PELMATOZOAN ECHINODERMS AS HOSTS TO PIT-FORMING PARASITES: PALEOECOLOGICALLY SIGNIFICANT EXAMPLES FROM THE MIDDLE PALEOZOIC

Stalked echinoderms (pelmatozoans) served as hosts to enigmatic endoparasitic organisms that produced distinctive pits commonly associated with pathological swelling and malformations. The paleontological significance of these embedment structures, known as Tremichnus, is herein illustrated by several informative Silurian and Devonian examples. Silurian examples highlight interactions between pit-forming organisms and encrusting bryozoans. Calyxes of the monobathrid camerate crinoid Eucalyptocrinites from the Wenlock-age Massie Formation of southeastern Indiana preserve the first documented examples of parasitic pits that were overgrown by laminar stenolaemate bryozoans after death of the host. In contrast, an articulated theca of the diploporitan Holocystites sp. from the same stratigraphic unit records a laminar bryozoan colony that ends sharply at the edge of a pit, being unable to overgrow the depression despite having sufficient thickness to do so. This suggests that the pit was still occupied by the parasite, which persisted after the death of the host. As the pit-forming organisms derived nutrition from the water column rather than the host echinoderm’s tissues, the parasite was able to transition from an endoparasitic lifestyle to an endolithic one, switching from using the theca as an involuntary living domicile to a “pre-bioeroded” bioclast. Although Devonian examples of Tremichnus are uncommon in general, examples illustrate the strong host-specificity of symbionts. A pluricolumnal of the monobathrid camerate crinoid Trybliocrinus flatheanus from the Emsian-age Coladilla Formation of Spain is heavily infested (20+ pits). Despite being a common and robust fossil in this region, this is the only documented parasitized specimen; more importantly, infestation of a eucalyptocrinacean host demonstrates the persistence of this family as hosts from the Silurian. Likewise, a pit-bearing crown of the ichthyocrinid flexible crinoid Synaptocrinus indicates persistence of the infestation of this family as well. Although many aspects of Tremichnus formation and pelmatozoan infestation remain enigmatic, these discoveries provide new information and offer guiding questions related to the nature and ecological/evolutionary significance of this unique host-parasite relationship.