Parasitism and predation on Paleozoic crinoids.

Abstract

Biotic interactions are widely recognized as agents of evolution, yet the nature, extent, and implications of many such relationships in the fossil record remain elusive. A classic example is the association between Paleozoic crinoids and platyceratid gastropods, characterized by platyceratid attachment over crinoid anal vents. The traditional interpretation for this interaction is that gastropods fed on crinoid excrement without detriment to crinoids. To test this hypothesis, measurements were collected for two Devonian crinoid species. Results indicated that in both species, snail-infested individuals were significantly smaller than uninfested individuals, falsifying the hypothesis that the crinoid-gastropod relationship was strictly commensal. Smaller size of infested crinoids is interpreted as a consequence of nutrient-stealing by gastropods. Moreover, the absence of platyceratids on the largest crinoids suggests that large size may have conferred immunity from permanent infestation. Several workers have suggested that platyceratid parasitism may have driven the evolution of anti-infestation structures such as anal tubes in fossil crinoids. Anal tubes prevented platyceratids from occupying crinoid anal vents, but they did not prevent infestation: gastropods attached to crinoid tegmina and accessed intercalical nutrients via drilling. Nevertheless, a survey of tubed vs. tubeless crinoids demonstrates significantly lower infestation in tubed crinoids. Drilling tubed crinoids likely was less cost-effective than infesting tubed crinoids. Moreover, phylogenetic analysis of the Compsocrinina suggests that tubes evolved repeatedly in platyceratid-infested clades, failing to reject the hypothesis that tubes evolved in response to platyceratid parasitism. Predation, another purported force of evolutionary change, was studied in a Mississippian LagerstAtten (Le Grand) using arm regeneration frequency. Nine percent of the crinoids contained at least one regenerating arm, with 27% regeneration observed in the most abundant species, <italic>Rhodocrinites kirbyi</italic>, a significantly higher frequency than observed in other species. <italic> Rhodocrinites kirbyi</italic> had the longest stalks, and individuals above median cup height (7 mm) displayed nearly 50% regeneration, whereas smaller individuals had only 2% regeneration. These patterns are consistent with a predator that attacks the most apparent prey. Regeneration frequency in fossil crinoids potentially provides a valuable, yet overlooked data source for testing hypotheses pertinent to the role of predation in the evolution of Phanerozoic marine life.