This is Mazon Monday post #300.  What’s your favorite Mazon Creek fossil?  Tell us at email:esconi.info@gmail.com.

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The Lauer Foundation just won’t let the Mazon Creek horseshoe crabs alone… first it was Euproops danae remains in bromalites (see Mazon Monday #283), now they’ve found evidence of ancient algal or parasitic infestation in another Euproops danae fossil from the Mazon Creek biota.

The paper “Unique, dimple-like exoskeletal structures suggest syn-vivo infestations in Late Carboniferous horseshoe crabs” was published in the journal Biology Letters. The authors are familiar to anyone interested in Mazon Creek science – Russell Bicknell, Jason Dunlop, Andrew Young, Bruce Lauer, Rene’ Lauer, and Victoria McCoy.

The fossil at the center of the study resides in the Lauer Foundation’s collection and was collected from the Mazon River. It is an unusually large Euproops danae, falling within the uppermost size class known for the species. What makes it extraordinary, however, are the 115 small, circular dimples covering the prosoma. These depressions are set into the exoskeleton and arranged in dense clusters. The authors interpret them as evidence of a syn-vivo infestation, likely microbial or algal in origin. If correct, this would represent the oldest known infection in any horseshoe crab.

The research team evaluated several possible causes. The dimples are far too uniform and numerous to be bite marks or injuries. Modern horseshoe crabs do suffer dorsal damage, but those wounds are much larger and typically puncture through the shell. These structures are shallow, small, and tightly grouped. They also don’t resemble post-mortem borings… worm tubes, bryozoans, fungi, and other bioeroders leave distinctive tubes, walled pits, or surface etching. Nor do the dimples match any known exoskeletal ornamentation in xiphosurids.

Instead, the authors note a striking resemblance to “shell disease” seen in modern Limulus polyphemus, where:

• green algae extend rhizoids into the cuticle
• bacterial or fungal pathogens form crater-like lesions
• lesions occur in clusters, especially on older individuals

Under magnification, the pattern on the Mazon Creek specimen closely matches these modern infestations. The fossil may therefore preserve a syn-vivo microbial or algal infection more than 307 million years old.

The interpretation becomes even more interesting when considering the animal’s life history. Because the fossil is so large—and because heavy fouling only builds up after the final molt. The specimen may represent a terminal-moult individual, a fully mature adult that would no longer shed its exoskeleton. Modern horseshoe crabs slough off fouling organisms with each molt; persistent, dense infestations occur only once molting stops.

In living horseshoe crabs, heavy fouling:

• increases drag
• reduces burrowing efficiency
• limits feeding
• increases vulnerability to predators

The same pressures may have affected this Carboniferous crab.

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

Absrtract

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.

Introduction

Exceptional preservation in the fossil record presents rare insights into the history of life on Earth [1–3]. Sites exhibiting this level of detail are called Konservat-Lagerstätten and document soft-bodied anatomy that otherwise would be lost to rapid decay [3,4]. Evidence of skin, muscles, gut contents, central nervous systems and internal organs such as hearts and lungs [5–10],
as well as ecological interactions, including predation, parasitism, epibionts and organisms living within other hosts [11–16] are known from these sites. Konservat-Lagerstätten, therefore, provide a wealth of material for reconstructing the biology of extinct organisms [17], and renewed research of iconic Konservat-Lagerstätten continues to uncover novel palaeobiological data [7].

The Mazon Creek Konservat-Lagerstätte is renowned for its exceptional soft-tissue preservation of a unique and diverse biota, with fossils of over 465 animal and 350 plant species [18]. Among these is an abundant arthropod fauna. This includes a striking array of Xiphosurida—horseshoe crabs—with three known species: Euproops danae [19], Liomesaspis laevis [20] and Paleolimulus mazonensis [21]. Specimens of these xiphosurids have offered extensive insight into the anatomy, palaeobiology and evolution of the group (see [22–29]). Building on these records, we present evidence of possible interactions between the belinurid E. danae and infesting organisms—the oldest such example in the horseshoe crab fossil record.