A swollen crinoid pluricolumnal from the Upper Ordovician of northern Kentucky, USA: the oldest record of an amorphous paleopathologic response in Crinoidea?

Swollen or otherwise malformed crinoid remains are relatively common in the Phanerozoic. However, published reports typically describe swellings associated with a discrete pit, boring, embedment structure or encruster, and, moreover, are overwhelmingly from Silurian or younger strata. Here, the rare occurrence of an amorphously swollen crinoid pluricolumnal is described from the Upper Ordovician (Katian) Grant Lake Formation of north-central Kentucky, USA. This represents one of the oldest examples of crinoid malformation potentially resulting from interaction with a parasite or epibiont, one of the oldest examples of swelling in a crinoid column, and likely represents the oldest record of amorphous swelling. The pluricolumnal is morphologically generalized, making definitive identification difficult. Potential candidates include the diplobathrid camerate Pycnocrinus and the large, morphologically aberrant disparid Anomalocrinus. Regardless, if generated by an antagonistic biotic interaction, this specimen seems to support the hypothesis that crinoids with large calyxes and relatively complex arm morphologies were preferentially utilized as hosts for parasites and commensals over crinoids with simpler morphologies in the Ordovician.