Trilobites within nautiloid cephalopods RICHARDARNOLD DAVIS,R. H.B.FRAAYE AND CHARLES HEPWORTH HOLLAND Davis,R.A., Fraaye, R.H.B. & Holland,C.H. 2001 03 15: Trilobiteswithin nautiloid cephalopods. Lethaia,Vol.34, pp.37– 45. Oslo.ISSN 0024-1164. ‘Sheltered preservation’of the remainsof trilobites within the shells ofnautiloid cepha- lopodsis not especially uncommon. In mostcases, of course, both the trilobitesand the nautiloidswere dead,and the association,merely dueto post-mortem happenstance. However, onthe basisof state ofpreservation and occurrence, anumber oflive indivi- dualsof the trilobite genera Acidaspis,Flexicalymene, and Isotelus fromthe Ordovician ofthe United States andof Alcymene and Encrinuraspis fromthe Silurianof Wales and the Czech Republic seem to have entered conchs ofdead cephalopods, presumably for refuge. &Behaviour,nautiloid cephalopods, taphonomy,Trilobita. RichardArnold Davis [[email protected]], College ofMount St. Joseph, 5701 Delhi Road,Cincinnati, Ohio, 45233-1670, USA; R. H.B.Fraaye[oertijdmuseum.degroene- [email protected]],Oertijdmuseum ‘ DeGroene Poort’ , Bosscheweg80, NL 5283WB Boxtel, TheNetherlands; and Charles HepworthHolland [[email protected]],Department of Geol- ogy,Trinity College, Dublin 2, Ireland; received 9th December, 1999, revised 21st October, 2000. Emptycephalopod shellson thesea oorwould have body-chamber (or otherwiseclosely associated with beenideal hidingplaces for animalsin danger of theconch). However, examples of thisphenomenon predation.This would have beenespecially so for have notbeen extensively discussed or well illustrated. potentialvictims unable to burrowand hidein thesoft Hence,this paper. sedimentsof asea oor.Indeed, in some environ- Whenone is presentedwith specimens of trilobites ments,empty cephalopod shellsmight have beenthe incephalopod body-chambers,the question immedi- onlysuitable shelters from predators. Onthe other atelyarises as towhether the trilobites were alive hand,such empty conchs might have provided ideal withinthe cephalopod conchsand, if so, what they lairsfrom which a predator could have pouncedon weredoing there. passingprey. Variouskinds of organicmaterial potentially could Anumberof occurrencesof organismsthat befound within a givencephalopod conch.The most apparently soughtshelter within conchs of Mesozoic obviouspossibility, of course,is the remains of the ammonoidshave beenreported. Forexample, Matsu- cephalopod thatsecreted the shell (for example,jaw moto& Nihongi(1979) and Maeda (1991) described elementsand radulae (Nixon1996)) and,perhaps, its ammoniteswithin ammonites; Fraaye &Ja¨ger (1995a) unhatchedeggs or hatched progeny.Aside from such discussed shesin like circumstances; and Fraaye & remainsor productsof thecephalopods themselves, Ja¨ger (1995b) reported onlobsters within ammonite thereare at leastsix types of whathas beencalled conchs. ‘sheltered preservation’that might occur within Present-dayanalogues involving other classes of cephalopod conchs: molluscshave beenreported too.Walker (1992), for example,documented hermit crabs ingastropod 1. Crop/stomach/gutcontents of thecephalopod conchs.Schmitt (1965, p. 142) reported onamphipod animal(e.g. Fraaye &Ja¨ger 1996). crustaceansof thegenus Siphonectes inshells of the 2. Organismsthat lived withinthe living cephalopod scaphopod Dentalium.And reef-dwelling stomato- (for example,as endoparasites). pods commonlyuse empty gastropod shellsas 3. Organismsthat deliberately entered shells of dead domicilesand as broodingsites (Reaka et al. 1989; cephalopods for food,refuge, ecdysis,reproduction, Zuschin& Piller 1983). Moreover,Brett (1977) lodging,and soon(e.g.Mikulic 1994). discussedand gured atrilobitewithin a closed 4. Foodor othermatter brought in bythe organisms brachiopod shell fromthe Devonian of NewYork. itemizedabove (e.g.Ja ¨ger &Fraaye 1997). There have beenoccasional reports of Palaeozoic 5. Fecal matter,shed exoskeletons,spawn, and soon, orthoconiccephalopods fromEurope and North depositedor left bysuch organisms (e.g. Fraaye & Americathat each have oneor moretrilobites in the Ja¨ger 1995b; Kaiser& Voigt1983). 38 RichardArnold Davis et al. LETHAIA34 (2001) Fig.1. Encrinuraspisbeaumonti (Barrande,1846) in Sphooceras truncatum (Barrande,1868) Flower, 1962. Encrinuraspisbeaumonti horizon, KopaninaFormation (Ludfordian Stage, Ludlow Series,Silurian). Zadn õ´ Kopanina(Dlouha Hora),Czech Republic (NarodniMuseum, Prague, Czech Republic, number L16830; gured specimen[the trilobite]:Barrande, 1872, pl.9, gs.24– 26; S Ï najdr,1990, pp.206– 207; KrÏ õ´z, 1992, pl.1, g.18), 1.1. £ 6. Organismsor parts of organismscarried into (1971), inthe realm of palaeontology,it commonly cephalopod shellsby post-mortem transportation isa problemto establish that the ‘ host’actually was (e.g.Luppold et al. 1984, pl. 1, g. 3). alive atthe time of theassociation. Fraaye and Ja¨ger (1995a, b)have describedand illustratedexamples of There are manykinds of relationshipsbetween and shesand decapod crustaceansin thebody-chambers amongliving things. ‘ Symbiosis’, as usedby Cheng of Jurassicammonites from Germany and England. (1967) and manyothers, has beenemployed for ‘all Forthese associations, they used the term ‘ inquilin- typesof heterospecic associations,excluding preda- ism’. Incaseslike these Jurassic examples, and ofthose tion,during which there exists physical contact or oftrilobitesfound in the body-chambers of nautiloid intimateproximity between the two members’ (Cheng cephalopods, whichwe describebelow, it isclear that 1967, p. 4). Withinthis general category,a signicant the shesand arthropods inquestionwere associated boundaryhas beendrawn betweenrelationships withthe shells of dead cephalopods. Thus,strictly involvingmetabolic dependence and thosenot invol- speaking,the term ‘ inquilinism’is inappropriate, vingsuch dependence (for example,mutualism and although itisthe closest technical term in general use. parasitism,which do, versus commensalism and phoresis,which do not).However, as Gotto(1969) noted,‘ Unfortunately,authors ndthe greatest difculty in using these terms in a preciselysimilar manner,and thesame association will often be Material describedunder different headings according tothe Encrinuraspisbeaumonti in Sphooceras authorityconsulted’ (Gotto 1969, p. 14). truncatum Thispaper reportson whatappear tohave beenlive trilobiteswithin the body-chambers of dead cephalo- Specimennumber L 16830 inthe Narodni Museum, pods.Gotto (1969, p. 15) usedthe term ‘ inquilinism’ Prague, isan orthoconic nautiloid from the Encrinur- for ‘organismswhich live together,one within the aspis beaumonti horizon,Kopanina Formation (Upper other,the former usingthe host animal mainly as a Silurian,Ludfordian Stage)of theCzech Republic;it is refuge’. Similarly,Morton (1989, p. 10) applied about18 cmlong (Fig. 1). The cephalopod consistsof ‘inquilinism’for ‘thoseassociations in which one arelatively completebody-chamber (11 cmlong) with animallives within another, doing the host little or no anincomplete phragmocone. The trilobiteis 3.4 cm harm,but simply using it as aplace of moreor less longby 1.8 cmin maximum width and islocated permanentrefuge’ . Bothof theseare extensionsof the almostat thecentre of thebody-chamber. The useof theterm ‘ inquiline’by Butler (1879) for animals trilobiteis oriented with its anterior end pointed thatlive withina gall along withthe insect larva that adapically and withits longitudinal axis about 15 ° to formed it(this, according to The OxfordEnglish thatof thecephalopod. The thoraxand thepygidium Dictionary (Simpson& Weiner1989, 1992), isthe are connected.The cephalon isslightlyseparated from originalzoological useof theterm ‘ inquiline’). thethorax, with three segments missing; this disloca- Anunderlyingassumption of all oftheseis that both tionat the cephalo-thoracic jointis indicative of a theinquiline and thehost are alive at thetime of the body-upright moultprocedure, as describedby Speyer association.However, as pointedout by Holland (1985, p. 246, gs.7h– l). LETHAIA34 (2001) Trilobites within nautiloid cephalopods 39 Fig.2. ?Treptoceras sp. with Flexicalymenemeeki (Foerste, 1910). No primarydata, but almostcertainly from the Cincinnatian(Upper Ordovician)of the Cincinnati,Ohio, USA area(Orton Geological Museum,Department of Geological Sciences, OhioState University,Columbus, Ohio, USA; OSU 50329), 2. £ Thisspecimen was reported originallyby Barrande cephalopod. The left sideof thecephalon issomewhat as ‘…conserve´ dans l’inte´rieurde la grande chambre eroded, butcan be seen to overlap the rstthoracic d’un exemplairede Orthoc.truncatum ’(1872, caption segmenton that side. topl. 9, gs.24– 26). However,the position of the Thesecond trilobite is about 2.4 cmlong and is trilobitewithin the cephalopod wasnot illustrated by adjacent totheadapertural endof thecephalopod. Itis Barrande or inlater works(S Ï najdr 1990, pp. 206–207; orienteddorsum-outward, with the cephalon adaper- KrÏ õ´z1992, pl. 1, g. 18). tural and withthe plane of symmetryabout 50 ° to the axisof theconch. The pygidiumis curved downward. The cephalon isexed slightlydownward and isbent Flexicalymene in ?Treptocerasduseri totheright, so that its posterior overlaps the rsttwo Describedbelow are threeorthoconic nautiloids from thoracicsegments on thatside. There isanotherslight theUpper Ordovicianof theCincinnati region that bendto the right after thefourth thoracic segment. have trilobitesof thegenus Flexicalymene associated Behindthe right eyeare twocracks thataffect the withthem. The cephalopods are specimensof