Even 300 million years ago, horseshoe crabs couldn’t escape pesky parasites.
A fossil horseshoe crab dotted with mysterious dimples hints at an ancient infestation by algae or parasites while the creature was still alive, researchers report in a study published December 2025 in Biology Letters. This rare find from Illinois’ famous Mazon Creek fossil site marks the oldest known case of such an infection in horseshoe crab history, shedding light on long-standing ecological battles in Earth’s distant past.
Horseshoe crabs, those ancient marine survivors often called living fossils, have roamed ocean floors for hundreds of millions of years. But even in the Late Carboniferous period — when swamps and deltas dominated landscapes — these arthropods faced familiar foes. Modern horseshoe crabs deal with algae, bacteria, fungi, and parasites that erode their shells or burrow in, sometimes causing pitted lesions or shell disease. Now, a single specimen of the extinct species Euproops danae suggests similar troubles plagued their ancestors.
The fossil, unearthed along the Mazon River and preserved in an ironstone concretion, shows a well-formed horseshoe crab about 6.5 centimeters long from head to tail spike. What stands out are 115 small, pit-like dimples clustered across the front and middle of its head shield, or prosoma. These indentations, ranging from 0.18 to 0.69 millimeters wide, don’t appear on the body or tail, and they’re embedded in the exoskeleton rather than sitting on top.
To uncover the dimples’ story, the team compared the fossil to other horseshoe crab specimens and modern examples. They ruled out natural shell patterns, injuries, or postmortem changes like worm burrows. Instead, the pits resemble erosion from green algae or bacterial infections seen in today’s horseshoe crabs, where tiny organisms dig into the outer shell layer. “The proliferation of opportunistic microbial or algal colonizers may have been facilitated by fluctuating salinity regimes and elevated organic content within the deltaic palaeoenvironment,” the researchers suggest.
This infested individual was likely an adult at the end of its growth spurt. Modern horseshoe crabs shed their shells as they grow but stop molting once sexually mature, allowing fouling organisms to build up over time. The fossil’s large size — among the biggest for its species — and heavy dimpling suggest it had reached this terminal molt stage, making it vulnerable to prolonged infestation. “Heavy fouling in modern horseshoe crabs increases drag, impairs mobility and reduces burrowing efficiency,” the team notes, adding that similar issues likely shortened the ancient crab’s lifespan by raising predation risk or limiting food access.
While parasites and epibionts have turned up in other ancient arthropods like trilobites and sea scorpions, this is the first clear sign in the horseshoe crab lineage. It underscores how Konservat-Lagerstätten like Mazon Creek, with their exquisite soft-tissue preservation, reveal hidden slices of paleoecology—from gut contents to predator-prey tussles. As researchers dig deeper into these sites, more tales of ancient ailments may emerge, painting a fuller picture of life’s enduring struggles.
Citations: Unique, dimple-like exoskeletal structures suggest syn-vivo infestations in Late Carboniferous horseshoe crabs. DOI: 10.1098/rsbl.2025.0565.
