Host specific pit forming epizoans on silurian crinoids
Lethaia 11(3): 217-232
Circular-parabolic pits occur commonly on the endoskeletal remains of certain Paleozoic crinoids [Ichthyocrimus laevis, Synchirocrinus? typus, Eucalyptocrinites spp., Asaphocrinus ornatus, Lecanocrinus macropetalus]. Detailed study of several hundred specimens, representing about 30 pelmatozoan spp from the Upper Silurian Rochester Shale of New York [USA] and Ontario [Canada], reveals that such pits occur exclusively in 7 spp. of crinoids. There are consistent differences in the morphology and orientation of holes occurring on the different crinoid species. This suggests that distinct epizoan species settled selectively on given hosts. The relationship between the hole-producing epizoans and crinoid hosts is inferred to have been a form of dependent commensalism. Preliminary surveys of other Paleozoic crinoid assemblages reveal similar host-selectivity by pit-producing epizoans. Crinoid-epizoan pairs apparently co-evolved through considerable spans of geologic time as related genera and species of different ages, from Silurian to Pennsylvania, exhibit similar pits.