A Prejudiced Review of Ancient Parasites and Their Host Echinoderms: CSI Fossil Record or Just an Excuse for Speculation? Recognizing the presence of a parasite and identifying it is a relatively straightforward task for the twenty-first century parasitologist. Not so the pursuit of ancient parasites in fossil organisms, a much more difficult proposition. Herein, Boucot''s seven-tiered scheme of reliability classes is applied as a measure of confidence of the recognition of putative parasitism in two echinoderm classes, Upper Palaeozoic crinoids and a Cretaceous echinoid (high confidence is 1, low confidence 7). Of the five examples, the parasitic(?) organism is preserved in only two of them. A zaphrentoid coral on the camerate crinoid Amphoracrinus may have robbed food from the arms (Category 1 or 2B). A pit in what appears to be a carefully selected site on the disparid crinoid Synbathocrinus is associated with a growth deformity of the cup (Category 4). Multiple pits in an Amphoracrinus theca are also associated with a deformed cup, but it is more difficult to interpret (Category 4 or 7). Some specimens of the camerate crinoid Neoplatycrinites have circular grooves or depressions posteriorly, presumably produced by coprophagic/parasitic platyceratid gastropods (Category 1). Site selectivity of pits in the echinoid Hemipneustes places them preferentially adjacent to respiratory tube feet (Category 4). From these examples it is deduced that sparse infestations of borings or epizoozoic organisms permit a more confident interpretation of organism/organism interactions; dense accumulations, possibly following multiple spatfalls, mask such patterns.