View - Kowalewski, M. - Virginia Tech

BAUMILLER AND GAHN—PARASITISM ON MARINE INVERTEBRATESespecially with the two drilled taxa, Nucleocrinusand Heteroschisma, and the documented drillingof crinoids by platyceratids led the authors to arguethat the blastoid holes were produced by thesegastropods (Fig. 7). They suggested that the rarepresence of doubly-drilled blastoids and theoccasional incomplete/healed holes argued againstpredation, and instead for a long-term association.In addition, the position of the holes away fromthe anal vent was used to argue against coprophagy;instead, a hypothesis of nutrient-stealing wasproposed. This parasitic hypothesis is yet to betested explicitly, but if it stands up to scrutiny, theblastoid-platyceratid interaction may be worthexamining in the context of escalation.One other example of a platyceratidechinodermassociation is found in the Silurian andinvolves the platyceratid Naticonema and thecystoid Caryocrinites (Clarke, 1908; Bowsher,1955; Kluessendorf, 1983). As in other instancesof infested pelmatozoans, the gastropod is foundattached to the theca of the cystoid; this has beeninterpreted as reflecting the coprophagous habit ofthe gastropod and thus a commensal or mutualisticrelationship. However, based on the evidence fromcrinoids and blastoids, a parasitic interpretation isequally plausible.CONCLUSIONThe list of examples of parasitism in the fossilrecord is extensive. Distinguishing parasitism frompredation, commensalism, or mutualism requires(1) evidence of a long-term relationship betweentwo organisms, (2) demonstration of how theinteraction benefits the parasite, and (3)demonstration of how it is detrimental to the host.Only in exceptional cases can these requirementsbe fulfilled. One example that provides muchinformation on parasitic interactions involvesplatyceratids and crinoids. This association hasrecently been re-interpreted as parasitic rather thancommensal. The fact that platyceratids weregeologically long-lived (Ordovician to Permian),abundant, occurred in a broad range of marinesettings, and are known to be associated with avariety of taxa (crinoids, blastoids, cystoids,brachiopods), implies that the parasitic habit mayhave been common during the Paleozoic.Furthermore, because platyceratids have beenFIGURE 7—Distribution of blastoid genera parasitized by platyceratids during the Paleozoic. Solidbars represent co-occurrences of crinoids and platyceratids; open bars represent inferred associationsbased on trace fossils (platyceratid growth scars). The upper curve represents Paleozoic blastoid genericdiversity based on data from Sepkoski’s unpublished compendium.205