study, Hopkins and her colleague Jin-Bo Hou from Nanjing University explored recently discovered specimens of the well-preserved trilobite Triarthrus eatoni from upstate New York. These fossils, notable for their shiny pyrite replacement, revealed an additional, previously unidentified leg beneath the head.
“This remarkable style of preservation allows us to directly observe 3D appendages in hundreds of specimens from the underside of the animals, much like examining the appendages of horseshoe crabs on a beach by flipping them over,” noted Hou.
Through comparisons with another trilobite species, the well-preserved Olenoides serratus from the Burgess Shale in British Columbia, Hopkins and Hou have suggested a model illustrating how the appendages were attached to the head in relation to the grooves in the exoskeleton. This model clarifies the previous discrepancies, indicating that the trilobite head comprised six segments: one segment linked to the origins of the eyes and five additional segments associated with one pair of antennae and four pairs of walking legs.
This study builds upon earlier analyses conducted by Hou and Hopkins on Triarthrus eatoni, which indicated that the walking legs contain tiny respiratory structures (gills) and that some spines on these legs serve to keep the gills clean.
