View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002feed from the inhalant current created by the ciliaryactivity of the host (Thorson, 1965), or by usingtheir pseudoproboscis to divert particles capturedby the host to its own mouth (Pernet and Kohn,1998). Thus, in our scheme, “nutrients” benefit theparasite, and their loss is a detriment to the host.As Table 1 indicates, examples of parasitismsuch as the bivalve-capulid association, where theidentity of the interacting organisms and the natureof the benefit/detriment can be reliably assessed, arerare in the fossil record. Most common are instanceswhere a trace fossil, or a growth abnormality, isinterpreted as having been parasite-induced, wherethe identity of the parasite cannot be ascertained,and where the beneficial effects on the parasite andthe detrimental effects on the host are poorlyconstrained. The case of the Pliocene bivalve,Isognomon, serves as an illustration of such parasiticinterpretations. Savazzi (1995) reported sevendifferent types of anomalies in the hinge region of14 of 24 specimens of Isognomon maxillatus fromthe Upper Pliocene of Italy. The size and shape ofsome of the cavities suggested that they wereproduced by a worm-like organism. According toSavazzi (1995, p. 136), “…a polychaete could be areasonable candidate,” and a “…sipunculid couldalso be a possible candidate.” However, because theobserved teratologies, or malformations, find noexact modern analogs, we categorize the identityof the parasite as “inferred” (category 3).The Isognomon example also illustrates theproblems in assessing the benefit of an associationto the parasite and its detrimental effects on thehost. The benefit to the infesting organism mayseem obvious because the host is at least providingthe infester with a suitable life position orprotection, but unless the infester’s effect on thehost can be shown to be detrimental, such aninteraction could represent mutualism orcommensalism. For Isognomon, Savazzi (1995, p.137) chose parasitism, which he claimed was“much more likely,” because the position of someof the anomalies would have made it difficult forthe infesting organism to access sea water directlywhile allowing it to tap into the digestive systemor the hemocoele of the bivalve. In our scheme,we leave the question of benefit/detriment forIsognomon as undetermined (“?”), in part becauseSavazzi (1995) did not specify whether it involvednutrients, tissues, or something else that wasbenefiting the parasite and causing harm to the host.Although one might infer that the parasite wasstealing nutrients from the host, we consideredthose fossil examples for which no modern analogfor parasitism had been presented, and no explicittest was used to reject commensalism ormutualism, as “undetermined.”In our classification scheme, the vast majorityof inferred cases of parasitism are “undetermined”with regard to benefit/detriment (Table 1). Ingeneral, the inference of parasitism in these casesis based on accepting “reaction” features asdetrimental to the host. Reaction features, such aspearls, galls, blisters, and deformations, do indicatethat the host responded biotically to the infester,but whether such a response placed the infestedindividual at a selective disadvantage relative touninfested conspecifics has rarely been explored.Of course, it is plausible to assume that theenergetic costs associated with the “reaction” mayplace infested individuals at some disadvantage,but such costs may be quantitatively, and thusselectively, trivial. Thus, even the swollen,distorted, and excavated stems of crinoids causedby the reaction to some infester that have beenclaimed as cases of “true parasitism” (Pickett, 1973,p. 342), remain enigmatic and may be moreappropriately treated as commensal (Franzen,1974; Brett, 1978, 1985; Baird et al., 1990).Many of the examples in Table 1 illustrate theproblems faced by paleontologists in distinguishingbetween parasitism, commensalism, andmutualism—problems that have long beenrecognized (Conway Morris, 1981; Savazzi, 1995).Distinguishing between predation and parasitismcan prove equally difficult. Complete boreholes inthe tests of fossil organisms have generally beeninterpreted as predatory, especially whenmorphologically similar holes are produced byextant predators, such as muricid or naticidgastropods. However, when such boreholes precedethe known first appearance of modern predatory198