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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
BAUMILLER AND GAHN—PARASITISM ON MARINE INVERTEBRATESresponse to parasitism. A phylogenetic analysisusing 26 genera of monobathrid camerates showedthat: 1) the most parsimonious distribution required4 independent events of tube evolution, and 2) thatthe tubeless sister taxa of tubed crinoids wereinfested by platyceratids, while the tubed crinoidswere generally uninfested.The results of the Gahn and Baumiller (2001)study were consistent with the hypothesis that thetube evolved in response to parasitism; but escalationgoes a step further and invokes an arms race. Thus,adaptations that counter the effectiveness of enemiesshould be rendered ineffective by morphological orbehavioral evolution of those enemies. Didplatyceratids counter the evolution of exaggeratedanal tubes with such changes? Baumiller (1990)reported numerous individuals of the Mississippianbatocrinid crinoids, Batocrinus icosidactylus andBatocrinus irregularis, with circular, cylindrical totapered holes penetrating the plates of the tegmenat the base of a long, slender, multi-plated tube withan anal opening at its apex (Fig. 5). A sectionedspecimen of another tube-bearing Mississippianbatocrinid, Macrocrinus mundulus, infested by aplatyceratid, revealed a hole in the base of the tubedirectly beneath the gastropod. The presence of U-shaped attachment scars, rare instances of multipleholes, and healed (or incomplete) holes was used asevidence that the holes in crinoids did not representpredation, but parasitism. Furthermore, it indicatesthat the tube was not a foolproof strategy of escapefrom infestation by snails, as it could be counteredby drilling. If platyceratid drilling evolved inresponse to the evolution of the tube, the “tit-fortat”would represent a case of escalation.PLATYCERATIDS ASPARASITES ON OTHER TAXACrinoids are not the only echinoderm groupassociated with platyceratids. Several instances ofplatyceratids attached to blastoid calyxes areknown, including two Devonian and twoMississippian examples (Fig. 6). Levin and Fay(1964) described several specimens of Diploblastuskirkwoodensis (Mississippian), each with a smallFIGURE 5—Examples of Mississippian batocrinidcrinoids with drillholes. Scale bar = 0.5 cm.1, Macrocrinus mundulus (P19426), lateral view ofspecimen. Note the long anal tube extendingbeyond the tips of the arms and the platyceratidpositioned on the tegmen between the arm bases.2, Photomicrograph of ground section of samespecimen. 3, Batocrinus icosidactylus (P19402),lateral view showing drillhole at base of broken-offanal tube. 4, Batocrinus icosidactylus, lateral viewshowing two complete drillholes near base of analtube. 5, Batocrinus icosidactylus (P19394), lateralview of partial calyx with complete drillhole. 6,Batocrinus irregularis (P19393), lateral view ofspecimen with a robust tube penetrated by drillhole.All specimens housed in the Field Museum ofNatural History, Chicago, Illinois.203
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THE PALEONTOLOGICAL SOCIETYPAPERSVo
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The Paleontological Society Special
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AuthorsRICHARD K. BAMBACHBotanical
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THE FOSSIL RECORD OF PREDATIONMicha
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INTRODUCTIONTHE FOSSIL RECORD OF PR
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KOWALEWSKI—ANALYTICAL METHODSTHE
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PALEONTOLOGICAL SOCIETY PAPERS, V.
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THE PALEONTOLOGICAL SOCIETY PAPERSP
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