Paterson, J.R., Jago, J.B., Brock, G.A. & Gehling, J.G., 2007. Taphonomy and palaeoecology of the...

alaeoecology 249 (2007) 302–321www.elsevier.com/locate/palaeo

Palaeogeography, Palaeoclimatology, P

Taphonomy and palaeoecology of the emuellid trilobiteBalcoracania dailyi (early Cambrian, South Australia)

John R. Paterson a,⁎, James B. Jago b, Glenn A. Brock a, James G. Gehling c

a Centre for Ecostratigraphy and Palaeobiology, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australiab School of Natural and Built Environments, University of South Australia, Mawson Lakes, SA 5095, Australia

c South Australian Museum, Division of Natural Science, North Terrace, Adelaide, SA 5000, Australia

Received 15 September 2006; received in revised form 2 February 2007; accepted 12 February 2007

Abstract

Monospecific assemblages of the trilobite Balcoracania dailyi occur in lower Cambrian strata within the Adelaide Geosynclinein South Australia. Biostratinomic data from single bedding plane assemblages within the Warragee and Coads Hill Members of theBilly Creek Formation and White Point Conglomerate reveal a range of taphonomic signatures from census to within-habitat, time-averaged assemblages. These assemblages are interpreted as having inhabited protected, shallow, marginal marine environments.Size–frequency distributions, coupled with taphonomic data, show that the Warragee Member census assemblage represents aliving population in a physically stressful environment within a tidally-influenced lagoon, while the original population structure ofthe Coads Hill Member and White Point Conglomerate assemblages has been lost due to varying degrees of taphonomicoverprinting. Integration of taphonomic, stratigraphic and sedimentological data supports the interpretation of B. dailyi asrepresenting an opportunistic species. A preserved body cluster from the Warragee Member assemblage is considered tocharacterise a congregation formed for the purpose of synchronous reproduction and ecdysis, representing one of the oldestexamples of gregarious behaviour in the arthropod fossil record. Furthermore, by analogy with modern horseshoe crabs, the highnumber of juveniles (i.e., protaspides and early meraspides) within the same assemblage are believed to be constituents of a nurserywithin the intertidal zone, with adults migrating into the shallows to copulate and spawn. Preserved moult ensembles from theCoads Hill Member and White Point Conglomerate have enabled the description of exuviation techniques for B. dailyi.© 2007 Elsevier B.V. All rights reserved.

Keywords: Trilobita; Emuellidae; Early Cambrian; Palaeoenvironment; Behaviour; Moulting

1. Introduction

Balcoracania dailyi Pocock (1970) is a member ofthe family Emuellidae, an unusual group of trilobites

⁎ Corresponding author. Current address: Division of EarthSciences, School of Environmental Sciences and Natural ResourcesManagement, University of New England, Armidale, NSW 2351,Australia.

E-mail address: [email protected] (J.R. Paterson).

0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2007.02.004

endemic to the tropical region of East Gondwana duringthe early Cambrian (Brock et al., 2000; Paterson andEdgecombe, 2006). This species was first described byPocock (1970) from the lower Cambrian White PointConglomerate that outcrops to the west of CapeD'Estaing on the north coast of Kangaroo Island,South Australia. Pocock also described B. flindersi, ajunior subjective synonym of B. dailyi (fide Patersonand Edgecombe, 2006), from the Warragee Member ofthe Billy Creek Formation at Balcoracana Creek in the

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303J.R. Paterson et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 249 (2007) 302–321

Flinders Ranges. Paterson and Edgecombe (2006)described and illustrated specimens of B. dailyi fromthe Coads Hill Member of the Billy Creek Formation atReaphook Hill.

Emuellids have previously been interpreted as thesister group of all other trilobites with facial sutures(Lauterbach, 1980, 1983, 1989; Zhang et al., 1980) orgrouped with either Olenellida (Hahn, 1989) or Red-lichiida (Pocock, 1970). Phylogenetic discussion hasemphasised their peculiar trunk tagmosis, including aplethora of thoracic segments; Paterson and Edgecombe(2006) recorded 103 thoracic segments in B. dailyi,the maximum number known in any trilobite. How-ever, Paterson and Edgecombe (2006) recently proposedthat the Emuellidae are a derived clade within theRedlichiina.

The present study reviews the stratigraphic andgeographic occurrence of B. dailyi from South Aus-tralia, and investigates in detail the taphonomic dataavailable from single bedding plane assemblages fromthree localities, including: the type locality on KangarooIsland; Balcoracana Creek in the Flinders Ranges; andReaphook Hill. These data, coupled with palaeoenvir-onmental interpretations, form the basis of investiga-tions into various palaeoecological aspects of thisspecies, such as population dynamics, opportunism,gregarious reproductive behaviour, and exuviation.

2. Materials and methods

Specimens of B. dailyi (preserved as internal andexternal moulds) utilised in this study come fromcollections housed at the South Australian Museum,Adelaide (prefix SAMP). Studied specimens arerepresentative of single bedding plane assemblagesfrom: (1) the Warragee Member of the Billy CreekFormation, originally collected by C.R. Dalgarno and J.E. Johnson and subsequently studied by Pocock (1970);(2) Unit G of the Coads Hill Member of the Billy CreekFormation, collected by JGG; and (3) the type locality ofB. dailyi in the White Point Conglomerate, west of CapeD'Estaing, Kangaroo Island, collected by JRP and JBJ.These bedding plane assemblages have been used forstatistical analysis. Locality and stratigraphic informa-tion are discussed below.

Sagittal cephalic lengths for size–frequency data weremeasured to the nearest 0.05 mm using Vernier calipers.Size–frequency histograms were constructed on 0.5 mmincrements. In cases where specimens with a valueequating to the boundary of each 0.5 mm class (e.g. 5.00),numbers were split evenly, with half going into the nextclass above (e.g. 5.0–5.5), and the other half going into

the class below (e.g. 4.5–5.0). Horizontal (facingdirection) and vertical (convex-up and -down) orienta-tions of exoskeletons were measured only from a singlebedding surface of the Warragee Member, as other bed-ding assemblages occur in jointed and fragmentary, oftenfissile, shales and siltstones. As the original horizontalorientation of the specimen illustrated in Fig. 3 isunknown, horizontal orientations of individuals weremeasured with respect to an arbitrarily defined “north”vector.

3. Stratigraphic and geographic occurrence of B.dailyi

B. dailyi occurs in lower Cambrian strata (Pararaiajaneae Zone; late Botoman) in only a few areas of theAdelaide Geosyncline (Arrowie and Stansbury Basins)of South Australia (Fig. 1). B. dailyi is known from theWhite Point Conglomerate and Emu Bay Shale thatoutcrop at various localities on the northern coast ofKangaroo Island (Pocock, 1970; Daily et al., 1980;Gravestock and Gatehouse, 1995). Specimens areabundant within various horizons throughout the shalesand siltstones in the upper portion of the White PointConglomerate at the type locality to the west of CapeD'Estaing, and on the western side of Emu Bay, east ofCape D'Estaing (Pocock, 1970, text-figs 1, 2; Dailyet al., 1980). Specimens from the coeval Emu BayShale are known to occur at Emu Bay (JRP, personalobservation), and Big Gully where they are extremelyrare (Paterson and Jago, 2006). Unlike the typicallymonospecific assemblages of B. dailyi from the basalmembers of the Billy Creek Formation, the White PointConglomerate and Emu Bay Shale contain severalother trilobite taxa. These include: Emuella dalgarnoi,Emuella polymera, Estaingia bilobata, Holyoakiasimpsoni, Megapharanaspis nedini and Redlichiatakooensis (Pocock, 1964, 1970; Bengtson et al.,1990; Nedin, 1995a; Paterson and Edgecombe, 2006;Paterson and Jago, 2006). Occurrences of B. dailyi inthe White Point Conglomerate and Emu Bay Shale arecommonly monospecific within single bedding assem-blages, although other taxa (usually E. bilobata andR. takooensis) are seldom associated.

Specimens of B. dailyi in the Warragee Member ofthe Billy Creek Formation in the Flinders Ranges occurin sparse, but richly fossiliferous shale and siltstonehorizons throughout the unit, which has a thickness of upto 370 m (Pocock, 1970; Moore, 1979, figs 3, 4). Thesefossiliferous horizons appear to contain only monospe-cific assemblages of B. dailyi, apart from the presence ofhorizontal and subvertical burrows (sometimes found

Fig. 1. Geographic distribution of Balcoracania dailyi in South Australia. (1) Flinders Ranges. (2) Reaphook Hill. (3) Northern coast of KangarooIsland.

304 J.R. Paterson et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 249 (2007) 302–321

associated) and rare Diplichnites at various horizons(Moore, 1979; JRP, personal observation).

B. dailyi occurs in the shales and siltstones of threeinformal units (G, H, J) of the Coads Hill Member of theBilly Creek Formation at Reaphook Hill (Moore, 1980;Gravestock and Cowley, 1995; Paterson and Edge-combe, 2006). Specimens are abundant within a singlebedding plane of Unit G, approximately 43 m above thebase of the unit in section RH-A of Moore (1980, figs 2,13) and in a 10 cm-thick buff calcareous siltstonehorizon in section RH-E (Gehling, 1971; Moore, 1980,fig. 2); the horizon in the latter section has been repeatedby the N–S-trending fault mapped by Moore (1980,fig. 2). A single horizon within the basal portion of the

3 m-thick Unit J also contains abundant specimens ofB. dailyi (Moore, 1980, figs 4, 13). Both monospecificoccurrences of B. dailyi in Units G and J are associatedwith rare brachiopod fragments. Fragments of B. dailyiare rare in the middle to upper portion of Unit H (Moore,1980; Gravestock and Cowley, 1995).

4. Taphonomy

The bedding plane assemblages of B. dailyi at eachof the localities discussed above typically preserveboth articulated and disarticulated individuals (Fig. 2).The assemblages have been interpreted as follows:the Warragee Member (Billy Creek Formation) sample

Fig. 2. Relative abundance of Balcoracania dailyi sclerites from single bedding plane assemblages at various localities in South Australia. Nindicates number of sclerites, not the number of individuals. (A) 1.5 cm-thick shale bed from the Warragee Member (Billy Creek Formation),approximately 151 m above the base (and ∼7.5 m below the lowest of three prominent tuff beds), in a tributary immediately south of BalcoracanaCreek, Flinders Ranges (see Pocock, 1970, text-fig. 1); ⁎ denotes all counts that represent articulated protaspides and early meraspides. (B) 10 cm-thick calcareous siltstone bed from Unit G, section RH-E, Coads Hill Member (Billy Creek Formation), Reaphook Hill (see Moore, 1980, figs 2, 3).(C) 5 cm-thick micaceous siltstone bed from the type locality, White Point Conglomerate, west of Cape D'Estaing, Kangaroo Island (see Pocock,1970, text-figs 1, 2).


represents a census assemblage; the Coads Hill Member(Billy Creek Formation) sample represents a partcensus, part time-averaged assemblage; and the WhitePoint Conglomerate sample represents a within-habitat,time-averaged assemblage (sensu Kidwell and Bosence,1991; Kidwell, 1993; Brett and Baird, 1993).

4.1. Warragee Member, Billy Creek Formation

Biostratinomic data from the Warragee Membercensus assemblage (Figs. 2A and 3) strongly suggestmass mortality followed by a very brief quiescentperiod when the assemblage was subaqueously exposedon the substrate prior to burial. This is evidenced by ahigh percentage of articulation (71.3%), with over 15%

of sclerites representing complete exoskeletons, manyof which display the dorsal imprint of the in situhypostome. It is possible that some, or all, of thecephalothoraces represent complete exoskeletons, sincethe posterior portion of many specimens is incompletebecause they are preserved on the edges of discontin-uous sediment laminations due to the uneven split ofthe layers when sampled. The assemblage also containsa high percentage of juvenile (protaspid and meraspid)stages (Fig. 5A). Taphonomic experiments on modernarthropods have demonstrated that an exoskeleton willbegin to decay and disarticulate within hours to daysafter death or ecdysis (Plotnick, 1986; Allison, 1986;Speyer, 1987; Plotnick et al., 1988; Briggs and Kear,1994; Babcock and Chang, 1997; Babcock et al., 2000).

Fig. 3. Census assemblage of Balcoracania dailyi from the Warragee Member of the Billy Creek Formation, Balcoracana Creek, Flinders Ranges;SAMP41456. (A) Latex cast of a body cluster of predominantly complete individuals of similar size. Scale bar is 5 cm. (B) Facing directions ofindividuals showing no obvious preferred orientation. (C) Proportion of exoskeletons that are convex-up and convex-down within the body cluster.


Therefore the degree of articulation, coupled with thelack of fragmentation, abrasion and reorientation,suggests that the Warragee Member assemblage isautochthonous and was rapidly buried soon after death,with minimal pre-burial disturbance (Brett and Baird,1986; Speyer and Brett, 1986; Speyer, 1987, 1991). Theproportion of disarticulated sclerites within the assem-blage (Fig. 2A) may be due to either minor reworkingbefore burial (discussed below), or that some indivi-duals had begun the process of exuviation (see Section6.3).

Orientation data of individuals suggest that there wasan apparent absence of strong currents (Fig. 3B),although gentle currents (b10 cm/s) may have beenpartly responsible for the inversion of many individuals(Fig. 3C). There have been many explanations forinversion (dorsum-down orientation) in trilobites, in-cluding swimming upside-down, decay, ecdysis, scav-

enging, bioturbation, escape postures (cf. Hughes andCooper, 1999), and mechanical processes, such astraction currents and the “falling leaf effect” (see Speyerand Brett, 1985, pp. 99–100; Speyer, 1987, pp. 214–217, and Brett et al., 1999, pp. 293–295, for anoverview). Speyer (1985) suggested that inversion, atleast among clustered Phacops rana (= Eldredgeopsrana, fide Struve, 1990) from the Devonian HamiltonGroup of New York, was related to ecdysis and probablyaided in apolysis (pre-ecdysial separation of cuticle fromintegument tissue). By analogy with Limulus andmodern crustaceans, loose enrolment in Eldredgeopsassisted with the inversion of the body and wasproceeded by an erratic flexure of the tergite againstthe substrate. This would have aided the dislocation of ajoint at the cephalothoracic juncture and subsequentemergence of the animal. A similar scenario of inversionto aid ecdysis has been suggested for inverted exuviae of


“Homotelus” bromidensis from the Ordovician ofOklahoma (Karim and Westrop, 2002). It is unlikelythat B. dailyi inverted its body to assist with moultingfor two reasons: (1) the inverted individuals in theWarragee Member assemblage (Fig. 3A) unlikelyrepresent exuviae, as these exoskeletons appear to befully articulated, in particular, at the cephalothoracicjuncture and facial sutures (see Section 6.4 regardingexuviation techniques); and (2) aspects of the morphol-ogy, such as the fused macropleural unit with excep-tionally long spines and long, strongly tapering thorax(Fig. 8D, F), would inhibit enrolment in order to achievethe inverted position.

Inversion resulting from escape or upside-downswimming postures, scavenging and bioturbation canalso be dismissed for the Warragee Member assemblageof B. dailyi. Regarding the inverted swimming posture,we agree with Speyer and Brett (1985, p. 100) that ‘it ismost unlikely that mobile, nektonic organisms would beforced to the seafloor and buried en masse.’ Further-more, an inverted swimming posture would not explainthe individuals preserved dorsal-side up. Hughes andCooper (1999) suggested that inversion in Flexicaly-mene from the Upper Ordovician Kope Formation inKentucky may represent an escape posture with theindividual becoming entombed in the process. This isbased primarily on the inclined position of numerousindividuals relative to bedding and in various flexureposes. Inverted specimens of B. dailyi from theWarragee Member assemblage are not preserved inthis manner. Inversion due to scavenging seems unlikelyin the Warragee Member assemblage for two reasons:(1) fossiliferous horizons in the Warragee Member typi-cally contain only monospecific assemblages of B. dailyi,although this species is known to co-occur with recog-nised predator–scavengers such as Anomalocaris in theEmu Bay Shale on Kangaroo Island (Paterson and Jago,2006); and (2) many inverted, fully articulated specimensshow no obvious signs of disarticulation or breakage, aswould be expected if disturbed by scavengers or predators(Babcock, 2003). Bioturbated sediments are commonthroughout the Warragee Member and have been foundassociated with specimens of B. dailyi, but some assem-blages containing inverted individuals occur in finelylaminated shales (e.g., Fig. 3) that show no evidence ofbioturbation.

Inversion of individuals in the Warragee Memberassemblage is most likely the result of decay andmechanical processes. It has been suggested thattrilobite corpses generated a considerable amount ofgas during visceral decay (Hicks in Walcott, 1881;Beecher, 1894; Speyer and Brett, 1985; Speyer, 1987).

During the brief period of time between death andburial, probably only a few hours, decay gas wouldbuild up beneath the exoskeleton as soft parts werebroken down by decomposition and necrolysis, causingthe carcass to become partially buoyant. This buoyancywould make the exoskeleton susceptible to overturningby even the slightest of bottom currents (Speyer andBrett, 1985). These gentle currents may have also beenresponsible for disturbing moult ensembles or carcassesthat had undergone considerable decay, as evidenced bythe occurrence of disarticulated sclerites within theassemblage (Fig. 2A).

4.2. Coads Hill Member, Billy Creek Formation

The preservation of the B. dailyi assemblage fromUnit G of the Coads Hill Member indicates that therehas been an initial period of minor reworking ofsclerites, followed by a rapid burial or obrution event(sensu Brett et al., 1997) that buried the living pop-ulation with the reworked sclerites. The initial period ofreworking is evidenced by the considerably high per-centage (67.9%) and density of disarticulated sclerites(Figs. 2B and 4A), and some minor shape sorting basedon the ratio of cranidial and librigenal sclerites (Fig. 2B).However, this reworking appears to have been minimaldue to the paucity of size sorting (Fig. 5B) and frag-mentation, and no preferred concavo-convex orientationof sclerites (Speyer and Brett, 1986; Speyer, 1987,1991). This time-averaged accumulation of sclerites andthe associated living individuals, many of which ap-peared to be moulting at the time, were suddenly buried.This interpretation is based on the association of dis-articulated sclerites with articulated individuals (e.g.,Fig. 4B), in addition to the occurrence of completelyreworked sclerites (Fig. 4A), undisturbed exuviae(Fig. 7A, D, F–H) and fully articulated carcasses(e.g., Paterson and Edgecombe, 2006, fig. 7.7) pre-served in the same bed, and in some cases lamina. Asnoted by Brett and Baird (1986, p. 213), moult en-sembles ‘provide outstanding indicators of relativelyrapid burial under undisturbed, low-energy conditions…because individual elements would be dissociated byeven weak currents’, and this has been well documentedin many case studies on trilobite taphonomy (e.g.,Fortey, 1975; Speyer, 1985; Speyer and Brett, 1985;Schumacher and Shrake, 1997; Hickerson, 1997; Karimand Westrop, 2002; Hunda et al., 2006; Brett et al.,2006).

A comparable taphonomic scenario to that describedabove has been documented by Hunda et al. (2006) onassemblages of Flexicalymene retrorsa from the Upper

Fig. 4. Assemblage of Balcoracania dailyi from Unit G of the Coads Hill Member, Billy Creek Formation, Reaphook Hill. Scale bars=1 cm. (A)Time-averaged accumulation of disarticulated sclerites; latex cast, SAMP42086. (B) Aggregation of exuviae and disarticulated sclerites; latex cast,SAMP42087.


Ordovician Mt. Orab shale bed of the ArnheimFormation, Ohio. The clay and silt layers containingF. retrorsa are believed to represent a series of stackedevent beds that resulted from rapid deposition associatedwith distal storms. The trilobite assemblages from theMt. Orab bed and the Coads Hill Member share manytaphonomic characteristics. In general, both illustrate asimilar degree of articulation, 22.7% in the Mt. Orab bedversus 32.1% in the Coads Hill Member. These

assemblages also show little evidence of transport,based on the lack of fragmentation, abrasion, and nopreferred concavo-convex orientation, although bothassemblages show some degree of shape sorting (Hundaet al., 2006, Table 1). The assemblages of F. retrorsafrom the clay layers of the Mt. Orab bed appear to havebeen buried in the same manner as the assemblage ofB. dailyi from the Coads Hill Member (discussedabove). Hunda et al. (2006) proposed, amongst other

Fig. 5. Size–frequency distributions for bedding plane assemblages of Balcoracania dailyi from South Australia. (A) Warragee Member of the BillyCreek Formation, Balcoracana Creek, Flinders Ranges; modified from Pocock (1970, text-fig. 3). (B) Coads Hill Member of the Billy CreekFormation, Reaphook Hill. (C) White Point Conglomerate, west of Cape D'Estaing, Kangaroo Island.


alternative explanations, that at the time of burial theautochthonous clay layer assemblages – representing anassociation of articulated exoskeletons, moult ensem-

bles and disarticulated sclerites – contained a mix oftransported individuals with others that were living andmoulting at the final site of deposition.


Schumacher and Shrake (1997) reported a trilobiteassemblage from the Upper Ordovician WaynesvilleFormation of southwestern Ohio, containing an associ-ation of articulated carcasses and moult remains withdisarticulated sclerites, the latter often encrusted withbryozoans and worm tubes indicating long periods ofseafloor exposure. Schumacher and Shrake (1997,p. 150) suggested that both types of preservation in agiven stratigraphic interval indicate that the bed was‘deposited either in one episode of high sedimentationthat incorporated both living and disarticulated remainsof trilobites or in a number of high-sedimentation eventspunctuated by periods of little or no sedimentation.’ Thebiostratinomic evidence from the Coads Hill assemblagewould imply that the former depositional scenarioproposed by Schumacher and Shrake (1997) is morelikely.

4.3. White Point Conglomerate

Biostratinomic characteristics of the assemblage of B.dailyi from the White Point Conglomerate clearlyindicate that it represents a parautochthonous, within-habitat, time-averaged assemblage. Kidwell (1993,p. 278) defined these assemblages as ‘time-averagedover a period of relative environmental stability, so thatonly individuals from a single temporally persistentcommunity are mixed.’ They are often characterised by amixture of preservation states depending on environ-mental conditions, being of low density within ahomogeneous sedimentary body, and containing anassociation of apparently indigenous but ecologicallydisparate forms (Kidwell and Bosence, 1991). TheWhitePoint Conglomerate assemblage contains a low densityof predominantly disarticulated sclerites (93.1%) thatdisplay a considerable degree of shape sorting (Fig. 2C),with few sclerites showing signs of minor fragmentationand breakage. This suggests that the assemblage hasbeen reworked to some extent (Brett and Baird, 1986;Speyer, 1983, 1987; Mikulic, 1990). The assemblagealso contains rare, fragmented thoracic segments ofR. takooensis. As mentioned above, assemblages ofB. dailyi are typically monospecific, so the association ofRedlichia fragments suggests that some transportationand faunal mixing have taken place. However, thepresence of articulated individuals (6.9%) and the singleoccurrence of a librigena attached to the rostral platesuggest that there were pulses of rapid burial throughoutthe duration of reworking and deposition of the bed.Furthermore, there has not been a significant amount ofsize sorting within the assemblage (Fig. 5C), suggestingthat sclerites were not subjected to persistent currents

resulting in winnowing and extensive transportation(Westrop, 1986; Mikulic, 1990; Westrop, 1992; Westropet al., 1993; Westrop and Rudkin, 1999).

5. Palaeoenvironmental setting

5.1. Billy Creek Formation

Moore (1979, 1980, 1990) interpreted the fine-grained sediments of the Warragee and Coads HillMembers of the Billy Creek Formation as having beendeposited along the shoreline of an epeiric sea.Sedimentological evidence provided by Moore (1979,1990) suggests that the fine-grained sediments of theWarragee Member were probably deposited within thesubtidal, intertidal and supratidal zones of a protectedlagoonal system. The unit is dominated by laminatedshale (including shaly redbeds) and siltstone with minorinterbeds of coarse siltstone and fine sandstone that aretypically evenly laminated, but rarely display ripplelaminations and flaser bedding. Desiccation cracks(sometimes associated with assemblages of B. dailyi),halite casts, rare anhydrite, and minor interbeds ofstromatolitic dolomite occur throughout the WarrageeMember, indicating infrequent subaerial exposure andevaporite formation. The arthropod trace Diplichnites,as well as horizontal and subvertical burrows, are alsofound in the shales and siltstones (Moore, 1979, 1990;JRP and GAB, personal observation).

The Coads Hill Member of the Billy CreekFormation at Reaphook Hill was deposited under similarconditions to that of the Warragee Member to the west.Moore (1980) noted the occurrence of B. dailyi in threeof the nine informal units of the Coads Hill Member, i.e.,Units G, H and J. Unit G is similar to the WarrageeMember in that it is dominated by laminated shale andsiltstone with minor interbeds of dolomite (oftenstromatolitic) and rare siltstones displaying asymmetri-cal ripple marks; desiccation cracks and halite casts arealso common. Unit H is dominated by dark grey, foetidlimestones, with the lower and upper portions of the unitcontaining shale and stromatolitic dolomite, and thepresence of desiccation cracks and halite casts. Unit Jcomprises evenly laminated shale and fine siltstone withminor carbonate interbeds. Moore (1980) interpreted thedepositional environments of these informal units asfollows: (1) Unit G was deposited on subtidal tointertidal mudflats subject to frequent marine inundationand reducing conditions; (2) Unit H accumulated in asemi-restricted, very shallow marine environment withsporadic subaerial exposure and occasional dysaerobicepisodes indicated by the foetid limestones with high


organic content; and (3) Unit J marks a period of fine-grained clastic deposition in a relatively open, shallowmarine environment. While we regard these units of theCoads Hill Member as having formed in a shallow,marginal marine environment, we consider Units G, Hand J to have been deposited in a protected, tidally-influenced lagoon with occasional episodes of evaporiteand carbonate formation, and phases of dysoxia and/oranoxia.

5.2. Kangaroo Island

The sedimentology and stratigraphy of the KangarooIsland Group, which includes the White Point Con-glomerate and Emu Bay Shale, have been documentedin considerable detail by Daily et al. (1979, 1980). Theyinterpreted these sediments as having been deposited ona shoreline, along which fan deltas interfingered withtidal deposits. Their ‘fine-grained facies association’ isbased on various characteristics, including the ‘associ-ation of desiccation cracks with a marine infauna andepifauna, and the interbedding of ripple laminatedmarine sandstone and coarse siltstone with varyingproportions of mudstone’ (Daily et al., 1980, p. 394).The desiccation cracks are commonly associated withredbeds, indicating subaerial exposure. The fossiliferoussediments containing B. dailyi are interpreted asrestricted, shallow, marginal marine deposits, possiblywithin an embayment (Daily et al., 1979, 1980; Nedin,1995b).

The Emu Bay Shale at Big Gully consists predom-inantly of dark grey to black laminated (often pyritic)shales, in contrast to the grey-green, laminated shalesand siltstones common at other localities, such as thetype section at Emu Bay (Daily et al., 1980). It isthought that the unique sequence of the Emu Bay Shaleat Big Gully was deposited in a fluctuating dysoxic–anoxic microenvironment formed below wave base in adeep nearshore basin (Conway Morris and Jenkins,1985; Nedin, 1995b). This unique environment fa-voured the preservation of the Emu Bay ShaleLagerstätte that occurs only at Big Gully (see Patersonand Jago, 2006, references therein).

6. Palaeoecology

The taphonomic and palaeoenvironmental datapresented above demonstrate that B. dailyi was aninhabitant of protected, shallow, marginal marineenvironments. The rare occurrence of B. dailyi in thedysoxic–anoxic microenvironment of the Emu BayShale Lagerstätte at Big Gully is probably allochtho-

nous. Conway Morris and Jenkins (1985, p. 168)suggested that ‘trilobites and other free-living creaturesmay have been asphyxiated when they swam or crawledinto the top waters or edges of this toxic ‘pool’.’ Gainesand Droser (2005) and Gaines et al. (2005) recentlysuggested that some Cambrian Lagerstätten depositedunder cyclic dysoxic and anoxic conditions maypreserve in situ biotic assemblages living underoxygen-stressed conditions, meaning that some of themore common benthic constituents of the Emu BayShale Lagerstätte may be preserved in situ during pe-riods of bottom water oxygenation. However, B. dailyiwas an unlikely inhabitant of this stressed environ-ment, based on its rarity at Big Gully and commonoccurrence in well-oxygenated shallow marine environ-ments in other areas, thus was likely to have sufferedthe fate suggested by Conway Morris and Jenkins(1985).

Assemblages of B. dailyi are usually monospecific,yet other rare faunal constituents such as brachiopodsand tubular burrows – the latter indicating the presenceof soft-bodied organisms – demonstrate that B. dailyiformed part of a community in most areas. An extensiveexamination of large collections from the White PointConglomerate and Emu Bay Shale in the SouthAustralian Museum by JRP and JBJ found only twohand specimens that contained B. dailyi in associationwith E. bilobata and R. takooensis. It is possible thatthese taxa were part of a single community, buttaphonomic processes resulting in faunal mixing (asmentioned above) cannot be ruled out.

6.1. Population dynamics

Size–frequency histograms were constructed foreach of the bedding plane assemblages (Fig. 5) inorder to elucidate their population dynamics. Cautionmust be taken when interpreting the population struc-ture of fossil assemblages as they have commonlysuffered taphonomic loss and time-averaging. This isespecially true for trilobite and other arthropodassemblages, with the added complication of dealingwith exuviae. In a study on decapod crustaceans,Hartnoll and Bryant (1990) demonstrated that size–frequency distributions of assemblages of not onlyexuviae but carcasses will have a different distributionto the living population from which they were de-rived. This has serious implications for studies ontrilobite population dynamics and reiterates the im-portance of understanding the taphonomic signatureof an assemblage before meaningful conclusions canbe drawn.


6.1.1. Warragee Member, Billy Creek FormationThe size–frequency histogram of B. dailyi from the

Warragee Member assemblage (Fig. 5A) differs consid-erably from those of the Coads Hill Member and WhitePoint Conglomerate. In living populations, positively-skewed distributions suggest high infant mortality andsubsequent relatively lower mortality. This situation istypical in populations where there is a high initialspatfall and high mortality as larvae and juvenilesstruggle to establish themselves when exposed to avariety of physical stresses and predators (Hallam, 1972;Dodd and Stanton, 1981; Brenchley and Harper, 1998).This is a highly probable situation for the Warrageeassemblage as the environment is relatively unstable,with evidence of fluctuating water levels and hypersa-line conditions. Taphonomic data from the Warrageeassemblage show that the living population was notkilled by an obrution event, but that there was minimalpre-burial disturbance, including a supposed absence ofpredator–scavengers, thus preserving the censusedliving population. Hartnoll and Bryant (1990) illustratedthat even the preservation of decapod carcasses canshow a different distribution to that of the livingpopulation (positively-skewed versus normal, respec-tively), although these results were based on assumingrelatively constant recruitment and mortality rates in asteady-state population. It is important to keep in mindthat positively-skewed sample distributions can resultfrom normal or positively-skewed population structures,depending on the degree of taphonomic loss within afossil assemblage, especially if an arthropod assemblagehas a large constituent of exuviae (Cummins et al., 1986;Sheldon, 1988; Hartnoll and Bryant, 1990). Thecombined taphonomic and palaeoenvironmental evi-dence would tend to suggest that the Warrageeassemblage represents the true structure of the livingpopulation under stressed environmental conditions.Other trilobite assemblages representing censused livingpopulations showing a positively-skewed distributionthat may have been living under environmentally-stressed conditions include Triarthrus eatoni from theUpper Ordovician Frankfort Formation of New York(Cisne, 1973a,b; Whittington and Almond, 1987; Briggset al., 1991; Etter, 2002; Whiteley et al., 2002; see alsoBrett et al., 2003, 2006 regarding the inferred habitat ofthis species from near Cincinnati and Ontario, respec-tively), and Elrathia kingii from the Middle CambrianWheeler Formation of Utah (Gaines and Droser, 2003;R.R. Gaines, pers. comm. 2006). This type ofpopulation structure resulting from extrinsic stresseshas also been documented for a variety of otherorganisms, e.g., the modern suspension feeding bivalve

Mulinia lateralis (Levinton and Bambach, 1970), theJurassic epibyssate pectinid bivalve Parvamussiumpumilum (Fürsich et al., 2001), and a variety ofPalaeozoic brachiopods from Indiana and Illinois(Richards and Bambach, 1975).

Another potential explanation for this type ofpopulation structure is that perhaps the Warrageeassemblage represents a nursery. A population of thehorseshoe crab Tachypleus tridentatus from AventuraBeach (intertidal flat) in Palawan in the Philippinescontains only juveniles, with the absence of adultsexplained by the fact that they occupy deeper waters andare only found near the beach during the breedingseason (Almendral and Schoppe, 2005). The presence ofadults in the Warragee assemblage may represent acongregation of individuals coming to breed within theintertidal zone of a lagoon; this will be explored furtherin Section 6.3.

Pocock (1970) suggested that the Warragee assem-blage resulted from the living population being killedsuddenly during a mass mortality event that was con-nected with volcanic activity, evidenced from the sev-eral tuff horizons within the member. However, thetuffaceous horizons and bedding assemblages ofB. dailyiwithin the Warragee Member do not closely occur to-gether stratigraphically. Assemblages commonly occurabove the tuff layers (Moore, 1979, fig. 4), contradic-ting the order of events that would result in mass mor-tality from volcanic activity. Furthermore, Dalgarno(1964, p. 139), who originally collected the specimensdescribed by Pocock (1970), reported the trilobites oc-curring ‘approximately 25 ft below the lowest of threetuffaceous bands in the lower part of the Billy CreekFormation near Balcoracana Creek.’ If volcanic activitywere responsible for the mass mortality, one wouldexpect to find the tuff horizons occurring immediatelyabove the fossiliferous horizons. Other possible causes ofdeath of theWarragee population will be discussed below(Section 6.3).

6.1.2. Coads Hill Member, Billy Creek FormationThe size–frequency distribution of B. dailyi from the

Coads Hill assemblage shows a slightly positive skew(Fig. 5B). This assemblage creates a complication wheninterpreting the population structure because the taph-onomic evidence suggests that it contains a juxtaposi-tion of time-averaged material associated with moultensembles and complete carcasses. This taphonomicmixing may explain the slight positive skewness of thedistribution, but it is difficult to determine whether theoriginal population structure of the census (moultensembles + carcasses) assemblage may have been


normal or positively-skewed. In order to gain a betterresolution, the size–frequency distribution of the CoadsHill assemblage was distributed into two separatehistograms, one of the time-averaged portion (Fig. 6A)and the other of the census portion (Fig. 6B). Thedistribution for the time-averaged portion of theassemblage shows a normal or Gaussian distributionthat is typical of transported and/or time-averagedassemblages. The census portion of the assemblagealso shows an apparent normal distribution, suggestingthat the population was in a steady state. However,Hartnoll and Bryant (1990) suggested that under lowmortality conditions, normal distributions for trilobiteassemblages could result from an accumulation ofcorpses and exuviae showing selective preservation inthe small to median size classes. Yet, the size–frequencydistribution of the census portion of the Coads Hill

Fig. 6. Size–frequency distributions for the Coads Hill Member assemblag(B) Census portion of the sample.

assemblage (Fig. 6B) could be interpreted as the truestructure of the living population corresponding toenvironmental conditions, as the Coads Hill assemblage(from Unit G) possibly inhabited more environmentally-stable waters – based on the co-occurrence of rarebrachiopods – when compared to the assemblages fromthe Warragee Member.

Similar size–frequency distributions have beendocumented for trilobite assemblages that display asimilar taphonomic signature to that of the Coads Hillassemblage (Fig. 5B), i.e., a mixture of both time-averaged and census accumulations. Hunda et al. (2006,fig. 7) illustrated the effect of including differenttaphonomic accumulations (time-averaged and census)into a single size–frequency histogram by incorporatingall measured specimens of F. retrorsa from the 0.46 m-thick Mt. Orab shale bed of the Arnheim Formation,

e of Balcoracania dailyi. (A) Time-averaged portion of the sample.


Ohio; the result being a normal distribution. Brezinski(1986) recorded a monospecific assemblage of Ampyx-ina bellatula from the Upper Ordovician MaquoketaGroup of Missouri containing 97 complete individualsand a considerable number of partially articulated anddisarticulated sclerites. While Brezinski interpreted thissample as a census assemblage, Sheldon (1988,pp. 302–303) believed the assemblage represented atime-averaged accumulation based on the high propor-tion of associated moult remains and disarticulatedsclerites. Nevertheless, Brezinski's (1986, fig. 4) size–frequency distribution included only the completeindividuals, representing the living population structureshowing a normal distribution, presumably in a steadystate. Brezinski interpreted this occurrence of Ampyxinaas an opportunistic assemblage inhabiting shallowwaters with restricted circulation resulting in oxygendeficiencies, hypersalinity and abundant organic con-centrations. This interpretation, when coupled with thesize–frequency data, is unusual, as Brezinski (1986,p. 319) himself stated that this distribution is ‘not whatone would expect for an opportunistic species inasmuchas one would expect a right-skewed distribution.’ Heattempted to explain this by suggesting that previouspopulation studies of interpreted opportunists werebased on time-averaged rather than censused popula-tions. Other explanations may be that if A. bellatula isan opportunist, the assemblage is time-averaged assuggested by Sheldon (1988), or that it is not, in fact, anopportunist but happened to populate a region ofnortheastern Missouri during a time of environmentalstability and that the assemblage may characterise agregarious monospecific aggregation (sensu Speyer andBrett, 1985). Regarding the latter explanation, theassemblage of A. bellatula was recovered from across-laminated, dolomitic siltstone bed sandwichedbetween shale layers containing euhedral pyrite. Thereis no mention of the A. bellatula bed containing pyriteor any other evidence that would suggest low-oxygenconditions.

6.1.3. White Point ConglomerateThe size–frequency distribution of B. dailyi from the

White Point Conglomerate assemblage shows a normal(Gaussian) distribution (Fig. 5C). As noted above, thistype of distribution is typical of either: (1) steady-statepopulations, e.g., certain decapod crustaceans (Hartnoll,1978; Hartnoll and Bryant, 1990); or (2) transportedand/or time-averaged assemblages that have undergonesome degree of taphonomic loss, such as the trilobitesfrom the Ordovician Teretiusculus Shales of centralWales (Sheldon, 1988; see also Hartnoll and Bryant,

1990 for a discussion of Sheldon's paper). Thebiostratinomic data clearly indicate that the assemblagehas been reworked with minor transportation, implyingthat the size–frequency distribution is simply ataphonomic artifact.

6.2. B. dailyi as an opportunistic species?

The stratigraphic, sedimentologic, geographic andtaphonomic evidence provides some support for theinterpretation of B. dailyi as being an opportunisticspecies. Levinton (1970) provided seven criteria to aid inthe identification of opportunists in the fossil record.B. dailyimeetsmost of these criteria, including: numericaldominance (85–100%) of the assemblage; presence inthin but widespread isochronous horizons although it maybe patchy within those horizons; occurs in severalhorizons with barren intervals in-between; aggregationof individual species in clusters (= Warragee assemblage,see Section 6.3); and random orientation and lack of sizesorting of specimens in individual beds, in order toeliminate the possibility of numerical abundance resultingfrom factors such as mechanical sorting or low sedimen-tation rates. However, Levinton's two other criteria do notnecessarily apply to B. dailyi, i.e., opportunistic speciesare abundant in several otherwise distinct faunal assem-blages, and that they appear in great abundance in atypicalfacies. Regarding the former, assemblages of B. dailyi aretypically monospecific and are rarely associated withother biota (as mentioned above). Levinton commentedthat the occurrence of opportunists within a distinctcommunity is due to their eurytopism. It is uncertain as towhether B. dailyi was eurytopic, as it appears to berestricted to very shallow water environments. Although,one could argue that its distribution in shallow, tidally-influenced, marginal marine environments would cover awide range of conditions, implying that B. dailyi was aeurytopic organism. To address the other criterion ofabundance in atypical facies, B. dailyi is only abundant infine-grained, siliciclastic, shallow water facies.

In general, opportunists represent species that canadapt to unstable, variable environments often domi-nated by physical stress (Levinton, 1970; Hallam, 1972;Dodd and Stanton, 1981), as demonstrated above forB. dailyi. Moreover, opportunistic species are typicallyr-strategists. While it is extremely difficult to assessgrowth rates for fossil arthropods (see Sheldon, 1988;Chatterton and Speyer, 1997, pp. 208–209; Hunt andChapman, 2001, for discussion), B. dailyi displays othercharacteristics typical of an r-strategist, such as a smallbody size (with a known maximum exoskeletal sagittallength of 37 mm) and high fecundity. Hence, the

Fig. 7. Size–frequency distribution of the body cluster of Balcoraca-nia dailyi from the Warragee Member assemblage.


evidence presented above would tend to favour theinterpretation of B. dailyi as an opportunist.

6.3. Gregarious behaviour?

Behavioural palaeobiology is a relatively young fieldin trilobite studies, pioneered by S.E. Speyer in hisseminal papers onMiddle Devonian trilobite assemblagesfrom New York (Speyer, 1985, 1987, 1990; Speyer andBrett, 1985). Since then only a limited number of papershave dealt with the gregarious behaviour (or clustering) oftrilobites in any detail (e.g., Hughes and Cooper, 1999;Fortey and Owens, 1999; Suzuki and Bergström, 1999;Davis et al., 2001; Karim and Westrop, 2002; Clarksonet al., 2003; Chatterton et al., 2003).

By analogy with extant marine arthropods, Speyerand Brett (1985) and Speyer (1985, 1990) suggested thatwell preserved trilobite clusters from the MiddleDevonian Hamilton Group were the result of gregariousbehaviour related to moulting and reproduction. Theyproposed that these trilobites (in particular Elredgeopsrana) assembled into monospecific, size-segregatedclusters (forming ‘body clusters’) and moulted (produc-ing ‘moult clusters’) prior to en masse copulation. Theydefined a ‘body cluster’ as a densely clustered mono-specific assemblage containing numerous, complete,size-segregated individuals ≤2 cm from the next closestneighbour; although, Hughes and Cooper (1999) foundthat having individuals ≤2 cm apart was not a usefuldefinition in their study, with their mean distancebetween individuals being 4.4 cm. The definition of‘moult clusters’ is essentially the same as ‘body clus-ters’, but most individuals represent moult ensemblesand dissociated sclerites.

Other possible reasons for clustering in trilobiteshave been suggested, such as mechanical aggregation,feeding and stress-related behaviour in response toenvironmental disturbance (Speyer and Brett, 1985;Hughes and Cooper, 1999). However, these explana-tions can often be eliminated based on taphonomic,morphometric and taxonomic evidence. Firstly, me-chanical aggregation can be ruled out as it is unlikelythat currents would create a monospecific, size-segre-gated cluster of articulated individuals, especially if theyshow no preferred facing orientation, as is the case forthe Warragee cluster of B. dailyi (Figs. 3 and 7).Secondly, feeding and stress-related behaviour alsoseem doubtful for much the same reasons; argumentsagainst these alternative explanations are presented indetail by Speyer and Brett (1985, pp. 97–98).

The assemblage of individuals from the WarrageeMember illustrated in Fig. 3 is considered to represent a

body cluster formed for the purpose of synchronousecdysis, mating and possibly spawning. The assemblagecontains 39 individuals preserved in a single laminawithin an area of approximately 120 cm2, with thefurthest distance of one individual from another being10.4 mm. Although this assemblage does not display ahigh density of individuals like the body clusters ofE. rana (Speyer and Brett, 1985, figs 3A, 4, 6) and“Homotelus” bromidensis (Karim and Westrop, 2002,figs 1, 6), it certainly falls within the realm of otherinterpreted body clusters (Speyer, 1990, Table 32;Hughes and Cooper, 1999; Babcock, 2003, Table 3).The size–frequency distribution of the cluster indicatesthat the individuals are size-segregated (Fig. 7).Cephalic lengths (sag.) range between 3.5 and 7.0 mm,with over 29% of the individuals restricted to the 5.5–6.0 mm size class.

Interpretation of the Warragee body cluster (Fig. 3),and the assemblage in general, is perhaps best discussedby analogy to horseshoe crabs coupled with theavailable taphonomic and sedimentological data. Sincethere are no modern exemplars of trilobites, in order toinfer some aspects of the palaeobiology of B. dailyi wehave chosen Limulus polyphemus (Chelicerata: Xipho-sura) because it inhabits similar environments to that ofB. dailyi, i.e., shallow marine, especially tidally-influenced, environments. If we assume Speyer andBrett's (1985) sequence of events to be true for alltrilobites (i.e., congregation N moulting N copulation),then the Warragee body cluster represents a pre-ecdysialgathering of sexually active individuals. It is possible


that some individuals had already begun the act ofexuviation, as evidenced by the small percentage ofaxial shields within the assemblage (Fig. 2A) and theproportion (28.7%) of disarticulated sclerites (Fig. 2A).Speyer (1985) showed that some body clusters occa-sionally include small proportions of moulted remains.The single bedding assemblage from the WarrageeMember was probably preserved in the intertidal zone ofa lagoon; Moore (1979, 1990) reported other assem-blages from the Warragee Member associated withdesiccation cracks, which suggests that they hadventured as far as the waterline and had becomestranded by the receding tide, leaving them exposedon the intertidal flat. It is possible that B. dailyimigratedfrom deeper waters into the intertidal zone for thepurposes of moulting and reproduction in the samemanner as modern horseshoe crabs.

Adult Limulus migrate at high tide during spring andearly summer to breed and spawn in the mid to upperintertidal zone (Rudloe, 1980; Shuster, 1982; Shusterand Botton, 1985; Ehlinger et al., 2003). Femalesexcavate a shallow nest near the waterline in the mid toupper intertidal zone and deposit up to 20,000 eggs. Theeggs are fertilized by sperm released by an attached maleand by one or more satellite males that typicallycongregate around the nesting couple (Penn andBrockmann, 1994; Ehlinger et al., 2003). Certain risksare involved with this type of reproductive strategy inthat they must locate suitable beaches, move to theupper reaches of the intertidal zone, copulate and spawn,and as the high tide recedes, they must move seaward.Individuals will often become stranded and althoughthey can usually survive for longer than one tidal cycle,if they are stranded near the high tide line and if tidalamplitude is decreasing, mortality is inevitable (Bottonand Loveland, 1989; Penn and Brockmann, 1995). It hasbeen suggested that this risky reproductive strategy in anunstable environment is compensated by high fecundityand low adult mortality (Botton and Loveland, 1989).The same factors may explain the strongly positively-skewed size distribution of the Warragee assemblage(Fig. 5A), as discussed in Section 6.1.1. As mentionedin Section 6.1.1, the Warragee assemblage mayrepresent a nursery, due to the high percentage ofjuveniles (Fig. 5A), with the body cluster representingthe return of the breeding population. It is common forhorseshoe crab nursery sites, usually intertidal flats, tocontain only larvae and juveniles, with adults returningduring the breeding season (Botton et al., 2003;Almendral and Schoppe, 2005).

If the Warragee assemblage does represent a nurseryand breeding population of B. dailyi, it is uncertain as to

why this species adopted this particular reproductivestrategy. Breeding and nest-site selection in Limulus arestrongly influenced by water chemistry, sedimentgeochemistry and tidal cycles, yet predation, inter- andintraspecific competition probably also play importantroles (Babcock, 2003). The environmental distributionof B. dailyi suggests that it may have been euryhalineand eurythermal, i.e., tolerant of widely changingsalinities and temperatures, respectively. It is interestingto note that Limulus larvae are more tolerant of suddenhyposalinity shock than adults and juveniles. Ehlingerand Tankersley (2004) suggest that this is probably dueto the fact that intertidal nesting areas undergo rapidfluctuations in salinity. Larvae that have a highertolerance to fluctuating salinity than juveniles andadults are also known in other arthropods, such asdecapod crustaceans (e.g., Anger, 1991; Diesel andSchuh, 1998). Perhaps hyposalinity shock is onepossible scenario relating to the mortality of theWarragee body cluster, although other extrinsic factorssuch as oxygen and temperature fluctuations may havebeen responsible.

If the interpretation of the Warragee body cluster ofB. dailyi as a gregarious behavioural aggregation isindeed correct, it reinforces the notion of Karim andWestrop (2002) that the occurrence of clustering in awide diversity of species from a variety of phylogenet-ically distant clades indicates that it is likely to havebeen a behavioural characteristic of trilobites in general.Furthermore, the Warragee cluster represents one of theoldest examples of gregarious behaviour in the arthro-pod fossil record (cf. Chatterton et al., 2003; Erickson,2004).

6.4. Exuviation

While there is a considerable amount of literature onmoulting in trilobites (see Henningsmoen, 1975;Whittington, 1997, pp. 152–158; and Brandt, 2002 foran overview), there are a limited number of detailed casestudies on exuviation techniques in Cambrian trilobites(e.g., Henningsmoen, 1957; Whittington, 1980; McNa-mara and Rudkin, 1984; McNamara, 1986; Whittington,1990, 1995; Geyer, 1996; Chatterton and Ludvigsen,1998; Clarkson et al., 2003).

We propose that B. dailyi moulted in a similar wayto other Cambrian trilobites, such as Paradoxidesand Ogygopsis (see McNamara and Rudkin, 1984;and subsequent revision by Whittington, 1990), andLabiostria (see Chatterton and Ludvigsen, 1998; al-though they suggest various modes for this taxon). Thismode of exuviation involves the flexure of the body

Fig. 8. Exuviae of Balcoracania dailyi. All scale bars=3 mm. All specimens come from the Coads Hill Member, Billy Creek Formation, ReaphookHill, except B, C and E, which are from the White Point Conglomerate, west of Cape D'Estaing, Kangaroo Island. (A) Almost complete exoskeletonshowing inverted and rotated left librigena, slightly dislodged right librigena, and dislodged cranidium from thorax; internal mould, SAMP42088. (B)Partially complete axial shield showing inverted and rotated librigenae; internal mould, SAMP40267. (C) Aberrant specimen showing articulatedcephalon and first prothoracic segment inverted beneath the remaining thorax; internal mould, SAMP40273. (D) Almost complete axial shield; latexcast of external mould, SAMP42089. (E) Moult ensemble consisting of a partial thorax, dislodged cranidium, and discarded hypostome and invertedleft librigena; latex cast of external mould, SAMP42090. (F) Complete thoracopygidium; latex cast of external mould, SAMP42091. (G) Almostcomplete axial shield with dislodged cranidium and first prothoracic segment; latex cast of external mould, SAMP42092. (H) Moult ensembleconsisting of an almost complete axial shield with dislodged cranidium and thorax telescoped beneath, a displaced portion of the opisthothorax, and adiscarded left librigena; latex cast of external mould, SAMP42093.



above the substrate with the anterior margin of thecephalon buried in the substrate; the long macropleuralspines, with their tips pushed into the substrate, mayhave been used to hold the body in the arched positionand prohibit posterior movement. The opening of thecephalic sutures would have then allowed the anterioremergence of the post-ecdysial animal, dragging theaxial shield (sensu Henningsmoen, 1975) forward,causing one or both of the librigenae to becomeinverted and rotated with respect to their in situ position(Fig. 8A, B) and, in some instances, dislodging thecranidium from the thorax (Fig. 8A, E, G, H). As notedby Whittington (1990), forward emergence was essen-tial to this process in order to withdraw the genal andpleural, especially macropleural, spines.

In some cases, the animal may have shed thelibrigenae, rostral plate and hypostome elsewhere eitherbefore and/or after emerging from the remainder of theaxial shield. This interpretation is based on the commonoccurrence of articulated axial shields of B. dailyiwithout the librigenae, rostral plate and hypostomepreserved in association (Fig. 8D, G). A cursory glanceof the trilobite literature indicates that the occurrence ofonly the axial shield is relatively common, as it is knownfor a variety of other Cambrian taxa, such as Xystridura(Öpik, 1975, pl. 15, figs 1, 4, pl. 22, fig. 2), Bathyur-iscus (Robison, 1964, pl. 83, fig. 11; Robison, 1967, pl.24, fig. 25; Gunther and Gunther, 1981, pl. 11, fig. C)and Xingrenaspis (Yuan et al., 2002, pl. 48, figs 1–5, 7,8), and a range of post-Cambrian species (McNamaraand Rudkin, 1984). Specimen SAMP42090 of B. dailyi(Fig. 8E), representing a moult ensemble (sensu Speyer,1985), provides further support for the pre- and/or post-ecdysial shedding of cephalic sclerites, with thefollowing sequence of events possibly explaining theexuvial configuration: (1) the hypostome and left(inverted) librigena are shed first; (2) the animalmoves slightly anterolaterally; and (3) it then dislodgesthe cranidium and rotates it to the right-hand-side as itmoves forward to completely emerge from the thorax.

A unique specimen ofB. dailyi (Fig. 8C, SAMP40273)that has the articulated cephalon and first prothoracicsegment inverted beneath the thorax may represent anindividual that has performed a rather violent flexure ofthe body, resulting in the integument between the first andsecond prothoracic segments weakening and breakingbefore the opening of the cephalic sutures had taken place,thus causing the animal to emerge in a Salterian mode ofexuviation. Furthermore, there appears to be no sign of thesecond, third and fourth prothoracic segments in thisspecimen, which may have been carried away with thenewly moulted animal and subsequently shed elsewhere.

A similar scenario was described and illustrated for aspecimen ofOryctocephalus from the Stephen Formation,British Columbia (McNamara and Rudkin, 1984,fig. 13A). McNamara and Rudkin (1984) rightly pointedout that this particular specimen of Oryctocephalusprobably represents an aberrant ecdysial event – as ap-pears to be the case for the specimen of B. dailyi(SAMP40273) – based on the ecdysial modes in otherspecimens of Oryctocephalus (e.g., McNamara andRudkin, 1984, fig. 13B; Whittington, 1995, pl. 2, figs 1,3, 6, 7, pl. 4, fig. 1; Yuan et al., 2002, pl. 19, figs 1, 2).

Acknowledgements

This research was funded by a Macquarie UniversityResearch Fellowship (MURF) grant to JRP and aNational Geographic Committee for Research andExploration Grant (#7918-05) to JRP, GAB and JBJ.We wish to thank Sarah Laurence (South AustralianDepartment of Environment and Heritage) for organis-ing permission to collect from the type locality onKangaroo Island; Dean Oliver for drafting Fig. 1; DennisRice (South Australian Museum) for arranging the loanof specimens; Greg Edgecombe (Australian Museum)and John Merrick (Macquarie University) for comment-ing on an earlier draft of the paper; and Nigel Hughes andSteve Westrop for their constructive reviews.

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