Taphonomy of Isocrinid Stalks: Influence of Decay and Autotomy Tomasz K

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Marine & Environmental Sciences Faculty Articles Department of Marine and Environmental Sciences

1-1-1995 Taphonomy of Isocrinid Stalks: Influence of Decay and Autotomy Tomasz K. Baumiller Harvard University

Ghislaine Llewellyn Harvard University

Charles G. Messing Nova Southeastern University, [email protected]

William I. Ausich Ohio State University Find out more information about Nova Southeastern University and the Halmos College of Natural Sciences and Oceanography.

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Recommended Citation Baumiller, Tomasz K., Ghislaine Llewellyn, Charles G. Messing, and William I. Ausich. "Taphonomy of isocrinid stalks; influence of decay and autotomy." Palaios 10, no. 1 (1995): 87-95.

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skeletalmorphology. In recentyears, however,studies on soft-partanato- myof stalks of living crinoids (Grim- Taphonomyof Isocrinid Stalks: mer et al., 1984a, b, 1985; Grimmer and Holland, 1990; Holland et al., Influenceof Decay and Autotomy 1991) have provideda new perspec- tive to paleobiologistsstudying crinoid stalks. For example, Donovan (1989) used the absence of muscles in stalksof livingcrinoids and hard- TOMASZ K. BAUMTTILTER a characteristicmorphology and is part morphologyof fossilsto argue Departmentof Earth and Planetary the site of autotomy. that fossilcrinoid stalks also lacked Sciences, Althoughstalk shedding by au- muscles;Baumiller and Ausich(1992) Harvard University, totomymay contributeto the ob- used the distributionof ligamentsin 20 OxfordSt., servedpatterns, decay experiments livingisocrinids to interpretthe pat- Cambridge,MA 02138, U.S.A. on isocrinidstalks, bothin situ and tern of fragmentationof Mississip- in the lab, suggestthat post-mortem pian stalks. These examples illus- GHISLAINE LLEWELLYN disarticulationalso results in non- tratehow data on livingcrinoids can Departmentof Earth and Planetary random fragmentation.Thus both help generate paleontological hy- Sciences, processes,autotomy and post-mor- potheses. Harvard University, tem decay, contributeto the ob- This studyis a furtherexample of 20 OxfordSt., served pattern of fragmentation. this approach.We used data on the Cambridge,MA 02138, U.S.A. Underlyingboth processes is the or- morphologyand behaviorof living is- ganization of soft tissues at synos- ocrinidsto test the hypothesisthat CHARLES G. MESSING toses. there is a preferredsite of disartic- Nova UniversityOceanographic ulation in theirstalks. To test this Center, hypothesiswe examinedstalks of ex- 8000 North Ocean Drive, INTRODUCTION tantisocrinids and subjecteda series Dania, FL 33004, U.S.A. of them to a set of experimental The recentuse of submersiblesfor treatments.Having establishedthat WILLIAM I. AUSICH the studyof stalked crinoids has pro- a preferredsite ofdisarticulation ex- Departmentof GeologicalSciences, duceda wealthof data on theecology, ists,we thenexamined Mesozoic and Ohio State University, morphology,and biostratinomyof Cenozoic isocrinidstalks to deter- 155 S. Oval Mall, these animals (Macurda and Meyer, mineif, in thesefossil specimens, the Columbus,OH 43210, U.S.A. 1974; Roux, 1980a, b; Conan et al., site of disarticulationwas the same 1981;Messing et al., 1988;Holland et as in livingisocrinids. al., 1991;Baumiller et al., 1991;Llew- ellynand Messing,1994). Isocrinids ISOCRINID STALK PALAIOS, 1995, V. 10, p. 87-95 have been the focus of most of the MORPHOLOGY, AUTOTOMY, ecological and biostratinomic re- AND BIOSTRATINOMY Stalks of isocrinidcrinoids are dif- search, whereas the morphologyof ferentiated into cirri-bearingcol- representativesof all 5 livingorders The soft-and hard-partstructure umnals (nodals) and columnals ofcrinoids has been studiedin detail of isocrinidstalks has been studied lacking cirri (internodals). This (Grimmer et al., 1984a, b; 1985; in detail by several investigators skeletaldifferentiation allowed us to Grimmerand Holland,1990; Holland (Macurda and Meyer, 1975; Roux, test whetherstalk fragmentationis et al., 1991). These data have given 1974, 1975, 1977; Donovan, 1984; random or whetherit occurs pref- us a better,though in comparisonto Grimmeret al., 1985). The isocrinid erentiallyat a specificarticulation. otherinvertebrate groups still rather stalk is organizedinto two distinct Our analyses indicate that the pat- rudimentary,understanding of living regions:a shortproximal growing re- ternsof fragmentationin multicol- crinoids,which is criticalfor inter- gionin whichnew columnals develop, umnal segmentsof extant isocrinids pretingfossil crinoids. Fossil crinoid and a distalregion in whichcolumnal collected by submersible,by dredg- stalks,for example, have oftenbeen arrangementand size remain con- ing,and in sedimentsamples, as well used forsystematic, taxonomic, and stant.Two typesof columnals can be as those found as fossils,are non- biostratigraphic purposes (Glu- distinguishedin thestalk: the nodals, random. The preferred plane of chowski,1981; Hagdorn,1983, 1986; which bear appendages called cirri, fragmentationcorresponds to the Moore and Jeffords,1968; Schubert and the internodals,which lack cirri. synostosis,the articulationbetween et al., 1992; Simms,1989; Stukalina, In externalappearance the isocrinid a nodal and the internodaldistal to 1988). These approaches have gen- stalk consistsof repeatingsegments it. In isocrinidsthis articulation has erally relied on interpretationsof (noditaxes),each composed of a set

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of internodalsbounded on its distal ie, 1984). The physicalproperties of nodal as the distalmost (terminal) end by a cirri-bearingnodal (Fig. 3A the MCT's, such as stiffnessand vis- columnal. in Baumillerand Ausich,1992). Each cosity,may be controlledby the or- segment contains approximately ganism; autotomymay involve the LABORATORY EXPERIMENTS equal numbersof columnals,except activereduction of the stiffnessand/ for the proximalregion where new or viscosityof the ligament.It is not The decay experimentsconducted internodalsare intercalated. knownwhy or howfrequently autot- by Baumillerand Ausich(1992) were The regularpattern of the nodi- omy of stalk segments occurs. If limited in scope: only four incom- taxis skeletalmorphology is also re- sheddingof distal stalk segmentsis plete stalk segmentswere used, the flectedin the organizationof isocrin- frequent,the majorityof multicol- specimenshad been frozenprior to id softtissues (Grimmer et al., 1985). umnal segments (pluricolumnals) the experiments,the complete se- In each noditaxis,long ligamentfi- found in the sediment should ter- quence of disarticulationwas ob- bers,called through-goingligaments, minatedistally at a nodal ratherthan servedover a period of only 6 days, pass throughthe stereomof the in- an internodal. withonly a singlespecimen observed ternodalsand into the nodal where Alternatively,a patternof nodal- duringan additional 15-day period theyterminate (Fig. 3B in Baumiller terminatedpluricolumnals could be and under modified experimental and Ausich, 1992). Only shortliga- producedby another mechanism: the conditions,and onlya singlespecies mentfibers, called intercolumnallig- differentialresistance of synostosial was tested. In an attemptto extend aments,penetrating no deeper than and non-synostosialarticulations to and confirmthose results,the decay tensof microns into the stereom, con- decay. Baumillerand Ausich (1992) sequencesof 12 specimensof two dif- nect the nodal to the adjacent distal arguedthat synostoses, because they ferentisocrinid species [(Endoxocri- internodal,called the infranodal. This lack thicker, through-goingliga- nus parrae (Gervais)and Chladocri- patternis then repeatedin the sub- ments and because they may bear nus decorus (Carpenter)] were ob- sequent noditaxis.Thus the stalk is special structuresfacilitating autot- servedfor up to 60 days (Fig. 1). The subdividedinto noditaxes, each held omy,would be the firstto disarticu- specimens were placed in 60-liter, togetherby long ligamentsand con- late followingdeath, thus producing aerated, sea-watertanks and main- nectedto othernoditaxes by articu- nodal-terminatingpluricolumnals. tained in the dark at either18 or 20? lations bearing only short ligament Decay experimentson fourshort stalk C. They were monitoreddaily and fibers.The lattertype of articulation fragmentsof the isocrinidCenocri- the patternsof fragmentationwere is called the synostosis.The synos- nus asterius (Linne) confirmedthis recorded.The resultslargely confirm tosisis easilydistinguished from oth- prediction:the stalk segmentsdis- the conclusionsof the previousstudy er typesof articulations (symplexies) articulatedfirst at the synostosialar- (Baumiller and Ausich, 1992): the in livingand fossilisocrinids because ticulations,while the otherarticula- synostoses fragmentedbefore the of its relativelyfeatureless skeletal tionsremained intact for several days symplexies (articulations between morphology(Hagdorn, 1983; Dono- longer. internodals) did, yielding isolated van, 1984). These results, and the observa- noditaxes; but unlike the previous Observationsof living isocrinids in tionsof livingcrinoids in situ and in study in which all synostosesfrag- situ and in the lab have shownthat the lab, suggestthat isocrinidpluri- mentedbefore any ofthe symplexies distal parts oftendisarticulate from columnalsin modernsediments and did, in thisinstance some synostoses the restof the stalk (Baumilleret al., as fossilsshould be characterizedby remainedintact longer than several 1991). When such disarticulationoc- nodal-terminatedsegments. Alter- of the symplexies. curs,it invariablydoes so at a syn- natively,if the processes of differ- To statisticallytest whethersyn- ostosis. As a result,the most distal entialdecay and/orshedding are un- ostoses have a higherprobability of columnalof the stalk is a cirri-bear- important,the pluricolumnalsfound fragmentation,the observed patterns ingnodal. The disarticulationof parts should be terminatedat a random of disarticulationwere comparedto of the stalk in a animal has underthe null living columnal,i.e., eitherat a nodal or at patternsexpected hy- been interpretedas autotomyand the pothesis of an equal probabilityof an internodal.Since, in a givenstalk, synostosisis judged to be the artic- for all articulations thenumber of internodals exceeds the fragmentation ulation specialized for autotomy (H0: probabilityof synostosialbreak numberof nodals (Roux, 1974,1977; Emson and Wilk- by approximately = probabilityof symplectialbreak). ie, 1980;Wilkie and Emson,1988; Oji, an orderof magnitude, a randompat- To obtainthe patternsexpected un- 1989). ternof disarticulation would produce der the null model, the disarticula- The details of how crinoidsautot- manymore pluricolumnals terminat- tion of each of the 12 stalks used in omize stalk segmentshave not been ingin internodalsthan in nodals.The the experimentswas simulatedby a workedout, but it is believedthat the goal of this studyis to test whether QuickBASIC program.The articu- mutable collagenous tissue (MCT) the pluricolumnalsin experimental lations at which fragmentationoc- composingthe ligamentsmust be in- and naturalsettings, and those pre- curred were selected by a random volved in this process (Emson and servedas fossils,exhibit the expected number generator and a running Wilkie,1980; Motokawa, 1984; Wilk- non-randompattern, i.e., with the count was kept of how manyof the

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breakswere synostoses (the initial ra- Sp4 Sp5 tio of synostosesto symplexieswas 100 1:10 for Endoxocrinus parrae and 1:14 for Chladocrinusdecorus). As the numberof breaks in the simulation reached a stage of disarticulation correspondingto that in the experiment,a comparisonwas made 80 betweenthe simulatedand the ob- servedpattern. If the numberof broken synostoseswas smaller in the 0) simulation, the simulation was 411 countedas "non-random";otherwise 0) it was counted as "random." This Q 60 procedurewas continueduntil the E numberof simulated breaks corre- 0 spondedto the maximumnumber of U breaksin the givenexperiment. The 4-1 entireprocedure was repeated 1000 C D times, keeping "non-random"and 40 "random" scores. If after1000 sim- ulationsthe "random" score was low- L. er than 50 (correspondingto a "non- random"score greater than 950), the null hypothesiswas rejectedat P < 0.05.By repeatingthe simulations for 20 each of the 12 stalks,it was possible to determinefor which, if any, stages of disarticulation the "random" model could be rejected. The results,shown in Figure2, sug- 0 gest that in almost all stages of dis- articulationin nearlyall stalks,the 0 10 20 30 40 50 60 "random"model can be rejected,i.e., that synostoseshave a higherprob- Decay days abilityof disarticulation.Generally, FIGURE 1 -Results ofdecay experimentson Endoxocrinusparrae and Chladocrinusdecorus. the modelcannot be rejectedonly in The total numberof synostosial (Sto) and symplectial(Spo) articulationswas counted in each the initialand finalstages of disar- specimenat thestart of the experiment. The degree offragmentation of each typeof articulation ticulation.But this is as is plottedthrough time. Fragmentation is expressed foreach specimen as the ratioof number expected, of articulationsto the totalnumber of such articulations: = St = sincefor the firstbreak the ran- fragmented synostoses Stds,,t/ very Sto; symplexies = Sp = Spsar,/Spo.The subscripts correspond to differentindividuals. C. dom probabilityof it corresponding decorus at 20? C: St1, Spl; St2, Sp2; St3, Sp3; St7, Sp7: E. parrae at 20? C: St4, Sp4; St5, to a synostosesis 1/10(0.1) forEn- Sp5; St6, Sp6; E. parrae at 18? C: St8, Sp8; St9, Sp9; C. decorus at 18? C: St10, Sp10; doxocrinusparrae and 1/14(0.07) for St11, Sp11; St12, Sp12. Chladocrinusdecorus, i.e., forboth taxa it is higherthan the 0.05 thresh- old forrejecting the randommodel. Likewise,as disarticulationproceeds the resultingpattern of pluricolum- lected in the vicinityof living iso- to completionpast thestage at which nals should be dominatedby those crinids. all synostoseshad been brokenand terminatingon a nodal (Fig. 3A). In the in situ experiment,we col- symplexiesbegin to disarticulate,the lectedlive Endoxocrinusparrae and randomprobability again approach- IN SITU EXPERIMENTS AND Chladocrinusdecorus via a suction es 1/10for E. parrae and 1/14for C. STATISTICAL ANALYSES hose apparatus on the submersible decorusand the randommodel can- JohnsonSea Linkin 420-430m south not be rejected. To determinewhether the pre- of West End, Grand Bahama Island In conclusion,the results of the de- dictedpattern of stalk segmentster- (see Llewellynand Messing,1994 for cay experimentsconfirm that the minatingon nodals can be observed studysite details). Specimens of both synostosisis the plane of post-mor- in nature,we performedan addition- specieswere placed, either live or sac- tem weakness,that it preferentially al experimentand examineddredged rificed(by freezing),inside panty- disarticulatesduring decay, and that specimensand sedimentsamples col- hose (Hanes?) legs,tied off,weight-

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1 est segment(3 columnalsin our ex- @01 --# 1 ample), what is the probabilitythat 0 -*-#2 2 of these segmentswould terminate >%0 -o-# 3 in a nodal? We can the bino- l apply --'-#4 mial equation to answer this ques- a..0 0.1 tion: 1^ ?, E - -#6 0.05 ^~~~~~~~~~~~~~~~~~...... -'*-# 7 n! = -3...... ^ -'--# 8 P(x) pn-x)q E.0 iH,' ../ . x!(n- x)! -- #10 0.01 whereP(x) is theprobability that the -.-#1 1 WI eventwill occur x timesin n trials;p ::'k .'/ / --*-#12 is the probabilitythat the eventwill occurin any giventrial; and q = 1 - p is the probabilityof the eventnot

- 0.001 I I occurring.In our x = n Ir^~~ |. I example, 2, 0 10 20 30 40 50 60 70 80 = 3, and, sincewe have made the as- sumptionthat all segmentsare of Total number of breaks equal lengthof 3 columnals,one of FIGURE 2-Comparison of patternsof fragmentationof simulatedstalks to experimentally whichis a nodal, p, the probability decayed stalks. The data representthe comparisonof 1000 computersimulations generated that a nodal lies at the distal end in underthe nullmodel (no differencesin probabilityof fragmentationbetween synostoses and any of thesesegments, is 1/3,while q, symplexies)to data fromlab decay experiments.The heavy horizontalline corresponds to 50 the probabilitythat an internodalis simulationswith an equal or a greaternumber of disarticulatedsynostoses than observed in at the distal end, is 2/3. The value of the decay experiments;for data below the line,the nullmodel is rejected at P < 0.05. The numbersin the legend correspondto the stalks listed in Figure1. P(x) in this exampleis 0.26; thus we cannotreject the null hypothesis and must conclude that the sample of three pluricolumnalsrepresents a ed from a plastic-coveredSCUBA nodal. Correspondingly,the proba- randompattern of breakage. The ap- weight,and returnedto thecollection bilitythat the pluricolumnaltermi- proach is conservativebecause the site. The pantyhosefabric allowed nates in an internodalis (x - 1)/x. shortestpluricolumnal is used to cal- ambientwater to circulatearound the For a segmentconsisting of 5 plates, culate the values of p and q; in the crinoid carcass while providing a a nodal and 4 internodals,there is a extremecase ofa pluricolumnalcon- means of retainingthe productsof 20 % probabilitythat the distalplate sistingof two plates,a nodal and an taphonomicbreakdown. Three spec- is a nodal; fora segmentwith 7 in- internodal,both p and q wouldhave imenswere recovered after 2 months ternodalsand a nodal, that proba- a value of 0.5. This approachthere- and two after5.5 months. bilityis 12.5%. The probabilitythat foreoverestimates the value ofp and The arms and calyx of all speci- both segmentsterminate at a nodal underestimatesthe value of q forall mens dissociated completely, but is 0.025, i.e., the productof proba- pluricolumnalsin the sample except even forthose remainingat the site bilitiesfor individual segments (0.125 the shortestone. The calculatedP(x) for5.5 months,some articulated stalk x 0.2). If a thirdpluricolumnal, con- is thereforemaximized, making it segmentswere recovered. In the ma- sisting of only three columnals,is more difficultto reject the null hy- jority (60/73) of recovered nodal- considered,the probabilitythat all pothesis. bearingsegments, the nodal was at threepossess a nodal at thedistal end The above approach can be ap- the terminalend (Table 1). is 0.008 (0.33 x 0.125 x 0.2). If these pliedto anysample of pluricolumnals To assess whetherthe observed threepluricolumnals constituted our by obtaining the values of n, the patternwas consistentwith a random sample,we would rejectthe null hy- number of pluricolumnalswith at ora non-randompattern of breakage, pothesisat P < 0.01, and conclude least a singlenodal, of x, the number we formulatedand tested a null hy- that the patternis more consistent ofpluricolumnals with a nodal at the pothesis in which the pattern was withthe scenariothat fragmentation distalend, and ofp, whichis theratio random.Under this randommodel, is more likely at the synostosis.If, of nodals to total numberof colum- everyarticulation has an equal prob- however,the third pluricolumnal ter- nals in the shortestspecimen in the abilityof fragmentation.For a plur- minatedat an internodal,testing for sample. For a sample in which all icolumnalconsisting of x plates,there non-randomnesswould prove more pluricolumnalscontain more than a is an equal probability(1/x) that any difficult.We chose the conservative singlenodal, p is just the reciprocal of the plates willlie at its distal end. approach and asked the following of the numberof plates in an indi- If onlyone of theseplates is a nodal, question: assuming all three seg- vidual noditaxis. there is a 1/x probabilitythat the mentswere of equal lengthand that Whenapplied to the data fromthe pluricolumnalwill terminate in a the lengthcorresponds to the short- pantyhoseexperiment, the statistical

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test confirmsthe initial impression: whentaken together, 60 of 73 nodal- bearing segments terminatedin a nodal,and even underthe mostcon- servativeassumption of 2 columnals/ segment,the randommodel was rejected at P < 0.001 (Table 1). The synostosisis thus the preferredsite of disarticulation.

PLURICOLUMNALS IN SEDIMENT SAMPLES, AND STALKS OF DREDGED AND COLLECTED SPECIMENS Sediment samples were collected in the same area and depth and by thesame meansas thelive specimens in the pantyhose experiment(see Llewellynand Messing,1994 forde- tails ofsampling). The submersible's standardcollecting buckets were cov- ered with 0.0625-inch2plastic mesh whichretained the coarse sediment fraction.Following ascent, retrieved sedimentwas preservedin alcohol and subsequentlywashed througha standardseries of sieves.The majorityof retrieved crinoidal material (>- 2 mm) consistedof individualcrinoid columnals;pluricolumnals were rare. The suctionapparatus is sufficiently powerfulto pickup completeisocrin- ids and even pieces of rubble,so it is unlikelythat individual columnals were preferentiallysampled. Of the retrievednodal-bearing pluricolumnals, 25 terminatedat nodals and 1 at an internodal(Table 2). Again,usingthe binomial test described above, the pattern of disarticulationwas non-randomwith respectto articulation type: synostoseswere more susceptibleto fragmentation. The preferreddisarticulation of synostosialarticulations was tested with two additional data sets con- FIGURE 3-A. Two examples of pluricolumnalsof Endoxocrinusparrae(top) and Chladocrinus sistingof dredged and collectedspec- decorus (bottom)following the decay treatment.Note thatthe proximalend in bothspecimens imensof isocrinids.The Museum of is an infranodaland the terminalend a nodal (scale bar = 0.25 mm).B. One of the specimens ComparativeZoology (Harvard Uni- of C. decorus collected duringthe Blake expedition.The terminalend is a nodal. (scale bar versity)houses isocrinidspecimens = 0.31 mm).C. A noditaxisof a Jurassicisocrinid terminating at a nodal. Pentacrinusscalaris m Goldfuss; MCZ 2463. (scale bar = 0.20 mm). D. An isocrinidpluricolumnal with nodal not at dredgedfrom a depth of 176-248 = in the vicinityof St. Vincent,eastern the terminalend. Pentacrinuscingulatus Goldfuss; MCZ 137.5. (scale bar 0.17 mm). Caribbean,by the Blake expedition in 1878-1879(MCZ 8; MCZ 232;MCZ 269). Severalof these were preserved specimensconsist of stalkfragments data undoubtedlyrepresent a sum- in alcohol,but a majoritywere dried. of variouslengths. Fragmentation of mationof processesincluding autot- Althoughmany crowns and armclus- the stalks may have occurredprior omy, post-mortemdecay, and me- tersare available,the majorityof the to,during, or after dredging, and these chanical breakage caused by

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TABLE 1-Results of in situ taphonomyexperiments. approach was applied to 180 specimens (crownswith column attached collectedvia the Termi- Terminat- of3 isocrinidspecies Probability JohnsonSea-Link in 265-530 m off Segments natingin ingin ofrandom withnodal nodal internodal fragmentation Grand Bahama Island (Messingand Llewellyn,in prep.). In 156 of these Chladocrinus decorus 15 15 0 3.1 x 10-5 specimens,the nodal was the termi- Chladocrinusdecorus 18 10 8 4.0 x 10-1 nal column (Table 4); this again in- Endoxocrinus parrae 15 11 4 6.0 x 10-2 dicates non-randomfragmentation Endoxocrinus parrae 10 10 0 1.0 x 10-3 evenunder the mostconservative as- Endoxocrinus parrae 15 14 1 5.0 x 10-4 sumption(P < 0.001). Total 73 60 13 1.1 x 10-8 The above examples demonstrate that in nature,as in the laboratory, a preferredplane of stalk disarticulation existsand correspondsto the TABLE 2-Patterns of fragmentationin specimens obtainedfrom sediment synostosis.It is not possible to dis- samples. criminatebetween those segments produced by autotomy,disarticula- tionor mechanicalbreakage, but the Termi- Terminat- Probability overallpattern is non-random. Segments natingin ingin ofrandom withnodal nodal internodal fragmentation DISARTICULATION Chladocrinus decorus 14 14 0 6.1 x10-5 PA'lTERNS IN FOSSIL Endoxocrinus parrae 12 11 1 3.1 x 10-3 ISOCRINID STALKS Total 26 25 1 4.0 x 10-7 Althoughcounts of stalkssubject- ed to experimentaland naturalcon- ditionsconfirm predictions based on TABLE 3-Patterns of fragmentationin specimens dredged by the "Blake" morphologicalanalysis, i.e., that syn- expedition. ostosesare the preferredsites of disarticulation,these results are notsuf- ficientto concludethat fossil isocrinid Terminat-Terminat- Probability stalksought to exhibitthe same pat- Segments ingin ingin ofrandom terns.Actualistic results may not be withnodal nodal internodal fragmentation applicable to fossil material for at least two reasons: biostratinomic Chladocrinus decorus 291 186 105 1.2 x 10-6 (1) patternscan be smeared or altered duringburial and fossilization,and (2) fossilizedtaxa may have had dif- TABLE 4-Types of columnals found at the distal end of isocrinidstalks ferent soft-tissuemorphology, re- collected by submersible. sultingin differenttaphonomic be- havior. To determinewhether the actualisticresults could be extended Termi- to the fossilrecord, we examinedcol- Terminat- nating Probability lections of fossil isocrinidpluricol- Number ingin in inter- of random umnals fromthe MCZ and the Na- ofstalks nodal nodal fragmentation tional Museum of Natural History, Chladocrinusdecorus 72 62 10 1.3 x 10-10 Smithsonian Institution (NMNH). These collectionsof stems are from Endoxocrinus 94 81 13 1.8 x 10-13 parrae Eu- Diplocrinus maclearanus 14 13 1 9.1 x 10-4 numerouslocalities throughout and in fromTriassic Total 180 156 24 0 rope range age to Eocene. The pluricolumnalsrep- resenta varietyof isocrinidspecies and wereprobably collected non-ran- domly;it is likelythat longerpluri- handling.We repeatedthe procedure (Fig. 3B; Table 3). Usingthe conser- columnalswere preferentially select- used withprevious data on thesestalk vative assumptionof 2 nodals/seg- ed. It is highlyunlikely, however, that segmentsand a majority,105 out of ment,the randommodel of breakage the collectorsexhibited any bias for a total of 186, terminatedat nodals was rejected(P < 0.001). The same segmentsterminating in nodals; in-

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stead, they probablycollected long TABLE 5-Patterns of fragmentationin fossil pluricolumnals. pluricolumnals indiscriminately. Since the of cirralscars al- presence Probability lows forthe recognitionof nodals in ofrandom fossils even when no cirri are pre- Segments TerminatingTerminating fragmen- served, the frequencyof segments withnodal in nodal in internodal tation terminatingin nodals versus internodals could be determinedfrom the NMNH collection 159 140 19 0 specimens.Thus, we wereable to re- MCZ collection 244 238 6 0 peat the procedureused in the pre- Total 403 378 25 0 vious experiments,to determinethe frequencyof the two types of segment.We selectedall isocrinidplur- icolumnalswhich allowed us to test applied; it is morelikely that the an- ofsoft-tissue organization must have thehypothesis that synostoses are the imal is activelyshedding parts of the been presentin Triassicisocrinids, as preferredsites of disarticulation. The stalk under unfavorableconditions. is suggestedby their patterns of frag- sample thus included all pluricol- Shedding has also been used to ex- mentation.Thus, amongthese early umnalsin whicha nodal was present plain why the isocrinidMetacrinus isocrinids, skeletal differentiation (Appendix 1). Just as in the actu- rotunduspossesses a relativelyshort (synostoses/symplexies)(Hagdorn, alisticexamples, the majorityof fos- stalk in spite of its unusuallyhigh 1983; Simms, 1988) and soft-tissue sil pluricolumnalsterminated at no- growthrate (Oji, 1989).Shedding may differentiationwere both present.A dals (384 of 410) (Fig. 3C, D; Table be supportedby the fact that there questionwhich we cannot answer with 5). The statisticalanalysis confirmed is no distal taper in stalks of large certainty,however, is whethersoft- that the patternwas not consistent isocrinidspecimens, whereas small tissue differentiationpreceded the witha randommodel even underthe individualsmay display a distaltaper evolutionof synostoses among the ar- most conservativeassumption of 2 in columndiameter. Since the most ticulates. Baumiller and Ausich columnals/segment(Ho rejectedat P distal columnalsare formedduring (1992) have suggestedthat some Mis- < 0.001). The synostoses,once again, early stages of ontogeny,their di- sissippiancrinoid stalks, while lack- proveto be the preferredsites of dis- ametershould be the smallest.The ing skeletal differentiation,pos- articulation. lack ofa distaltaper in largeindivid- sessed soft-tissueorganization anal- uals suggeststhat the oldest colum- ogous,or homologous,to the isocrin- DISCUSSION AND CONCLUSION nals have been shed. An alternative ids. They furtherargued that the explanation would require an in- abilityto autotomizeneed not be as- By investigatingmulti-columnal creasing proximal to distal radial sociated with a skeletal signature. segmentsfrom living and fossiliso- growthgradient. Shedding is a more Their conclusionneeds furthertest- crinidstalks, a non-randompattern likelyexplanation and deserves fu- ing, but if correct,would make the has been revealed. The majorityof ture attention. recognitionof autotomy in fossils isocrinidpluricolumnals terminate at The patternof post-mortemdis- muchmore difficult. Yet autotomyis a nodal, thoughthis columnalis un- articulationhas been demonstrated an importanttrait not onlybecause der-representedrelative to interno- withexperiments, both in situand in of its biomechanicaland paleoeco- dals in all isocrinids.At least two the lab. The actual mechanismre- logicalimplications, but also because mechanismscould be responsiblefor sponsible has not been identified; it couldprove to be a usefulcharacter generatingthis pattern: shedding, by Baumiller and Ausich (1992) pro- forphylogenetic analyses. However, autotomy,of distal noditaxesby the posed that symplexiesmay be more if this traitcannot be recognizedon livingorganism, or lower post-mor- resistantbecause of the presenceof purelymorphological criteria, other tem resistanceof the synostosisto through-goingligaments or because means must be used forits identifi- disarticulation. structurespromoting autotomy at cation.The patternof fragmentation Whethershedding of stalk frag- synostosesreduce the integrityof lig- methodused by Baumillerand Au- mentsoccurs in naturehas not been amentseven after death. Again, more sich (1992) is applicable onlyin very determined,but indirect evidence researchis requiredto identifywhich special circumstances, while the suggeststhat such sheddingdoes oc- of these two mechanismsis respon- methodsused in this studystill es- cur: the stalks of dredgedisocrinids sible. sentiallyrely on recognitionof skel- and those collected by submersible The greaterfrequency of nodal- etal differences.Other methodsfor commonlybreak and the breaksco- terminatingpluricolumnals may be recognizingautotomy, such as addi- incidewith a synostosis(Rasmussen, theresult of both processes, shedding tional non-randompatterns of frag- 1977).This couldbe attributedto the and post-mortemdisarticulation. mentationor geochemicalsignatures lowerstrength of this articulationas Both processesreflect the same un- associatedwith autotomy planes, may arguedby Rasmussen (1977), but the derlyingcause: the organizationof be availablebut need testingwith liv- stalk breaks even when no forceis softtissues at synostoses.This mode ing and fossilmaterial.

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ACKNOWLEDGMENTS GRIMMER,J.C., HOLLAND, N.D., and KUBOTA, Universityof Tokyo, Section II, v. 22, p. H., 1984a,The finestructure of the stalk 39-51. We thankF. Collier(MCZ) and J. ofthe pentacrinoid larva of a featherstar, RASMUSSEN,H.W., 1977,Function and at- Thompson(NMNH) foraccess to the Comanthusjaponica (Echinodermata: tachmentof the stem in Isocrinidaeand collections;the staff of Harbor Branch Crinoidea):Acta Zoologica (Stockholm), Pentacrinitidae:Review and interpre- OceanographicInstitute for help with v. 65,p. 41-58. tation:Lethaia, v. 10,p. 51-57. collectionof and GRIMMER,J.C., HOLLAND,N.D., and MES- Roux, M., 1974, Les principauxmodes d'ar- specimens samples; SING,C.G., 1984b,Fine structureof the ticulationdes ossicules du squelette des A. Pei fortechnical help; and H. Hag- stalkof the bourgueticrinid sea lilyDe- crinoides pedoncul6s actuels. Observa- dorn,M. LaBarbera,and 2 reviewers mocrinusconifer (Echinodermata: Cri- tions microstructuraleset consequences foruseful comments. This studywas noidea):Marine Biology, v. 81, p. 163- pour l'interpretation des fossiles: funded in part by grantsfrom the 176. ComptesRendu des Academiede Sci- National Science Foundation (EAR HAGDORN,H., 1983,Holocrinus doreckae n. ence,Paris, Serie D278, p. 2015-2018. 9104892 to W.I.A. sp. aus dem Oberen Muschelkalkund die Roux,M., 1975,Microstructural analysis of and T.K.B., and Entwicklungvon Sollbruchstellen inStiel the crinoidstem: University of Kansas EAR 9004232to C.G.M.). derIsocrinida: Neues Jahrbuch fur Geo- PaleontologicalContributions, Paper 75, logie und Paliontologie, Monatshefte,v. p. 1-7. REFERENCES 6, p. 345-368. 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APPENDIX 1 Disarticulationpatterns among fossilpluricolumnals.*

Museumof Comparative Zoology UnitedStates National Museum Catalog# Species A B C Catalog# Species A B C

112 Pentacrinuscingulatus 60 58 6 s2941 Pentacrinusagassizi 1 1 5 2139 P. tuberculatus 24 24 9 s2946 P. basaltiformis 3 3 3 1369 P. cingulatus 23 23 3 s2945 P. bajocensis 3 3 6 177 P. cingulatus 10 10 4 s2947 P. basaltiformis 3 3 4 3928 P. cingulatus 11 11 2 19510 P. basaltiformis 2 2 4 128 P. subteres 11 11 2 s2944 P. cingulatus 14 14 2 2349 P. subteres 9 8 2 s2953 P. nodulosus 1 1 11 9165 P. annulatus 6 6 6 No # P. tuberculatus 1 1 2 741 P. pentagonalis 9 9 3 s2954 P. pentagonalis 3 3 7 761 P. propinquus 4 4 6 No # P. pentagonalis 4 2 6 731 P. basaltiformis 14 14 3 19217 P. pentagonalis 43 30 2 No # P. scalaris 6 6 4 s2955 P. priscus 1 1 6 2459 P. scalaris 6 6 4 6031 P. sp. 8 8 5 129 P. subteres 8 7 3 9781 P. basaltiformis 3 3 8 1773 P. laevigatus 1 1 2 s2958 P. rupellens 4 4 4 2470 P. scalaris 8 8 5 23320 P. saevigratis 22 20 2 743 P. cingulatus 10 9 7 s2960 P. sigmariengesis 2 2 2 128 P. carinatus 1 1 7 1951 P. tuberculatus 4 3 6 2309 P. propinquus 2 2 6 s267 P. tuberculatus 4 4 10 1219 P. agassizi 4 4 3 194297 P. cingulatus 7 7 8 3881 P. sp. 3 2 5 23319 Isocrinuspropiquus 5 5 4 1779 P. subteres 1 1 7 s2974 L crista-galli 3 3 6 1221 P. annulatus 1 1 4 s2973 I. bavaricus 17 16 5 1389 P. scalaris 1 1 9 2450 P. cingulatus 3 3 4 2451 P. subteres 1 1 7 1478 P. basaltiformis 2 2 5 738 P. sp. 1 1 3 1470 P. propinquus 2 2 6 1217 P. sp. 2 2 8 949 P. basaltiformis 8 7 2

* Taxonomicassignments as on museumlabels. A: Totalnumber of pluricolumnals; B: number of nodal-terminating pluricolumnals; C: minimum number of columnals/pluricolumnal.

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