Reconstructing predation pressure on crinoids; estimating arm-loss rates from regenerating arms

The regeneration abilities of crinoids not only are important to understanding crinoid ecology, but also can serve as the basis for assessing the pressure exerted on crinoids by predators both in the Recent and in the geologic past. This is especially true of regenerating arms, because arm loss, and subsequent regeneration, is thought to result from interactions with predators, primarily fish. However, the commonly used regeneration-based proxy for predation pressure (proportion of individuals with regenerating arms) does not provide a measure of the rate at which those events occurred. Here we present a method for reconstructing the arm-loss rate per individual, a more direct proxy of predation pressure. This metric accounts for differences in arm length, arm number, and branching pattern, features highly variable among taxa, among environments, and through geologic time. Normalizing for those characters permits the transformation of observed proportions of regenerating arms to rates that can be compared across samples of morphologically distinct crinoids. Applying this method to a Recent crinoid (Cenometra bella) reveals that this shallow-water comatulid loses arms at a rate of about once every ten days. The same approach reveals that Mississippian shallow-water crinoids (Rhodocrinites kirbyi) experienced arm loss much less frequently, approximately once every 36 days, suggesting that predation pressure on crinoids today is greater than it was in the Mississippian.