Spatial Variation of Naticid Gastropod Predation in the Eocene of North-America Thor A

Western Washington University Western CEDAR

Geology Faculty Publications Geology

6-1995 Spatial Variation of Naticid Gastropod Predation in the Eocene of North-America Thor A. Hansen Western Washington University, [email protected]

Patricia H. Kelley

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Recommended Citation Hansen, Thor A. and Kelley, Patricia H., "Spatial Variation of Naticid Gastropod Predation in the Eocene of North-America" (1995). Geology Faculty Publications. 1. https://cedar.wwu.edu/geology_facpubs/1

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drillingfrequencies (mean = 30.3%, n = 4 samples) than did the Gulf Coast assemblages (mean = 18.0%, Spatial Variationof Naticid Gastropod n = 11 samples; Mann-Whitney U-test,P < 0.01). This resultis con- Predationin the Eocene of sistent with a trendfound by some previousworkers of an equatorward NorthAmerica decrease in naticid drilling.

INTRODUCTION THOR A. HANSEN = 49.58, P << 0.001), but there was The drillinggastropod family Na- Departmentof Geology, no significantbathymetric trend in ticidaehas figuredprominently in the WesternWashington University, drillingfrequencies among the five debate over escalation in predator- Bellingham,WA 98225 assemblages of the Moodys Branch. preysystems and in the studyof co- Drillingfrequency was notrelated to evolution.Vermeij (1987) used data PATRICIA H. KETTL,FY substrate type within the Moodys on drillingfrequencies to arguethat Departmentof Geologyand Branch. Drilling,however, was cor- biologicalhazards such as predation, GeologicalEngineering, related with species diversity of and aptationsto these hazards,have Universityof NorthDakota, Moodys Branch assemblages (r = increasedduring the Phanerozoic (the P.O. Box 8358, 0.50, P < 0.01). Drilling frequency "hypothesisof escalation"). Howev- Grand Forks,ND 58202 was significantlycorrelated with the er, the studies compiledby Vermeij percentage of naticids in each as- typicallyincluded one or a fewsam- semblage (r = 0.77, P < 0.001) and ples (taken fromsingle lithofacies), PALAIOS, 1995, V. 10, p. 268-278 the percentageof certainprey spe- whichwere then used to characterize cies ofthe bivalvefamilies Lucinidae thedrilling frequency of an entireage. Although,the fossil record of naticid and Corbulidae,and the gastropod Moreover,analysis of these limited gastropoddrilling has played an im- families Turritellidaeand Hippon- data necessarilymeant that drilling portant role in the controversyover icidae (r = 0.80, P < 0.001). These frequenciesfrom very differenten- predator-prey evolution, little is preferredspecies represented40% vironmentswere comparedthrough known about variation of drilling of the naticid victims,but only 16% time. Vermeijrecognized these defi- frequencieswithin single horizons or of the assemblages. ciencies in the existing literature how predation patterns are influ- Cook Mountain assemblages rep- whenhe called formore research into enced by environmentalvariables. resentedmarine inner to middle shelf drillingpatterns in the fossilrecord Withoutan understandingof spa- environmentsfrom Virginia, Ala- and cautioned that only "similar tial variation in drilling,temporal bama, Louisiana, and Texas. Nati- nonbiologicalenvironments" should patterns in drillingare difficultto cid drillingfrequencies of the entire be comparedthrough time (Vermeij, interpret.We surveyed27,554 spec- molluscan fauna ranged from6.8- 1987, p. 423). We know littleabout imens of molluscs from the Cook 38.7% with a mean of 22%. In the how naticid drillingpatterns vary Mountain interval (upper middle CookMountain, three preferred spe- acrossdifferent physical or biological Eocene) and Jackson Group (late cies (of the families Turritellidae, environmentsin the Recent;existing Eocene) of the U.S. Gulfand Atlan- Lucinidae, and Noetiidae), repre- informationis sketchyand largely tic Coastal Plain to documentspa- senting24% ofthe totalindividuals, anecdotal. Nevertheless, without tial variationin naticid drillingfre- accountedfor 44 % of the drilledin- knowledgeof the potentialrange of quencies. dividuals.As in the MoodysBranch drillingfrequencies within a single The Jackson Group assemblages Formation,Cook Mountain assem- horizon and how drilling is influ- fromthe MoodysBranch and Yazoo blages showed a significantcorrela- enced by environmentaland biolog- formationswere sampled along a tion betweenrelative abundance of ical variablessuch as substrate,water depth gradientin orderto compare preferredspecies and drillingfre- depth,latitude, and preyavailability, drillingfrequencies to environmen- quency of samples (r = 0.585, P < it is impossibleto assess the signifi- tal variableswithin a singleclimatic 0.05). The dependence of naticid cance of temporaltrends based on zone. There was a statisticallysig- drillingon the availability of pre- limitedareal studies. nificantdifference between drilling ferredprey species indicates a pos- We have undertakena comprehen- frequencyof the inner to middleshelf sible behavioral inflexibilityin na- sive survey of naticid drillingpat- Moodys Branch Formation(mean = ticidpredation patterns. terns in the Cretaceous and Paleo- 8.3%) and theouter shelf Yazoo For- The VirginiaCook Mountainfau- gene formationsof the U.S. Atlantic mation (mean = 21.2%; chi square na exhibited significantlygreater and Gulf coastal plains. Over 46,000

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specimensof molluscshave been ex- drillingfrequency from all sources MATERIALS amined from14 stratigraphiclevels (includingnon-naticid drillers) from of formationscale. Kelley and Han- stationsat 125 m, 175 m, and 225 m This study examinesspatial vari- sen (1993) reportedlargely on the waterdepth along a transecton the ationin naticidpredation within two temporal aspect of this study, as- westcoast ofBarbados. Nine species, well-sampledstratigraphic intervals sessing Cretaceous through Oligo- that each representedgreater than from the U.S. Gulf and Atlantic cene naticid predation in order to 1% ofthe molluscan population were coastal plains. The Cook Mountain evaluate the hypothesisof escalation presentat morethan one station.Six "interval" (upper middle Eocene, forthe naticidpredator-prey system. of these species showed an increase Claiborne Group) comprises the Concurrentlywith this temporalre- in drillingfrequency with depth (no roughly contemporaneous Piney search,we evaluatedspatial variation statisticaltests were performed). The Point Formationof Virginia,the up- of naticid drillingin two widelydis- trendis not convincing,however. Of per Lisbon Formationof Alabama, tributedhorizons: the middle Eocene the fourspecies commonto all three and the Cook Mountain Formation Cook Mountain Formationand its stations,only two showed a consis- of Louisiana and Texas; the upper stratigraphic equivalents (here tent trend of greater drillingwith Eocene intervalincludes the Moodys termed the "Cook Mountain inter- depth; drillingon one species was Branch and Yazoo formationsof val"), and the upperEocene Moodys most intense at the intermediate Louisiana and Mississippi (Jackson BranchFormation. The resultsof our depth,and the otherspecies showed Group). These stratigraphicinter- spatial surveyof naticid predation virtuallyno change. vals representtime spans of approx- were incorporatedinto our evalua- More informationis available on imately2-3 m.y.each (Hansen,1980). tion of temporaltrends (Kelley and latitudinalvariation in predatordi- All of the outcropsof these units Hansen, 1993). Here we documentin versityand drillingfrequencies. The expose richlyfossiliferous and un- detail our insightsinto how naticid diversityof all drillinggastropods in- consolidated clastic marine sedi- predation varied among different creases towards the equator (Mar- ments with aragonitic and calcitic lithofacies and biofacies within incovich,1977; Taylor and Taylor, shell mineralogiespreserved. Shell stratigraphichorizons. 1977). In accord withthis trend,na- preservationis excellent;most shells ticid drillingfrequencies on tropical are robustand sometimesretain orig- PREVIOUS STUDIES OF turritellidgastropods are higherthan inal coloration.Moodys Branchand GEOGRAPHIC VARIATION on temperatespecies (Dudley and Yazoo samples were taken from IN DRTT,T,ING Vermeij,1978). On the otherhand, a closely spaced dip and strike sec- surveyof bivalveprey from 10 trop- tions,allowing them to be placed in Previous works indicate that the ical and 7 coldtemperate sites in both a well-constrainedpaleoenvironmen- geographicvariability of Recent na- the Atlanticand Pacific Oceans re- tal framework.This cohesivenessof ticiddrilling frequencies can be wide; vealed an equatorwarddecrease in thesamples permitted comparison of forinstance, drilling frequency on the naticiddrilling frequency (Vermeij et drilling frequencies with environ- bivalve Codakia bella fromfive sites al., 1989). In addition,Paleocene tur- mentalvariables such as waterdepth on Guam varied from17-75% (Ver- ritellids from Alabama exhibited and substrate,and with biological meij, 1980). The same studyalso re- lowernaticid drilling frequencies (11 variables such as diversityand tax- ported geographicvariation in nati- of137 shellsdrilled) than did roughly onomiccomposition of the fauna. The cid drillingof the veneridGafrarium contemporaneousspecies in Mary- Cook Mountain interval includes pectinatum (0-19%, five sites on land and Virginia(33 of 179 shells samplesfrom widely spaced outcrops Guam), the cardiid Fragum fragum drilled;Allmon et al., 1990). along the Atlanticand GulfCoastal (0-23%, 5 sites), and the tellinid Thus, previousstudies on drilling Plain. Althoughthese outcropsare Quidnipagus palatum (4-18%, 5 patternsreveal that: 1) the range of environmentallyunrelated and dif- sites). However,sample size of each drillingfrequencies on a preypopu- ficultto place alonga depthgradient, taxon at most sites was relatively lation routinelyvaries from5-30% they cover a large latitudinalrange small (as fewas 21 valves; only two withinrelatively limited areas; 2) no and allow examinationof latitudinal of these samples had morethan 100 convincingtrend of drillingwith wa- variationsin drilling. valves). Drillingfrom all sources(in- terdepth can be detected;and 3) ev- CookMountain Interval.-In Vir- cludingnon-naticid drillers) on gas- idencefor a latitudinaltrend in drill- ginia,the Cook Mountaininterval is tropodsliving on sand or mud varied ing frequencies is contradictory. representedby the Piney Point For- from5.9-30.3 % in theeastern Pacific However,these studies generallyin- mation,exposed along the Pamun- and from5-38 % in thetropical west- volved relativelysmall sample sizes keyRiver. The PineyPoint is a clay- ern Pacific and Indian Ocean (Ver- and, in manycases, verylimited geo- ey, glauconitic,highly fossiliferous meij, 1987; no sample size given). graphicareas. We willdocument spa- sand (Ward,1984). Sedimentological Little is knownabout the variabil- tial trendsin the fossilrecord using and paleontologicalinformation in- ity of naticid predationwith depth a muchlarger data base overa broad dicate deposition in an inner-shelf in modernoceans. Sander and Lalli geographicarea and range of envi- environmentof less than 100m water (1982) studied species diversityand ronments. depth (Strickland,1984). Four sam-

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TABLE 1-Sample size (measured as numberof individuals,# IND), di- vironment;assemblage C, clay-rich versity(measured by R' = S - 1/logN), percentnaticids (% NAT), mean finesand on the innershelf; assem- grain size (GSIZE) and percent preferredspecies (% PREF) forthe Cook blage D, sandy clay on the western Mountaininterval. V, Virginia;A, Alabama; L, Louisiana; T, Texas. inner middle shelf; assemblage E, clay-richfine sand to sandy clay on Sample # IND R' % NAT GSIZE % PREF the eastern innermiddle shelf;and assemblageF, sandyclay on theouter H76-64-2V 194 12 4.1 0.50 22.7 middle shelf (Fig. 1). Of the thirty- H76-64-3V 259 12 4.3 0.33 30.2 threebulk samples,24 weresurveyed H76-64-5V 307 9 0 0.24 41.9 for drillholes:5 each from assem- H76-64-8V 236 8 0 0.22 19.1 blages A and B, 6 fromassemblage H76-31-3A 1234 14 7.9 0.35 73.0 C, 4 fromassemblage D, and 2 each H76-49-1A 260 13 2.3 0.41 1.9 fromassemblages E and F. H76-52A 210 14 3.3 0.34 4.3 Yazoo Formation. -The Yazoo H76-34-13A 286 9 5.6 0.14 10.5 Formationrepresents the deep-water H76-27-1A 316 13 2.2 0.29 7.8 facies of the Moodys Branch-Yazoo H76-76-1L 249 5 8.9 0.13 0 transgressivesequence. CynthiaPit H76-73-7T 282 7 11.3 0.05 0 sedimentswere deposited in an "out- H76-73-8T 636 9 13.4 0.28 0 er neritic"environment (Smith and H76-69-1T 224 3 21.9 0.09 0 Zumwalt,1987), and were included H76-69-2T 298 3 27.2 0.09 0 to broadenthe bathymetricrange of T 5 11.9 0.41 0 H76-69-4 245 our upper Eocene interval.Fossil re- coveryis generallypoor in the Yazoo as the shells tend to be chalkyand ples weretaken from the PineyPoint lagoonal environment(A.J. Scott, do not survivesieving. However, a Formation (samples collected from pers.comm., 1978). In total,15 sam- quarry of the Jackson Ready-Mix each formationare listedin Table 1). ples fromthe Cook Mountaininter- ConcreteCompany in the Yazoo For- The upper Lisbon Formation in val weresurveyed for drillholes. mation at Cynthia,Mississippi, has Alabama was collected froma road MoodysBranch Formation.-The exceptionallywell-preserved fossils cut near Barrytonin ChoctawCoun- Moodys Branch Formationis a thin that are easily sieved. Twelve bulk ty (sample numberH76-31-3), along (3-14 m thick),fossiliferous glauco- samples were processed fromCyn- thebanks of Little Stave Creek(H76- niticsand and sandyclay. The Moo- thia Pit. 34-13),and fromexposures on theAl- dys Branchis a transgressivemarine abama (H76-27-1) and Conecuh riv- shelf sand conformablyoverlain by METHODS ers (H76-49-1and H76-52). The sam- the outer neritic Yazoo Formation ples are veryfossiliferous sands and (Dockery,1976; Smith and Zumwalt, Sample Processing.-All of the clay-richsands representinga shal- 1987). collectionscome frombulk samples, low inner-shelfenvironment (Gard- Thirty-threebulk samples from which were wet sieved througha ner,1957). eightsections in the MoodysBranch 1-mmscreen. All shells werepicked Samples fromthe Cook Mountain wereprocessed for molluscs (washed, and identified,using the taxonomy Formationwere collected fromSa- picked,identified and counted) and of Palmer and Brann (1965, 1966), line Bayou in Louisiana (H76-76-1), subjectedto Q-Mode clusteranalysis with modifications and additions and the Trinity(H76-73-7 and H76- (Elder and Hansen, 1981). This anal- from Bretsky (1976) and Dockery 73-8) and LittleBrazos riversin Tex- ysisdelineated six distinctmolluscan (1977). This processof collection and as (H76-69-1,H76-69-2 and H76-69- assemblages(designated A-F, see Fig. identification preserved relative 4). The Cook Mountain is largelya 1). Depositionalenvironments of the abundances of species in the time- siltyor slightlysandy clay. Samples assemblageswere interpretedusing averagedfossil assemblages. Because fromLouisiana and theTrinity River stratigraphicrelationships and sedi- it was not possible to determinede- represent open-marine conditions mentologicand taphonomic infor- finitivelywhether incomplete speci- (Davis, 1962). Samples fromthe Lit- mation (e.g., grain size, mineralogy, menswere drilled, we used onlycom- tle Brazos Rivercontain a low-diver- sedimentarystructures, shell condi- plete or nearlycomplete specimens sitymolluscan assemblage with high tion and orientation,and tracefossil in our analysis. Tables 1 and 2 list juvenile mortality.The presence of assemblages; Elder and Hansen, the number of complete specimens pyrite and numerous crab concre- 1981). Environmental interpreta- surveyed for drillholes from each tions in this unit suggestlow oxygen tions indicatea generalonshore-off- sample; the study included 7749 conditions. In addition, the Cook shoretransect as follows:assemblage specimensfrom the Cook Mountain MountainFormation at LittleBrazos A, finesand in an open bay or lagoon; intervaland 17,364from the Moodys River is conformablyoverlain by assemblageB, mediumto finesand Branch. Because of low numbersof marsh clays,suggesting a nearshore in a nearshorewave-influenced en- specimens in the CynthiaPit sam-

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0 10 20 30 40 50 60 70 80 90 100 I I I I I I I I I I H-79-11-1 H-79-11-3 H-80-8-4 H-80-9-1 H-80-8-2 H-80-8-3

H-80-8-1-

H-80-10-2 H---80-10-4 H-79-13-1 A H-8o-1o-8 H-79-12-2 H-8-10-10 H-79-13-3 H-79-12-5 H-79-12-6 B H-79-12-1 H-79-12-3 H-79-12-4 H-79-10-4 H-79-10-5 H-76-75-2 D H-7-75-3 H-79-10-1 H-79-10-2 H-76-75-1 E H-79-11-6 H-79-11-7 F H-76-75-7 H-79-11-8 H-76-75-9 H-79-11-14

FIGURE1 -Cluster diagramfor Q modeanalysis of 33 samplesfrom the Moodys Branch Formation. Letters A-F correspondto assemblages discussed in text. pies,the 12 Yazoo sampleswere com- forcesthan are undrilledshells. Thus bilityof bias in this study,we ex- bined into one compositecollection, drilledbivalve shells could be under- aminedthousands of specimens from whichyielded 2441 specimens. representedin thesesamples because twoCook Mountainsamples, looking Roy et al. (manuscript)reported ofselective breakage. Roy et al. listed for degree of breakage and breaks thatmodern Mulinia shellsthat have severalcriteria to determinewhether throughdrillholes that would signify been drilled are significantlymore such a bias potentiallyexists in fossil weakeningof the shell by the hole. likely to break under compressive assemblages.To evaluate the possi- Althoughwe foundhigh numbers of

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TABLE 2-Sample size (measured as numberof individuals,# IND), di- tal numberof gastropodspecimens versity(measured by R' = S - 1/logN), percentnaticids (% NAT), mean plus one-halfthe numberof pelecy- grainsize (GSIZE) and percentpreferred species (% PREF) forthe Moodys pod valves) with one or more com- Branch and Yazoo formations.Letter after sample numberrefers to as- plete drillholes.Similarly, the fre- semblage; CP, CynthiaPit. quency of incompletedrillholes was calculated for each sample. Using techniquesof correlationand linear Sample # IND R' % NAT GSIZE % PREF regression,we thencompared the fre- H79-13-1A 180 13 0 0.19 11.1 quencies ofcomplete and incomplete H80-10-2A 157 10 1.9 0.28 35.4 drilling with latitude, depth, and H80-10-4A 114 13 0.9 0.18 24.6 meangrain size. We also relateddrill- H80-10-8A 105 11 2.9 0.50 16.7 ing to a varietyof biologicalfactors, H80-10-10A 357 15 1.4 0.21 14.0 includingspecies diversityof the as- H79-12-1B 143 11 0 0.37 22.7 semblage,percentage of naticids, and H79-12-3B 234 11 0 0.36 10.3 percentageof selectedprey species. H79-12-4B 289 8 0.4 1.00 6.2 H79-12-5B 166 9 0 0.70 8.1 RESULTS H79-12-6B 232 9 0 0.44 3.7 H79-11-1C 612 13 1.5 0.37 23.0 The drilling frequencies of the H79-11-3C 1296 14 0.8 0.29 18.8 Moodys BranchFormation were sig- H80-8-2C 740 12 0.3 0.26 13.1 nificantlydifferent from those of both H80-8-3C 989 11 0.1 0.21 5.5 the Yazoo Formation(chi square = H80-8-4C 718 13 0.4 0.40 10.6 49.58, P << 0.001) and the Cook H80-8-5C 329 12 2.4 0.51 21.9 Mountain interval (Mann-Whitney H76-75-1D 398 10 0.5 0.11 18.2 P < Neitherthe Cook 2.1 0.42 40.1 U-test, 0.001). H76-75-2D 380 13 Mountain intervalnor the 11 2.6 0.40 30.4 Moodys H79-10-1D 235 Branch Formationshowed a corre- H79-10-4D 418 8 0.5 0.20 14.4 H79-11-6E 389 12 0.8 0.30 18.8 lationbetween total drilling frequen- H79-11-7E 679 9 0 0.34 5.5 cy and sample size as measuredby H79-11-8F 82 9 0 0.26 21.5 the total numberof individuals(Ta- H76-75-4F 49 7 0 0.05 20.6 ble 3). The frequencyof complete CP 1977 15 7.6 0.04 12.9 drillholesfor 15 samplesof the Cook Mountain intervalranged from 6.8- 38.7% witha meanof 22 % (Table 4). Shellswith incomplete drillholes rep- fragmentedshells, we also foundsub- plete but nonfunctionaldrillholes resented0-2.6% of the fauna (mean stantialnumbers of broken shells with (those in whichthe ratio of innerto = 1.34%). Overalldrilling frequency completedrillholes (i.e., the breakage outerborehole diameter is less than (completeand incompletedrillholes) did not occur througha hole), argu- about 0.5; Kitchell et al., 1986), nor represented23.2% of the fauna for ingagainst such bias. The formations did we specificallynote edge drills the Cook Mountaininterval. The fre- we have studiedcontain some of the (Vermeijet al., 1989). quency of completedrilling for the best preservedfossil molluscan de- Grain Size.-An unwashed sub- 24 Moodys Branch samples ranged positsin theworld, with all shellmin- sample fromeach bulk sample was from3.0-21.9 %, witha meanof 8.3 % eralogiespreserved and no evidence dried, weighed, and wet sieved (Table 5). Incompletedrillholes were oftectonism or diageneticalteration. througha stackedstandard sieve set present in 0-11.0% of the fauna Because ofthe excellentpreservation of 2.0, 1.0, 0.5, 0.25, 0.125,and 0.063 (mean = 1.7%). Overall drillingfre- of these deposits, taphonomicbias mm.Residue on each sieve(including quencyrepresented 9.9 % ofthe fau- should be minimizedhere. shells) was dried and weighed.Total na ofthe MoodysBranch Formation. Data Tabulation.-Holes drilled weightof sieved materialwas sub- The Yazoo compositesample had a by naticidsgenerally are bevelledand tractedfrom initial sample weight to drillingfrequency of 21.2 % and 0.8 % parabolicin crosssection, in contrast obtainthe weight of the silt-clay frac- for complete and incompletedrill- to straight-sided,unbevelled holes tion. Mean grainsize was calculated holesrespectively, yielding an overall drilled by muricids. We examined as the cumulativesum ofthe relative drillingfrequency of 22.0%. specimensfor naticid drillholesand percentageby weight in each size We examined the relationshipof recordedwhether each drillholecom- fractionmultiplied by the median drillingto latitudeby comparing Gulf pletely penetratedthe shell or was grainsize of that fraction. Coast samplesof the Cook Mountain incomplete.Because of the magni- Data Analysis.-For each sample, interval with those from Virginia tude ofthe study(27,554 specimens), drillingfrequency was determinedas (Table 4). Frequencies of complete it was not practicalto identifycom- the percentageof individuals (the to- and incompletedrillholes were com-

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pared forthe fauna as a whole,as well TABLE 3-Correlation coefficients(r values) forlinear regressions of vari- as forindividual components such as ables against percentage of individualswith complete drills(bivalves and bivalves, gastropods,all turritellid gastropods combined). Asteriskindicates significance:* = P < 0.05, ** = gastropods,and Turritella nasuta, P < 0.01,*** = P < 0.001. the onlyspecies of Turritella present the Gulf Coast in both Virginiaand Cook Mountain MoodysBranch samples. The drillingfrequencies of = the total fauna forVirginia (mean size 0.11 0.28 = Sample 30.3%, n 4 samples) were signifi- Mean grainsize 0.40 0.18 cantlyhigher than those forthe Gulf % Naticids 0.31 0.77*** Coast samples (mean = 18.0%, n = % Bivalves 0.26 -0.69** 11 samples) as determinedby the % Preferredspecies 0.58* 0.77*** Mann-WhitneyU-test (P < 0.01). R' = (S - 1/logN) 0.40 0.50** Using the same test, no individual componentof the Cook Mountain faunadiffered significantly in drilling frequencybetween the Virginiaand systematicallywith depth when the Moodys Branch Formation, pre- Gulf Coast samples. total fauna, bivalves,or gastropods ferredprey species were Gonimyr- Predationintensity has beenshown wereconsidered for Moodys Branch taea curta and G. subcurta,Hippo- to affectdiversity in someliving com- AssemblagesA-F (Spearman's rank nix pygmaea, Caestocorbula waile- munities(Paine, 1966,1981).For each correlationcoefficient, rs, was nonsig- siana, and Turritellaalveata. These sample, we calculated species diver- nificantin all cases; Table 6). Several fivespecies accounted for 40% ofthe sityas R' = S - 1/logN(where S = widespread,frequently drilled spe- totalnumber of drilled individuals in numberof species and N = number cies also showed no trendin preda- the Moodys Branch Formationbut of individuals,Tables 1 and 2). The tionwith depth (Gonimyrtaeacurta, only 16% of the total numberof in- Moodys Branchshowed a significant G. subcurta, Corbula wailesiana, dividuals.In the Cook Mountainin- correlationof drilling frequency with Turritella alveata, and Hipponix terval, preferredprey species were R' (r = 0.50, P < 0.01). Withinthe pygmaea; Table 7). Gonimyrtaea Turritella sp., Lucina pomilia, and Cook Mountain interval,there was curtaand T. alveata exhibitedan in- Pachecoa decisa. These threespecies no significantcorrelation between to- crease between Moodys Branch as- accountedfor 44% of the drilledin- tal drillingfrequency and species di- semblagesA-E, but chi-squareval- dividuals in the Cook Mountain in- versity(Table 3). ues werenonsignificant in eithercase. tervaland 24% of the total number Drillingfrequency may also be re- There was no significantrank cor- of individuals.There was a signifi- lated to the abundance of naticids. relationof percentincomplete drill- cant correlation(Table 3) between The percentof naticidsin the Moo- holeswith depth for the bivalves, gas- the relativeabundance of preferred dys Branch and Yazoo Formations tropods,or forthe totalfauna (Table prey in a sample and total drilling was significantlycorrelated with the 8). However,the drillingfrequency frequencyof the sample in both the frequencyof completedrillholes (r = of the Yazoo assemblage (Cynthia MoodysBranch Formation (r = 0.80, 0.77, P < 0.001, Table 3). However, Pit), whichrepresented a deeperwa- P < 0.001; Fig. 2) and the Cook drilling frequencywas not signifi- ter environmentthan the Moodys Mountain interval(r = 0.585, P < cantlycorrelated with the percentof Branch Formation,was significantly 0.05; Fig. 3). naticidsin theCook Mountainfauna. greater than the overall Moodys To relate drilling and substrate Branchdrilling frequency (chi square DISCUSSION type, we performeda least-squares = 49.58) and greaterthan the drilling regressionof grain size against the frequency of any single Moodys The variabilityof drilling frequen- frequencyof complete and incom- Branch assemblage (chi-squareval- cy in Eocene fossilassemblages (es- plete drillingfor each sample from ues rangefrom 4.51 to 62.34). peciallythe Cook Mountaininterval, the Cook Mountain interval and Some ecological theory suggests in whichdrilling ranged from 6.8 to MoodysBranch Formation (Tables 1 thatpredators may choose their prey 38.7%) was similarto that seen in and 2). Drillingintensity and grain based on the abundance ofthat prey modernenvironments (5-38%; Ver- size werenot significantlycorrelated speciesin the community(Valentine, meij, 1987). The mean (8.3%) and for either stratigraphicunit (Table 1973). Withinboth the Cook Moun- range (4-21.9%) of drillingfrequen- 3). tain and Moodys Branch, however, cies in the Moodys Branch Forma- Samples for the Moodys Branch we found no overall correlationbe- tionwere significantly less, though the Formation were combined into as- tweenthe abundanceof each species values overlappedthose for the Cook semblages, which were compared and its drillingfrequency. Some spe- Mountaininterval (this drop in drill- along a depth gradient(Tables 6-8). cies weredrilled at frequenciesmuch ing frequency between the Cook The percentageof individualswith higherthan their proportionalrep- Mountain and Moodys Branch was complete drillholesdid not change resentationin the assemblage.In the associated with molluscan extinc-

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TABLE 4-Numbers of specimens withoutdrills (ND), withcomplete beveled drills(CD), withincomplete drills (IC) and withunbeveled drills(UN) forthe Cook Mountaininterval. % CD and % IC in farright columns indicateoverall percentages for numbers of individuals.Letter after sample numberindicates location of outcrop: Virginia(V), Alabama (A), Louisiana (L), and Texas (T).

Sample ND CD IC UN ND CD IC UN % CD % IC H76-64-2V 250 7 4 4 25 5 0 0 38.7 2.6 H76-64-3V 305 84 4 2 57 7 0 0 34.8 1.8 H76-64-5V 453 83 3 0 38 1 0 0 27.2 1.1 H76-64-8V 413 48 3 0 4 0 1 0 20.3 1.9 H76-31-3A 963 257 4 3 496 115 7 3 30.1 1.0 H76-49-1A 337 16 1 0 74 8 1 0 9.2 0.8 H76-52A 271 39 4 1 37 15 0 0 25.7 2.1 H76-34-13A 429 42 7 1 47 3 0 4 14.8 2.6 H76-27-1A 464 56 5 4 51 5 0 0 19.0 1.7 H76-76-1L 321 14 4 0 77 3 1 0 6.8 2.1 H76-73-7T 3 1 0 0 205 68 4 3 24.5 1.4 H76-73-8T 5 1 0 0 555 77 1 0 12.3 0.2 H76-69-1T 8 2 1 0 191 27 0 0 13.0 0.5 H76-69-2T 2 0 0 0 245 52 0 0 17.4 0 H76-69-4T 1 1 1 0 182 61 0 0 25.4 0.4

TABLE 5-Numbers of specimens withoutdrills (ND), withcomplete beveled drills(CD), withincomplete drills (IC) and withunbeveled drills(UN) forthe Moodys Branch and Yazoo formations.% CD and % IC in farright columns indicateoverall percentages fornumbers of individuals.Letter after sample numberindicates assemblage desig- nation and relativeposition on an onshore-offshoregradient.

Sample ND CD IC UN ND CD IC UN % CD % IC H79-13-1A 289 8 1 1 28 1 1 0 5.0 1.1 H80-10-2A 241 18 0 3 24 2 0 0 13.3 0 H80-10-4A 183 13 2 0 14 1 0 0 12.3 1.8 H80-10-8A 143 14 4 3 23 0 0 0 13.2 3.8 H80-10-10A 540 35 3 0 64 4 0 0 11.5 0.8 H79-12-1B 195 9 0 0 33 6 1 1 10.5 0.7 H79-12-3B 393 19 0 1 24 3 0 0 9.3 0 H79-12-4B 516 11 2 1 24 0 0 0 4.0 0.7 H79-12-5B 300 5 1 0 13 0 0 0 3.0 0.6 H79-12-6B 427 9 2 1 5 1 6 0 4.2 3.5 H79-11-1C 893 46 1 0 126 16 0 0 10.0 0.2 H79-11-3C 2166 99 2 16 130 24 0 0 9.2 0.2 H80-8-2C 1310 35 1 5 60 3 1 0 5.2 0.3 H80-8-3C 1809 47 43 7 32 4 0 0 5.1 4.3 H80-8-4C 1209 43 4 1 81 8 0 0 7.1 0.6 H80-8-5C 496 37 4 0 55 5 0 0 13.2 1.2 H76-75-1D 676 17 1 7 43 4 0 0 5.1 0.3 H76-75-2D 526 69 16 0 59 12 3 0 21.9 5.0 H79-10-1D 335 24 5 0 44 9 0 0 14.0 2.1 H79-10-4(D) 762 29 2 4 14 4 1 0 7.6 0.7 H79-11-6(E) 518 30 8 1 90 17 3 0 11.6 2.8 H79-11-7(E) 1159 64 23 5 48 3 2 0 10.1 3.7 H79-11-8(F) 115 7 2 1 12 0 7 0 9.9 11.0 H76-75-4(F) 84 3 3 1 2 0 1 0 6.5 8.2 CynthiaPit 837 86 4 1 1157 333 12 11 21.2 0.8

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tionsand climatechange; Kelley and TABLE 6-Percentage of individualswith complete drillsin the Moodys Hansen, 1993). However,the range Branch Formation.Assemblages A-F representa general onshore to off- of drillingfrequencies in both hori- shore gradient(F is furthestoffshore). Numbers of specimens are in pa- zons indicates that samples from rentheses.There is no significanttrend with depth as determinedby Spear- many differentpaleoenvironments man's Rank CorrelationCoefficient (differences of 0.1% were treated as are required to adequately charac- ties). terizethe drillingfrequency for a par- ticular time. Studies of fre- drilling Assemblage quency based on one or two samples froma singlelocality must be viewed A B C D+E F CP with great caution if these samples are used to representthe predation Bivalves 11.7% 5.6% 7.4% 10.9% 9.3% 18.5% intensity"characteristic" of a partic- (1501) (1892) (8273) (4281) (216) (928) ular time. Gastropods 9.9% 3.9% 11.2% 13.8% 0 22% We found a correlationof preda- (162) (259) (545) (356) (22) (1513) tion intensity with latitude. Al- Total 9.0% 5.7% 7.8% 10.9% 7.7% 21.1% though selected componentsof the fauna showed no significantdiffer- ences withlatitude, drilling frequen- depth and incomplete drillholes fauna rich in carnivores,including cies for the fauna as a whole were withinthe MoodysBranch. The lack high densitiesof naticids,is typical significantlygreater in Virginiasam- of a clear trendbetween drilling fre- of marine communities living at ples than those for the Gulf Coast. quency and depth on the Paleogene depthsgreater than 200 m,where pri- This result is in accord with obser- innerto middleshelf is in accordwith mary productivityis reduced and vations of an equatorwarddecrease our limited modern data base in suspension-feedingless important in drillingon modernbivalves (Ver- which the trend is vague at best (Hickman, 1984). This relationship meij et al., 1989) and Paleocene tur- (Sander and Lalli, 1982). We did, suggeststhat depth is relatedto drill- ritellids (Allmon et al., 1990). An however,find significantlygreater ingfrequencies only insofar as depth equatorwarddecrease in drillingcon- drillingwithin the outershelf Yazoo influencesthe percentageof naticids tradictsa perhaps morewidely held Formation. in an assemblage.Indeed, thereis a view that "drillinggastropod preda- The intensepredation pressure in significantcorrelation between the tors are strikinglymore diverse, and the Yazoo can be explained by the abundance of naticids and drilling they have a greaterimpact ecologi- highproportion of naticids in theYa- frequenciesin the Moodys Branch cally in the tropicsthan in the tem- zoo. The Yazoo assemblagecontains Formation.In addition,the signifi- perate zones" (Vermeij,1987, p. 172; a muchlarger percentage of naticids cantly higher overall drilling fre- Taylorand Taylor,1977; Vermeij and thandoes theMoodys Branch (7.58% quencyin the Cook Mountaininter- Dudley, 1982). vs. 0-2.85%) and manymore carniv- val, as compared with the Moodys The Moodys Branch and Yazoo orous gastropodsoverall, especially Branch, is mirroredby a greater Formationswere used to assess en- of the familyTurridae. A molluscan abundance of naticids (9%) in the vironmental and biological con- straintson drilling,because of their tightpaleoenvironmental control. In TABLE 7-Percentage of individualsof preferredspecies with complete addition,these samples representa drillsin the Moodys Branch and Yazoo formations.Assemblages A and B narrow latitudinal range, so that were combined to increase sample sizes (sample size in parentheses). X complicationsdue to variationin pa- not present. leoclimateare avoided. Species diversityand drillingfre- Assemblage quency were correlatedwithin the Moodys Branch Formation.This re- A+B C D+E F CP lationshipis consistentwith the hy- G. curta 15% pothesisthat predators may increase 18% 24% 0 18% diversitywithin communities by (156) (311) (132) (3) (11) pre- G. subcurta 39% 34.1% 43.8% X ventingmonopolization of resources 33% dominant (169) (387) (64) (6) by species (Paine, 1966). C. wailesiana 17.3% 9.7% 24.4 0 Depth did not correlatewith drill- 24.6% (127) (103) (443) (18) (505) hole frequencywithin the Moodys T. alveata 13.9% 18.4% 23.8% 0 X Branch Formationfor the faunaas a (72) (87) (63) (0) wholeor forselected widespread spe- H. pygmaea 5.9% 21.8% 16% 0 X cies (Tables 6 and 7). There was also (17) (124) (94) (22) no significantcorrelation between

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TABLE 8-Percentage of individualswith incompletedrills along an on- (Cost: benefitvalues are represented shore-offshoregradient in the Moodys Branch Formation.See Table 2 for by the ratio of drillingtime: energy sample sizes. gained, as reflectedby shell thick- ness: internalvolume; Kitchell et al., 1981; Kitchell, 1982; 1988, Assemblage Kelley, 1991.) An abundance of high-cost A B C D+E F CP preywould simplymake it too hard for many naticidsto make a living. Bivalves 1.3% 0.5% 1.3% 2.6% 4.6% 0.9% The correlationof drillingfrequency Gastropods 0.6% 2.7% 0 2.5% 36.4% 0.8% and percentageof preferred prey may Total 1.2% 1.0% 1.2% 2.6% 10.0% 0.8% also reflectthe behavioralstereotypy ofnaticids (Boggs et al., 1984;Kitch- ell,1986). Naticid prey selectivity may Cook Mountain.On the otherhand, Moodys Branch Formationor Cook developover evolutionary time based there is no correlationbetween the Mountaininterval. on cost-benefitfactors. In the short abundance of naticids and drilling The significant correlation be- term(within an individuallifetime), frequencywithin the Cook Mountain tweenpercentage of "preferredprey however,naticids may have difficulty interval. species" and drillingfrequency with- adjustingto the absenceof these pre- Assumingthat drillingfrequency in assemblagesof the Cook Mountain ferredspecies by targetingothers. is a functionof naticid abundance, and Moodys Branch (the only cor- Thus, whenpreferred species are ab- what factorscontrol the occurrence relationcommon to bothstratigraph- sent, overall drillingfrequencies are of naticids?Possible factorsthat we ic levels;Figs. 2 and 3) suggeststhat lower. (Ongoing work explores the consideredwere substratetype and naticiddrilling frequencies were par- cost: benefit relationshipsbetween availabilityof prey. Because naticids tiallycontrolled by the availabilityof preferredspecies and less frequently are infaunal gastropods that plow suitable prey species. The inability drilledprey.) throughthe substrate beneath the ofnaticids to compensatefor the lack There is a remarkableconsistency sedimentsurface, we expected that of preferredprey, by increasingpre- in the familiesthat were preferred as substratetype mightinfluence their dation on otherspecies, may be due preyby naticidsfor the entirePaleo- distribution.However, we found no to high cost: benefitvalues for the gene,a periodof over 30 millionyears relationshipbetween grain size and alternativeprey in the assemblages (Kelley and Hansen, 1993). The pre- drillingfrequency within either the where predation intensityis low. ferredspecies in the MoodysBranch Formation were from the families Lucinidae,Turritellidae, Corbulidae, Branch and Hipponicidae. In the Cook Moodys Mountaininterval, in- 25 preferredprey cluded speciesof the Lucinidae, Noe- tiidae,and Turritellidae.(All ofthese U preferredprey were slow-moving in- 20- faunalburrowers, with the exception of the hipponicid,which was a slow- moving,epifaunal, possibly commen- a) sal species.) In otherPaleogene ho- .1 15- rizons,there were typically only one a C U U U or twopreferred species in each level U U :3 and theywere always in the families U Lucinidae, Corbulidae, or Turritel- Eu 10- * urn * U lidae and Ex- 0- (Kelley Hansen, 1993). U amples are: Phacoides sp. (Lucini- U U dae) and Haustator gnoma (Turri- 5- * U U U tellidae) in the BrightseatFormation U U of Mesalia U Maryland (Paleocene), (Turritellidae)in the Bells Landing Member of Alabama (Paleocene), n ...... Turritella aldrichi from the Mat- 0 5 10 15 20 25 30 35 40 45 thewsLanding Memberof Alabama % PreferredSpecies (Paleocene), Corbula subengonata FIGURE 2-Percentage of fauna with complete drillsplotted against percentage of fauna and Vokesula aldrichi (both Corbu- comprised of preferredspecies (Gonimyrtaeacurta, G. subcurta, Hipponixpygmaea, Caes- lidae) in the Bashi Formationof Al- tocorbula wailesiana, and Turritellaalveata) forthe Moodys Branch Formation. abama (lower Eocene), Corbula ru-

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faripa fromthe Red BluffFormation of Mississippi (Oligocene),and Cor- CookMountain bula laqueata fromthe Mint Springs --vI/In Formation of Mississippi (Oligo- U cene). 35- U Such a long-termassault on these particularfamilies by naticids may reflectan inabilityof the preyto es- 30- U calate intoa highcost: benefitrange. U However,the Turritellidaeand Cor- ° 251I I. bulidae showed significantincreases ': in percentages of incomplete and ( U 20- U multiple boreholes fromthe Creta- ceous throughOligocene, suggesting U some increasein preydefensive abil- o 15- ity(Kelley and Hansen, 1993). Long- term be fur- preypreference may alsp 10- U ther evidence forbehavioral stereo- time typyby naticids on evolutionary 5- scales. This long term inflexibility withregard to preyspecies may ex- n the decline of naticid L plain drilling 0 10 20 30 40 50 60 70 80 frequenciesaround the Cretaceous- % Preferred Tertiary and Eocene-Oligoceneex- Species tinctionevents and (Kelley Hansen, FIGURE3-Percentage of faunawith complete drills plotted against percentage of fauna 1993). Naticids may have been too comprised of preferredspecies (Turritellasp., Lucina pomilia,and Pachecoa decisa) forthe stereotypedto respond rapidly to Cook Mountaininterval. large fluctuationsin the populations of preferredprey species duringex- tinctionepisodes. Only on an evolu- erfor Virginia samples of the Cook tionately high drilling frequen- tionarytime scale could naticidsde- Mountaininterval than forthose cies. The abundanceof these pre- velop new behaviorsto cope withthe fromthe Gulf Coast. Results are ferredprey species was signifi- restructured ecosystem. This re- consistentwith a previouslyre- cantlycorrelated with drilling fre- structuringmay also have presented ported equatorwarddecrease in quency, such that when these new opportunitiesfor escalation,as drillingfrequencies of molluscs. preferredprey were uncommon in in the Paleocene recovery,during 3) We foundno consistenttrend in a sample,overall drilling frequen- which naticid drilling frequencies drillingfrequencies with depth or cy was low. This relationshipmay morethan doubled overthose in the grain size within the Moodys reflecta behavioralinflexibility in Cretaceous (Kelley and Hansen, Branch Formation,although the naticidpredation patterns. 1993). deeper-water Yazoo Formation 7) Slow-moving infaunal species, exhibited a significantlygreater particularlyof the familiesTur- frequencyof drillingthan did the ritellidae,Corbulidae, and Lucin- CONCLUSIONS Moodys Branch. idae, werecommonly preferred as 4) There was a significantcorrela- preyin theseEocene assemblages 1) Frequenciesof drillingby naticid tion of drilling frequencywith and in other Paleogene forma- gastropods varied among geo- species diversity of Moodys tions. Such long-termvictimiza- graphicsamples withinthe mid- Branch assemblages. This result tion overtens of millionsof years dle Eocene Cook Mountaininter- is consistentwith the hypothesis may reflectthe prey'sinability to val (6.8-38.7%) and upper Eo- that predationpromotes high di- escalate out of danger. Alterna- cene Moodys Branch Formation versity. tively,predation may be a less ef- (4.0-21.9%) of the Gulf and At- 5) Drilling frequency was signifi- fective agent of selection than lanticCoastal Plain. Therefore,in cantly correlated with naticid otherfactors acting on these pre- orderto characterizethe drilling abundance within the Moodys ferredprey taxa. frequencythat existed at a par- Branch Formation.High drilling ticular time, samples froma va- frequenciesin the entire Cook ACKNOWLEDGMENTS rietyof paleoenvironments should Mountaininterval and the Yazoo be examined. Formationwere also accompanied We thank David Dockery for di- 2) Naticid gastropod drilling fre- by highproportions of naticids. rectingus to the CynthiaPit locality, quencies weresignificantly great- 6) Some species suffereddispropor- and David Haasl forassistance in the

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field.Ray Ferris,David Haasl, Ben Journalof Paleontology, v. 58, p. 1215- parative study of the mollusk commu- Farrell,and Elizabeth Akinsassisted 1234. nities on the shelf-slopemargin of Bar- in data tabulation. This was KELLEY, P.H., 1988,Predation by Miocene bados, West Indies: Veliger,v. 24, p. 309- study gastropodsof the ChesapeakeGroup: 318. supportedby National Science Foun- Stereotypedand predictable:PALAIOS, SMITH, S.M., and ZUMWALT,G.S., 1987, dation grant EAR 8915725 and a v. 3, p. 436-448. Gravityflow introduction of shallow wa- SummerResearch Award from West- KELLEY, P.H., 1991,Cannibalism by Ches- ter microfaunainto deep water deposi- ern WashingtonUniversity. apeakeGroup naticid gastropods: A pre- tional environments:Mississippi Geolo- dictableresult of stereotyped predation: gy,v. 8, p. 1-7. Journalof Paleontology, v. 65,p. 75-79. 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