The nature of the platyceratid–crinoid association as revealed by cross-sectional data from the Carboniferous of Alabama (USA)

The association of Paleozoic crinoids and platyceratid gastropods has drawn the attention of paleontologists for nearly 200 years. It has been variably interpreted as predatory, commensalistic, mutualistic or parasitic. Previous cross-sectional analyses of large populations of crinoids and platyceratids have favored the parasitic explanation for the interaction because of data suggesting that the average size of infested crinoids is less than that of their uninfested conspecifics. In this study, we examine a population of Dizygocrinus from the Carboniferous (Early Viséan) Monteagle Limestone of Madison County, Alabama, USA. Of the 198 specimens examined from a single lens, 37 showed evidence of infestation by way of attached gastropods or drillholes at the base of their anal tubes—slender, cylindrical extensions of the tegmen with the periproct at the distal end. In contrast to the results of previous studies, the average size of the infested Dizygocrinus in this study was greater than that of the uninfested subset. Using conceptual and numerical models, we demonstrate that despite the slightly larger average size of the infested crinoids, the gastropods diminished the growth rates of their hosts. Thus, parasitism is the best explanation for this biotic interaction. The presence of an anal tube among the Monteagle Limestone Dizygocrinus may have diminished the negative impact of their parasites by increasing the length of the hindgut, and as a consequence, increased crinoids’ ability to absorb nutrients.