LONG-TERM PARASITIC INTERACTIONS IN ANCIENT ECHINODERMS

Most parasitism involves interactions within the soft tissues of organisms, which presents an inherent bias against its preservation in the fossil record. Echinoderms are unusual in preserving much of the body within endoskeletal elements permeated with living stromal tissue, which is capable of responding to stimuli from settling epibionts. Hence, fossil echinoderms preserve some of the strongest records of paleoparasitism. This record is manifest in two exemplars that illustrate long-term interactions between pelmatozoan hosts and parasites. The interaction between coprophagous platyceratid gastropods and crinoids, long been viewed as an example of commensalism, may have bordered on parasitism. Moreover, with the Middle Devonian rise of predators, platyceratid gastropods may have "targeted" their hosts leading to a co-evolutionary trend toward increased spinosity. This interaction evolved within particular lineages of crinoids through tens of millions of years from Middle Ordovician to Late Permian.

The second interaction involves a clade of unknown soft-bodied, perhaps filter-feeding organisms and specific crinoid/cystoid hosts. The organisms produced a distinctive trace, termed Tremichnus by Brett (J. Paleontology, 1985), consisting of circular-parabolic pits that appear to be a combination of boring by the trace-maker and embedment owing to the growth of the living stereomic skeleton both on calyces and columns. There are several distinctive types of traces, based upon diameter-depth relationships and variation in size. In some cases, the crinoid hosts reacted by secreting excess stereom to form gall-like swellings. Evidently this was a harmful interaction for the host and there is strong evidence for host-specificity by the parasite suggesting that some crinoids may have possessed biochemical defenses that prevented settlement. As with the platyceratid interaction, parasites that produced Tremichnus co-evolved with crinoid hosts over prolonged intervals; examples are known from Middle Ordovician to at least Middle Jurassic time (~300 million years), surviving even the end Paleozoic extinctions, which very nearly caused the demise of the crinoid hosts. Clearly, interactions such as these developed early and persisted as "ecological standoffs" through multiple biotic crises.