Adaptive significance of indirect predatory effects in Middle Paleozoic crinoids
Indirect effects of predation are known to be important in structuring modern marine ecosystems, but have proven difficult to document at macroevolutionary timescales. An association between crinoids, snails, and fishes in the Middle Paleozoic may now offer a case in which the macroevolutionary consequences of indirect predation can be examined. A parasitic interpretation of the long-standing association between snails of family Platyceratoidea and their host crinoids has led to the hypothesis that the crinoid hosts suffered “collateral damage” from durophagous fishes preying on their parasites.
Here, we present evidence that the presence of defensive spines on the oral surfaces of Middle Paleozoic crinoids is an adaptive response to injuries incurred indirectly during the interaction between host, parasite, and predator. Crinoid genera in which snail infestation is more common are more frequently found with predator damage and more commonly bear spines on their oral surfaces. While the frequency of the crinoid-platyceratid interaction increased from the Devonian to the Mississippian, such oral spines were much more common in Silurian and Devonian crinoid genera than in those of the Mississippian, suggesting that the ecological disruptions of the Late Devonian mass extinctions decreased the adaptive significance of this three-way interaction to the crinoid hosts. We infer that the profound turnover of diversity and consequent changes in dominant predatory strategy among durophagous fishes in the Hangenberg event (end-Devonian) made oral spines obsolete as a defensive adaptation.