EXPLORING THE EVOLUTIONARY HISTORY AND TAPHONOMIC PROCESSES IN PARASITE-HOST INTERACTIONS – A CASE STUDY OF CIDAROID ECHINOID SPINE GALLS

Ecosystem restructuring in response to the modern biodiversity crisis is expected to change the landscape of parasite-host interactions, and understanding the ecological, taphonomic, and evolutionary processes acting on these interactions in the recent fossil record is of critical importance to inform mitigation efforts. Echinoid-targeting eulimid gastropod parasites exhibit a wide range of parasitic behaviors in modern marine environments and have likely parasitized echinoids from early in their evolutionary history. Domicile gall-forming parasitism by eulimids on cidaroid echinoid spines is relatively uncommon behavior, exhibited by some species of the eulimid genera Sabinella and Trochostilifer, targeting specific species of the cidaroid echinoid genera Eucidaris, Prionocidaris, and Stylocidaris. Cidaroid spines from these genera are common in Neogene and Quaternary fossil assemblages occurring near their living descendant populations, as is the case for tropical to subtropical populations of Eucidaris tribuloides in the western Atlantic and E. thourasii and E. galapagensis in the eastern Pacific. Herein we present a survey of Recent and fossil Eucidaris spines from these populations, to explore the evolutionary history of this association and the taphonomic processes effecting the preservation of spine galls. Despite observations that galling and encrustation are common in Recent populations, no galls nor extensive encrustation is observed in the fossil spines. No gall-forming eulimids have yet been reported from these fossil assemblages either, raising the possibility that this association is recently evolved. However, microtomography of the galls reveals that, though skeletonized, the stereom of the gall material is altered and of lower density when compared to the healthy portion of the spine, and tumbling experiments suggest galled spines are more prone to breakage. Considering this, we propose that there is likely extensive fossil record bias in preserving evidence of this biotic interaction, thus necessitating other methods for estimating parasitism intensity in recent fossil populations.