View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002specimens of two species of crinoids, Corocrinusand Gennaeocrinus, the authors will show thatinfested individuals were significantly smaller thanuninfested ones (Fig. 4). The differences areinterpreted as indicating slower growth rates orhigher mortality rates of infested crinoids. Thisdetrimental effect of platyceratids on their hostssupports the parasitic, nutrient-stealing hypothesis.Evolutionary consequences of platyceratidparasitism on crinoids.—The above observationsindicate that platyceratid gastropods represented abiological “hazard” to crinoids. In the context ofthe hypothesis of escalation (Vermeij 1977, 1980,1987), organisms that have enemies, be theypredatory or parasitic, are expected to respondevolutionarily to these hazards. Given that theassociation between crinoids and platyceratids wasgeologically long-lived, one might predict a crinoidresponse. A study by Gahn and Baumiller (2001)provides a test of this prediction.Gahn and Baumiller (2001) suggested that onepotentially effective anti-infestation feature ofcrinoids was a long, slender anal tube. Theseauthors argued that shifting the gastropod’s pointof access to the apex of the tube would, amongother things, have made it difficult for gastropodsto position themselves over the anus.To test the effectiveness of the tube as antiinfestationdevice, Gahn and Baumiller (2001)categorized crinoids as 1) tube-bearing, infested;2) tubeless, infested; 3) tube-bearing, uninfested;or 4) tubeless, uninfested—and showed that thedistribution of platyceratids was not independentof the presence of the tube (chi square P = 0.007).Platyceratids were associated almost exclusivelywith tubeless crinoids.Gahn and Baumiller (2001) also explored theevolutionary scenario that the tube evolved inFIGURE 4—Size distribution of platyceratid-infested (dotted curve) and uninfested (solid curve)specimens of Genneaocrinus variabilis from the Middle Devonian Bell Shale, Rockport, Michigan. Thedifference is statistically significant; infested individuals (N = 30) averaged 2.4 mm in height whereasuninfested specimens (N = 396) averaged 4.5 mm (t-test, P