Unique, dimple-like exoskeletal structures suggest syn-vivo infestations in Late Carboniferous horseshoe crabs

Exceptional preservation within the Mazon Creek Konservat-Lagerstätte has yielded unprecedented insights into Late Carboniferous flora and fauna including a wealth of information on extinct horseshoe crabs (Xiphosurida). Here, we document a unique specimen of the xiphosurid species Euproops danae that exhibits numerous dimple-like structures across the prosomal region. Comparison with modern horseshoe crabs suggests that these dimples may represent an algal or parasitic infestation that impacted the organism during life. This is the only known example of this infestation within the xiphosurid fossil record and provides evidence of life-stage-specific vulnerability, with dimpling indicating a terminal moult individual. These observations highlight the palaeobiological significance of pathological features within the fossil record and reinforce the value of Konservat-Lagerstätten in documenting ancient host–parasite interactions.