PARASITIC EMBEDMENT STRUCTURES (TREMICHNUS ISPP.) IN HEMICOSMITID RHOMBIFERANS: IMPLICATIONS FOR HOST-SPECIFICITY AND ECHINODERM PALEOECOLOGY

Parasitic embedment structures in echinoderm skeletons, herein placed in the ichnogenus Tremichnus, have been documented from a variety of Paleozoic pelmatozoan hosts. The occurrence, distribution, modes of emplacement, and paleoecology of Tremichnus in crinoids and diploporitan ‘cystoids’ have been relatively thoroughly studied; in contrast, parasitic pits in rhombiferans have not previously been the subject of detailed study. A large collection of blastozoans, including both diploporitans and rhombiferans, was recovered from the middle Silurian (Wenlock) Massie Formation of Indiana and used to test for patterns of infestation. Although large, relatively shallow pits are common in the holocystitid diploporitans, among rhombiferans, only the hemicosmitid Caryocrinites contained Tremichnus, with an infestation frequency of approximately 6%. Pits occur primarily on thecae; focused searching also revealed rare examples of pluricolumnals containing Tremichnus. Strong host-specificity is indicated by the restriction of embedment structures to small thecae attributable to an unidentified species of Caryocrinites and not in the co-occurring C. ornatus. This is consistent with study of large collections of C. ornatus from the approximately contemporaneous Rochester Shale of New York that yielded no evidence of Tremichnus despite the common occurrence of these pits in several species of crinoids present in the same beds. Tremichnus has also been discovered on a Middle Ordovician (Darriwilian) theca of the hemicosmitid Hemicosmites. This represents an age extension for parasitic embedment structures on blastozoan echinoderms and suggests that the association between the producers of Tremichnus and their hemicosmitid rhombiferan hosts was long ranging.