View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002spatial and environmental variation in drillingpredation in the middle Miocene of Central Europe.The sampling methodology allowed forcomparisons within provinces, between provinces(Boreal vs. Paratethys), and between facies (finegrainedvs. coarse-grained siliciclastics). Theyfound that unsuccessful and multiple drillholesoccurred more frequently in the Boreal provincethan the Paratethys province; the same facies alsoincluded molluscs with different drillingfrequencies—with as much as a three-folddifference between samples collected in adjacentsites from the same facies! They concludedunequivocally that spatial variation should beevaluated independently before any large-scaletemporal trends are inferred for predation. Clearlymultiple collections with emphasis on facies needto be included in the temporal analysis of escalatorypredation hypotheses.Shell ornamentation (spines).—Spinedevelopment requires extra amounts of calciumcarbonate in seawater, and shallow tropical marinewaters meet this requirement (Nichol, 1965;Stanley, 1970). Spines have inspired variedhypotheses concerning their function asantipredatory architecture (Table 5). Few studies,however, have focused on alternative hypothesessuch as whether spines are phylogenetic legaciesof shell building (as in cardiid bivalves), non-aptiveconstructional artifacts, exaptations, or adaptations.Few workers have endeavored to apply suchphilosophical rigor, and many have created adaptivescenarios. Another important possible function ofspines could be to increase surface area for thesettlement of epibionts, and for trapping debris thatcamouflages the shells (Vance, 1978; Feifarek, 1987;Stone, 1998). Stone (1998) has shown that spineson epifaunal bivalves deter the attack of muricidshell-drilling predators, but muricids can still borein areas where spines are absent. In the same study,spines were found not to deter predatory attacks bysea stars that engulf their prey. The rise of spinoseornamentation in bivalves predates the radiation ofthe predatory Muricidae in the Albian, and actuallyextends back to the late Paleozoic in thesuperfamily Pectinoidea (Stone, 1998).Molluscan conchiolin layers: Are theyantipredatory?—Conchiolin is the organiccomponent of molluscan shells composed ofproteins, polysaccharids, and glycosaminoglycans(Table 6) (Wilber and Simkiss, 1968; Gregoire,1972; Wainwright et al., 1982). The periostracumand non-calcareous operculae are composedchiefly of conchiolin, while the nacreous layersand other shell microstructures contain variousquantities of conchiolin. Thus, conchiolin has a veryold history, putatively stemming from the oldestshelled mollusc in the Cambrian Period. What ispuzzling, however, is that only a few groups ofmolluscs—chiefly the freshwater bivalves (e.g.,Margaritiferidae, Unionidae, Mutelidae), estuarineto marine bivalves (Corbiculidae and Solenidae),and a few marine species—have conchiolinrepresented as separate sheets within their shells(Taylor et al., 1969; Anderson, 1992; Harper, 1994).Conchiolin, as a protein, is thought to form at a highmetabolic cost to the organism—and, perhapsbecause of this, there appears to be an evolutionarytendency to lose conchiolin layers (references inKardon, 1998). Thus, there must be someevolutionary reason for maintaining conchiolin inmolluscan shells despite its high metabolic cost ofproduction (Table 6). It has long been hypothesizedthat the conchiolin sheets deterred predation bydrilling molluscan predators, such as naticid (Lewyand Samtleben, 1979) or muricid gastropods (Taylor,1970,1981), and most of the work done to test thishypothesis has focused on the corbulid bivalves(Fischer, 1963; Lewy and Samtleben, 1979; DeCauwer, 1985; Anderson et al., 1991; Anderson,1992; Harper, 1994). The corbulids (FamilyCorbulidae) are small, inequivalved bivalves with aglobose shell form, a single byssus thread, andshallow burrowing habits (e.g., Stanley, 1970). Theyfirst appeared in the Middle Jurassic, with thegreatest diversification taking place in theCretaceous and Eocene (e.g., Hallam, 1976).There appear to be three contrasting temporal“trends” related to whether conchiolin reducespredation. The first is that conchiolin doeseffectively reduce predation on corbulids throughtheir evolutionary history. Fischer (1963), for172