Adaptive morphologies and guild structure in a high-diversity bivalve fauna from an early Campanian...

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Cretaceous Research 33 (2012) 21e41

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Cretaceous Research

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Adaptive morphologies and guild structure in a high-diversity bivalve faunafrom an early Campanian rocky shore, Ivö Klack (Sweden)

Anne Mehlin Sørensen a,*, Finn Surlyk a, John W.M. Jagt b

aDepartment of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, DenmarkbNatuurhistorisch Museum Maastricht, de Bosquetplein 6-7, NL-6211 KJ Maastricht, The Netherlands

a r t i c l e i n f o

Article history:Received 10 March 2011Accepted in revised form 31 July 2011Available online 27 August 2011

Keywords:BivalvesPalaeoecologyRocky shoreLate CretaceousSweden

* Corresponding author. Tel.: þ45 35322401.E-mail address: anne@geo.ku.dk (A. M. Sørensen).

0195-6671/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.cretres.2011.07.004

a b s t r a c t

The bivalve fauna from a late early Campanian rocky shore at Ivö Klack (southern Sweden), comprisesjust over sixty species, a very high diversity in comparison to other Late Cretaceous and modern rockyshore bivalve assemblages. This high diversity is here considered to represent a reliable census of thefauna; only in part can it be explained by the cumulative effect of generations of bivalves inhabiting thiscoastal environment. The high density and diversity and the wide range of shell morphologies allowinterpretation of different modes of life in this variable environment with many contrasting habitats.Study of the functional morphology of bivalve shells and comparison with extant relatives has resulted ina subdivision of the fauna into seven guilds and five habitats. The bivalve fauna represents a within-habitat, time-averaged assemblage to which none of the species was introduced from adjacent envi-ronments. It includes some of the most northerly known, very small rudistid bivalves, in addition to theoldest known occurrences of Mytilus and Barbatia in association with rocky shores. Bivalves constitutedthe most important invertebrate group inhabiting the late early Campanian rocky shore at Ivö Klack, interms of diversity, density and biomass.

1. Introduction

Rocky shores provide many different habitats for the colonisingfauna and are therefore often characterised by a high faunal densityand diversity. Studies of ancient rocky shore faunas are few and farbetween, primarily because rocky shores are sites of erosion ratherthan of deposition, and the shelly fauna easily undergoes severefragmentation in such high-energy settings. However, goodexamples of ancient rocky shores do exist, with the encrustingfaunawell preserved (Surlyk and Christensen,1974; Lescinsky et al.,1991; Johnson and McKerrow, 1995; Desrochers, 2006). A well-exposed late early Campanian rocky shore is preserved at IvöKlack in southern Sweden; it yields an invertebrate fauna of bothhigh density and diversity (Surlyk and Sørensen, 2010). A biologicalinterpretation, comprising three zones, of the cementing epifaunahas been mapped out on large gneiss boulders and hummocks(Surlyk and Christensen, 1974). The highest zone is dominated bythe spondylid bivalve Spondylus labiatus (Wahlenberg, 1821), whilein the next, lower, zone on the near-vertical sides of the bouldersthe oyster Amphidonte haliotoideum (J. Sowerby, 1813) and theinarticulate craniid brachiopod Ancistrocrania stobaei (Lundgren,

1885) are predominant. The lowermost, photonegative zoneshows a predominance of serpulid species which occupy the over-hanging lower portions of the boulders and hummocks (Sørensenand Surlyk, 2010). The commonly excellent preservation of thisancient rocky shore fauna offers an opportunity to increase theknowledge of faunal composition, life habits and nature of theecosystem. Thus, the aim of the present paper is to interpret themode of life of the bivalve assemblages, to compare these withother Late Cretaceous and modern rocky shores faunas and tosubdivide the bivalves into guilds and preferred habitats in order tointerpret their ecological role on the early Campanian rocky shore.

2. Geological setting

The Ivö Klack site is a disused kaolin quarry, on the island of Ivöin lake Ivösjön, in the northern part of the Kristianstad Basin innorth-east Skåne, southern Sweden (Fig. 1). This basin was situatedat a palaeolatitude of approximately 50�N in a warm-temperate tosubtropical climate (Surlyk, 1997; Surlyk and Sørensen, 2010).Global sea level was in the order of 100 m higher than today(Kominz et al., 2008), and the basin was transgressed repeatedlyfrom the south, resulting in the formation of an archipelagowith low islands and peninsulas (Surlyk and Christensen, 1974;

Fig. 1. Simplified geological map of north-east Skåne (southern Sweden), showing the Kristianstad Basin and the locality of Ivö Klack (modified after Norling and Bergström, 1987).

A.M. Sørensen et al. / Cretaceous Research 33 (2012) 21e4122

Christensen, 1975). As the Late Cretaceous transgression pro-gressed, the deeply weathered basement at Ivö Klack was floodedand washed clean by waves, leaving a steep, irregular, boulder-strewn and hummocky rocky coast. The shore occupied the inter-tidal and subtidal zones with a maximumwater depth probably notmore than c. 5 m (Surlyk and Christensen, 1974). The succession atIvö Klack consists of Precambrian gneiss overlain by its weatheringproduct, kaolin. On top of the kaolin follow a few metres of Cam-panian quartz sand washed out from the kaolin and, in turn,overlain by coarse-grainedmarine carbonates composed of skeletalfragments of bivalves (with a predominance of oysters), bryozoans,brachiopods and echinoids (Surlyk and Sørensen, 2010). Belemnitesare abundant and show that the onlapping carbonates belong to theupper lower Campanian Belemnellocamax mammillatus zone, c.81e80 Ma (Christensen, 1975).

3. Material and methods

A large portion of the material studied was collected in1928e1929 by Alfred Rosenkrantz and in the early 1970s by FinnSurlyk and Walter Kegel Christensen; this is now housed at theStatens Naturhistorisk Museum, Copenhagen, with the prefix GMI(Geological Museum Ivö). The abundant, large-sized shells are wellpreserved, whereas juvenile specimens are almost absent, probablydue to shell breakage and fragmentation in this turbulent environ-ment. Bivalve shells are normally represented by isolated, dissoci-ated valves and many of the thick-shelled species reveal numeroussponge borings. A total of 19 species had aragonitic shells; these arerepresented by internal moulds which make proper taxonomicassignment rather difficult. One species, a trigoniid, is representedby external moulds as well. The abundance of moulds of formerlyaragonitic species is indicative of the slow dissolution of aragonite in

the turbulent, near-shore environment (compare Cherns andWright, 2000). Epifaunal encrustation of oyster muscle scars,subsequent to aragonite dissolution, indicates that dissolutionoccurred on the seafloor or under very shallow burial and subse-quent reworking of the sediment during storms, exposing the oystershells on the seafloor. The bivalve guilds and habitats are interpretedon the basis of studies of functional morphology of the shells anda comparison with modern relatives.

4. Results

The bivalve fauna from Ivö Klack comprises just over sixtyspecies, in around 40 genera, and inclusive of four indeterminateforms which are briefly described below and listed in Table 1.Commonest are oysters; these make up c. 90 per cent of all bivalveshells recorded in the present study. The assemblages are sub-divided into seven guilds, on the basis of tiering and postulatedmode of life (Fig. 2; Table 1). It has not been possible to assigna single indeterminate form, GMI 2602 (Fig. 5KeL) to any guild. Theepifaunal, free-lying guild comprises three species, the epifaunalcemented guild 14 (primarily oysters and spondylids), the epi-byssate guild 23, the semi-infaunal byssally attached guild one, theshallow infaunal guild 11, and the deep infaunal guild eight. Finally,the boring guild is documented by borings assignable to the ich-nogenus Gastrochaenolites Leymerie, 1842, found in oyster shells(Fig. 3). No shells of boring bivalves have been found, but at leastone species must have belonged to this guild.

Five habitats that represent different hydrodynamic conditionsand levels of light penetration are distinguished (Table 2). Theinterpretation of habitat preferences of the bivalve species listed isbased primarily on attachment strategy. A total of 20 speciesoccurred in the within-sediment habitat 1; they are all infaunal and

Table 1Abundance, shell composition and guild structure of the bivalve fauna from IvöKlack. GMI numbers indicate unidentified species. GMI 2602 is not included in thistable, since the guild could not be inferred. *Assignment to guild is tentative;a ¼ abundant (>100 specimens), c ¼ common (5 < specimens<100), r ¼ rare(specimens<5), A ¼ aragonite, C ¼ calcite.

Species Abundance Composition Guild

Neithea quinquecostata c CeA Epifaunal free-lyingguild*Vultogryphaea lacinata c CeA

*GMI 3501 r CeA

Amphidonte haliotoideum a CeA Epifaunal cementedguildAcutostrea incurva c CeA

Ceratostreon sp. r CeA*Exogyra? sp. r CeAGryphaeostrea canaliculata c CeAHyotissa semiplana r CeAPycnodonte vesicularis c CeARastellum diluvianum a CeABiradiolites suecicus c CeASpondylus labiatus a CeASpondylus latus r CeASpondylus truncatus c CeA*Spondylus lamellatus c CeASpondylus sp. A c CeA

Anomia? sp. r CeA Epibyssate guildMytilus sp. r CeABarbatia sp. c ABarbatia? sp. r AModiolus sp. A r CeALima ovata c CeACtenoides sp. r CeALimatula semisulcata c CeALimea sp. r CeAPseudolimea sp. r CeA*Camptonectes virgatus r CeADhondtichlamys pulchella c CeADhondtichlamys subarata c CeAMimachlamys undulata r CeALyriochlamys faujasi r CeAChlamys septemplicata c CeALyriochlamys dentata c CeALyriochlamys ternata r CeALycettia sp. r CeA*Plagiostoma hoperi r CeAIsognomon sp. r CeAPectinidae indet. r CeAGMI 3701 r ?

Modiolus sp. B c CeA Semi-infaunalbyssus-attached guild

Cardiinae sp. c A Shallow infaunal guildArctica sp. c AGranocardium? sp. r ACucullaea exaltata c ACucullaea sp. A c ACucullaea sp. B c ATrigonia sp. c AGlycymeris lens c AGlycymeris sp. A c A*Icanotia grosseplicata c ALiopistha sp. r A

Goniomya sp. c A Deep infaunal guildArcomya sp. c AArca? sp. r APanopaea regularis c ATellinidae? indet. c ?Laternulidae sp. r ATellina? sp. r AGMI 1715 r ?

no fossils ? Boring guild

A.M. Sørensen et al. / Cretaceous Research 33 (2012) 21e41 23

semi-infaunal. Seventeen species of byssate nestling and fissure-dwelling bivalves inhabited the protected photonegative habitat2, between and beneath hummocks and boulders and probably in

open spaces within bundles of marine plants. Thirty-five speciesrepresent the relatively protected habitat 3, between and beneathboulders, and these include byssally attached, cemented as well asfree-lying species. The semi-exposed habitat 4 hosted twenty-twospecies, inclusive of byssally attached, cemented as well as free-lying species. The most exposed habitat, habitat 5, was occupiedby twelve cemented and byssally attached species (Table 2).

4.1. The bivalve fauna

Size indications below include shell height (between umbo andventral margin) and length (measured from anterior to posterior).Identification of material has relied on various sources, inclusive ofNilsson (1827), Lundgren (1885, 1894), Holzapfel (1887e1889),Hennig (1897), Cox et al. (1969), Stanley (1970), Stenzel (1971),Dhondt (1972, 1973), Marquet (1982), Abdel-Gawad (1986), Mal-chus (1990), Waller and Marincovich (1992), Dhondt and Dieni(1993), Malchus et al. (1994, 1996) and Elder (1996).

Arcidae1. Barbatia sp. (Fig. 9B)

Internal moulds only; of medium size (length up to 55 mm);outline elongate, oval; long, near-straight taxodont hinge line;pallial line visible along entire ventral margin; outer surface withradial ribs.

2. Barbatia? sp. (Fig. 9H)

Internal mould only; one specimen known, of medium size(length 42 mm); outline triangular, suboval; only part of hinge linepreserved; pallial line visible along entire ventral margin. Incomparison to Barbatia sp. (see above), umbo less acuminate, widerand straighter; outer surface with radial ribs.

3. Arca? sp. (Fig. 9I)

Internal mould only; one specimen known, of large size (length81 mm); outline elongate; valve inflated; beak prominent andinflated; umbo positioned anteriorly; ventral margin concavecentrally; no dentition or ornament seen.

Cucullaeidae4. Cucullaea exaltata (Nilsson, 1827) (Fig. 6R and S)

Internal moulds only, of medium size (up to 53 mm in height);outline subtrigonal to subtrapezoidal; posterior margin obliquelytruncated; rounded posterior umbonal carina in both valves; pallialline prominent, visible along entire ventral margin; no gapes; noornament or dentition preserved.

5. Cucullaea sp. A (Fig. 6M and N)

Internal moulds only, of medium size (up to 40 mm in height);outline subtrigonal to subtrapezoidal; posterior margin obliquelytruncated; rounded posterior umbonal carina in both valves; pallialline prominent, traceable along entire ventral margin; no gapes.Compared to C. exaltata (see above), this has a deeper scar posteriorof the carina, a rounded postero-ventral margin and a moreacuminate beak; no ornament or dentition preserved.

6. Cucullaea sp. B (Fig. 6G and H)

Internal moulds only, of medium size (up to 49 mm in height);outline subtrigonal to suboval; posterior margin obliquely

Fig. 2. The seven bivalve guilds from Ivö Klack. Representing the free-lying epifaunal guild is Vultogryphaea lacinata (A); typical of the cemented epifaunal guild are Rastellumdiluvianum (B) and spondylids (C); representing the epibyssate guild are Modiolus sp. A (D) and Mimachlamys undulata (E); a typical member of the semi-infaunal guild is Modiolussp. B (F); representing the shallow infaunal guild is Cucullaea sp. A (G) and Arcomya sp. (H) is typical of the deep infaunal guild. The boring guild is represented by a boring in anoyster valve (see also Fig. 3).

truncated; rounded posterior umbonal carina in both valves. Nar-rower than C. exaltata and lacking a prominent pallial line. Noornament or dentition preserved.

Glycymerididae7. Glycymeris lens (Nilsson, 1827) (Fig. 6F)

Internal moulds only, of small size (up to 25 mm in height);outline subcircular; umbo orthogyrate and relatively straight; bothvalves evenly inflated; pallial line visible along entire ventralmargin, internally fluted ventrally; taxodont hinge well preserved;no ornament seen.

8. Glycymeris sp. A (Fig. 6P)

Internal mould only; one specimen known, of small size (32mmin height); outline subcircular; umbo orthogyrate and relativelystraight; ventral margin internally fluted; outer surface with radialribs. No dentition preserved.

Mytilidae9. Mytilus sp. (Fig. 8N)

Internal mould only; one specimen known, of medium size(length 60 mm); outline mytiliform; beak acuminate; anterioradductor scars small, yet distinct; valve inflated, ventral shellmargin straight and flattened; outer surface with concentricgrowth lines; no dentition preserved.

Fig. 3. Bivalve boring, assignable to the ichnogenus Gastrochaenolites, in a right valveof the oyster Rastellum diluvianum.

10. Modiolus sp. A (Fig. 8O)

Internal moulds only; of medium size (length up to 62 mm);outline modioliform; umbo obtuse and ventral margin curved;ligament area fairly long; both valves equally inflated; no dentitionor ornament seen.

11. Modiolus sp. B (Fig. 8H)

Internalmouldsonly;ofmediumsize (lengthupto52mm);outlinemodioliform, wider than Modiolus sp. A (see above); umbo obtuse,ventral margin more clearly curved than in previous form; ligamentarea fairly long; valve inflated; no dentition or ornament seen.

12. Lycettia sp. (Fig. 8I)

Internal mould only, one specimen known; of small size (length34 mm); outline mytiliform; beak more acuminate than in Mytilussp. (see above); sharp carina extending frombeak to postero-ventralmargin; valve inflated; ventral margin straight and flattened; outersurface with concentric growth lines; no dentition seen.

Isognomonidae13. Isognomon sp. (Fig. 9D and J)

Internal moulds only, of large size (at least 90 mm in width);beak area acuminate; only umbonal area preserved, but withcharacteristic numerous and regularly arranged ligament grooves,each about 3 mm wide and 9 mm long; no ornament preserved.

Pectinidae14. Camptonectes virgatus (Nilsson, 1827) (Fig. 8P)

Small sized (up to 11mm in height); outline drop shaped; valvesrelatively convex, with unequal auricles; umbonal angle 70�; rightanterior auricle elongate, wing like, with byssal sinus; left anteriorauricle near rectangular; posterior auricles smaller and moreobtusely angled; outer surface with radial striae, diverging fromumbo towards pallial and lateral margins; striae crossed byconcentric growth lines.

15. Lyriochlamys faujasi (Defrance, 1825) (Fig. 8J)

Large sized (up to 85mm in height); outline drop shaped; valvesprosocline and flattened; umbonal angle 75e83�; anterior auriclerather wide, elongate and wing like, with deep byssal sinus andcovered with concentric elevated lines which bend along byssalsinus; posterior auricle smaller, triangular and acutely angled;outer surface with 25e35 radial, divided ribs.

Fig. 4. Gryphaeid and ostreid oysters from Ivö Klack; all specimens natural size. AeB. Vultogryphaea lacinata (GMI 4101), left valve in outer and anterior views; CeD. Vultogryphaealacinata (GMI 4102), right valve in internal and external views; EeF. Gryphaeostrea canaliculata (GMI 4503), left valve in external and posterior views (attachment area facingupwards); G. Ceratostreon sp. (GMI 4201), left valve (attachment area facing upwards); H. Hyotissa semiplana (GMI 4107), right valve in internal view; IeJ. Pycnodonte vesicularis (GMI4505), right valve in internal and external views; KeL. Pycnodonte vesicularis (GMI 4504), right valve in internal and external views; MeN. Ceratostreon sp. (GMI 4203), left valve inexternal and internal views; O. Amphidonte haliotoideum (GMI 4105), left valve in internal view; PeQ. Amphidonte haliotoideum (GMI 4104), right valve in internal and externalviews; R. Pycnodonte vesicularis forma hippopodium (GMI 4502), right valve in internal view; SeT. Pycnodonte vesicularis forma hippopodium (GMI 4501), left valve in internal andventral views; UeV. Gryphaeostrea canaliculata (GMI 4401), right valve in external and internal views.

Fig. 5. Bivalves from Ivö Klack; all specimens natural size, except AeC, which are x 0.8. AeC. Rastellum diluvianum (GMI 6501), right valve in internal view (A), right valve in externalview (B) and ventral view (C), left valve at base; D. Acutostrea incurva (GMI 6401), left valve in external view; EeF. Gryphaeostrea canaliculata (GMI 4202), left valve in anterior andinternal views; G. Rastellum diluvianum (GMI 4901), left valve in external view; H. Tellina? sp. (GMI 1704), internal mould of right valve; IeJ. Exogyra? sp. (GMI 4506), right valve ininternal and external views; KeL. indeterminate (GMI 2602), internal mould of right valve in posterior and external views; MeN. Acutostrea incurva (GMI 6402), left valve in internaland external views.

Table 2The bivalve fauna and its link with five habitats at Ivö Klack. The habitats represent different hydrodynamic conditions and light penetration.

Increasing energy and illumination

Life habit Species Habitat 1Within-sedimentwith limitedturbulence

Habitat 2Protected photonegativeenvironments between andbeneath boulders and inspaces within bundles ofmarine plants

Habitat 3Relatively protectedenvironmentsbetween andbeneath boulders

Habitat 4Semi-exposed partsof the boulders andon the seafloor

Habitat 5Most exposed partsof the boulders withhigh waterturbulence

Byssate currentloving

Mytilus sp. X XModiolus sp. ALycettia sp.GMI 3701

Cemented by theentire left valve

Amphidonte haliotoideum X X XCeratostreon sp.Exogyra? sp.Pycnodonte vesicularisHyotissa semiplanaSpondylus sp. ASpondylus latusSpondylus truncatus

Cemented by a smallarea near the umbo

Acutostrea incurva X XGryphaeostrea canaliculataRastellum diluvianumBiradiolites suecicusSpondylus labiatusSpondylus lamellatus

Epifaunal, free-lying Neithea quinquecostata X XVultogryphaea lacinataGMI 3501GMI 3803

Byssate nestling andfissure dwellers

Barbatia sp. X XBarbatia? sp.Lima ovataCtenoides sp.Limea sp.Pseudolimea sp.Limatula semisulcataCamptonectes virgatusDhondtichlamys pulchellaDhondtichlamys subarataMimachlamys undulataLyriochlamys faujasiChlamys septemplicataLyriochlamys dentataPlagiostoma hoperiIsognomon sp.Pectinidae indet.

Semi-infaunal Modiolus sp. B XShallow infaunal Cardiinae sp. X

Arctica sp.Granocardium? sp.Cucullaea exaltataCucullaea sp. ACucullaea sp. BTrigonia sp.Glycymeris lensGlycymeris sp. AIcanotia grosseplicataLiopistha sp.

Deep infaunal Goniomya sp. XArcomya sp.Arcomya? sp.Panopaea regularisLaternulidae sp.Tellinidae? indet.Tellina? sp.GMI 1715

16. Lyriochlamys dentata (Nilsson, 1827) (Fig. 8L)

Large sized (up to 125 mm in height); outline drop shaped;umbonal angle 72e76�; right anterior auricle elongate andwing like,with deep byssal sinus covered with radial ribs; left anteriorauricle large, with slight byssal sinus; posterior auricles muchsmaller, obtusely angled and with few radial ribs; outer surfacewith numerous, commonly tripartite ribs with scabrousspinelets.

17. Lyriochlamys ternata (Münster in Goldfuss, 1833) (Fig. 8B)

Medium sized (32 mm in height), flattened; only a single valveknown; outline drop shaped; outer surfacewith rounded radial ribsand riblets, broad intercostal intervals with radial lines and well-developed, undulating concentric growth lines.

18. Dhondtichlamys pulchella (Nilsson, 1827) (Fig. 8E and F)

Small sized (up to 14 mm in height); outline rounded; umbonalangle 101�105�; right valve more convex than left; anterior auricleelongate and wing like, with deep byssal sinus in right valve;posterior auricle smaller and obtusely angled; outer surface witha large number of radially divided ribs.

19. Dhondtichlamys subarata (Nilsson, 1827) (Fig. 8Q and R)

Small sized (up to 13 mm in height); outline rounded; umbonalangle 90e97�; shell relatively convex, left valve more so; anteriorauricle elongate and wing like, with deep byssal sinus; posteriorauricle smaller and triangular; outer surface with a large number ofdivided ribs.

20. Mimachlamys undulata (Nilsson, 1827) (Fig. 8G)

Medium sized (38 mm in height); outline rounded; umbonalangle at first 90�, but widening ventrally to 95�; right valve fairlyconvex, auricles relatively small; both auricles triangular butanterior onewith deep byssal sinus on right valve and covered withradial ribs and pronounced concentric growth lines; outer surfacewith thin, close-set radial ribs crossed by concentric, irregularlydistributed growth lines. Only a single right valve available.

21. Chlamys septemplicata (Nilsson, 1827) (Fig. 8K)

Medium sized (up to 50 mm in height); outline drop shaped;umbonal angle 72e80�; valves prosocline and flattened; rightanterior auricle broad, with acute angle and byssal sinus; posteriorauricle smaller with an obtuse angle; outer surface with roundedradial ribs, broader intercostal intervals with finer radial ribs andfew, well-developed concentric growth lines.

22. Neithea quinquecostata (J. Sowerby, 1814) (Fig. 8C and D)

Medium sized (up to 50 mm in height); outline triangular; leftvalve cup shaped, right valve flattened; auricles small, triangularand of equal size; hinge with two divergent cardinal teeth, one oneach side of ligament pit; outer surface with six primary radial ribsbetween with are four intercostal ribs.

23. Pectinidae indet. (Fig. 8V)

Small sized (16 mm in height); outline rounded; umbonal angle90�; left valve convex, with anterior auricle relatively small andtriangular with deep byssal sinus; outer surface with pronounced

concentric ornament, with distinct sinuous imbrication pattern,continuing onto auricle. Only a single left valve available.

Anomiidae24. Anomia? sp. (Fig. 7O)

Medium sized (45 mm in height); outline rounded; single valveknownweakly inflated; outer surface with concentric growth lines.

Spondylidae25. Spondylus sp. A (Fig. 7B, K and Q)

Large sized (up to 100 mm in height); outline subrounded; rightvalve bulbous and outer surface with strong radial ribs, roughspines emerging from ribs; left valve deep and with large attach-ment area; free portion of left valve with long, prominent spinesfollowing ill-developed ribs and growth lines.

26. Spondylus labiatus (Wahlenberg, 1821) (Fig. 7A)

Mediumsized (up to50mminheight); outlineelongate tooval; leftvalve inflated, cylindrical and cemented to substrate along smallportion of shell (boulders and hummocks); right valve lid like; outersurfacewithstrongradial ribsandfewprominentgrowth lines.Speciescommonly found in clusters, with radial, petal-like arrangementpattern; easily recognised by strong radial ribs and elongate outline.

27. Spondylus latus (J. Sowerby, 1814) (Fig. 7C)

Medium sized (up to 50 mm in height); outline highly variable,but mostly elongate to oval; right valve inflated, cylindrical; outersurface of both valves with strong radial ribs and few, prominentgrowth lines, may merge lamellae fused to either substrate or otherepifauna; left valve probably cemented to substrate by largeattachment area and by lamellae.

28. Spondylus truncatus (Lamarck, 1819) (Fig. 7F, G and L)

Medium sized (up to 40 mm in height); outline variably sub-rounded; right valve gibbous with straight hinge line and smallauricles; outer surface with strong radial ribs, every fourth to sixthrib being more prominent with rough spines; left valve cementedto substrate by entire surface; shape of substrate recognisable onright valve (xenomorphic expression).

29. Spondylus lamellatus (Nilsson, 1827) (Fig. 7E)

Medium sized (up to 48 mm in height); outline highly variable,but mostly elongate to oval; valves inflated, cylindrical; outersurface with strong radial ribs and prominent growth lines, allmerging into lamellae.

Limidae30. Plagiostoma hoperi (Mantell, 1822) (Fig. 7P)

Medium sized (up to 35 mm in height); outline suborbicular tosuboval; opisthocline and length commonly slightly exceedingheight; beak anterior, with moderately long cardinal area, broadligament pit and obtuse auricles; outer surface radially striated,with weak concentric growth lines.

31. Lima ovata (Nilsson, 1827) (Fig. 7H)

Small sized (up to 33 mm in height); outline suborbicular tooval, height exceeding width; hinge margin relatively short;

Fig. 6. Bivalves from Ivö Klack; all specimens natural size. AeB. Cardiinae sp. (GMI 1601), internal mould of left valve in posterior and external views; CeD. Cardiinae sp. (GMI 1602),internal mould of right valve in external and posterior views; E. ‘Trigonia’ sp. (GMI 1402), rubber peel of external surface of right valve, showing ornament; F. Glycymeris lens (GMI1708), internal mould; G�H. Cucullaea sp. B (GMI 1504), internal mould of right valve in external and anterior views; IeJ. ‘Trigonia’ sp. (GMI 1401), internal mould of right valve inexternal and dorsal views; KeL. Granocardium? sp. (GMI 1705), internal mould of right valve in external and posterior views; MeN. Cucullaea sp. A (GMI 1502), internal mould ofright valve in external and anterior views; O. Liopistha sp. (GMI 1703), internal mould of right valve; P. Glycymeris sp. A (GMI 1716), internal mould; Q. Arctica sp. (GMI 1713), internalmould of left valve; ReS. Cucullaea exaltata (GMI 1501), internal mould of right valve in external and anterior views; TeU. Arctica sp. (GMI 1503), internal mould of left valve inexternal and posterior views.

Fig. 7. Bivalves from Ivö Klack; all specimens natural size. A. Spondylus labiatus (GMI 304), right valve in external view; B. Spondylus sp. A (GMI 401), right valve in external view; C.Spondylus latus (GMI 306), two left valves cemented together; D. Limatula semisulcata (GMI 302), right valve in external view; E. Spondylus lamellatus (GMI 308), right valve inexternal view; F. Spondylus truncatus (GMI 307), right valve in external view; G. Interior of a left valve of an indeterminate spondylid, possibly S. truncatus (GMI 101); H. Lima ovata(GMI 301), left valve in external view; I. Ctenoides sp. (GMI 1701), internal mould of left valve; J. indeterminate (GMI 1715), internal mould of right valve; K. Spondylus sp. A (GMI309), right valve in external view; L. Spondylus truncatus (GMI 305), right valve in external view; M. Pseudolimea sp. (GMI 310), right valve in external view; N. Limea sp. (GMI 303),right valve in external view; O. Anomia? sp. (GMI 3801), external view; P. Plagiostoma hoperi (GMI 311), right valve in external view; Q. Spondylus sp. A (GMI 21), right valve inexternal view.

Fig. 8. Bivalves from Ivö Klack; all specimens natural size, except P. A. indeterminate pectinid (GMI 3501), right valve in external view; B. Lyriochlamys ternata (GMI 3803), rightvalve in external view; C. Neithea quinquecostata (GMI 3503), left valve in external view; D. Neithea quinquecostata (GMI 3802), right valve in internal view; E. Dhondtichlamyspulchella (GMI 3605), external view; F. Dhondtichlamys pulchella (GMI 3602), right valve in external view; G. Mimachlamys undulata (GMI 3502), left valve in external view; H.Modiolus sp. B (GMI 1707), internal mould of right valve; I. Lycettia sp. (GMI 402), internal mould of left valve; J. Lyriochlamys faujasi (GMI 3601), external view; K. Chlamys sep-templicata (GMI 3401), right valve in external view; L. Lyriochlamys dentata (GMI 3402), in external view; M. indeterminate (GMI 3701), internal mould of left valve; N. Mytilus sp.(GMI 403), internal mould of left valve; O.Modiolus sp. A (GMI 1607), internal mould of left valve; P. Camptonectes virgatus (GMI 3504), left valve in external view; Q. Dhondtichlamyssubarata (GMI 3603), left valve in external view; R. Dhondtichlamys subarata (GMI 3604), right valve in external view; S�T. Biradiolites suecicus (GMI 4001), attached valves in topand lateral views; U. Icanotia grosseplicata (GMI 1605), internal mould of right valve; V. Pectinidae indet. (GMI 3702), left valve in external view.

Fig. 9. Bivalves from Ivö Klack; all specimens natural size. A. Panopaea regularis (GMI 1706), internal mould of right valve; B. Barbatia sp. (GMI 1710), internal mould of right valve; C.Goniomya sp. (GMI 1604), internal mould of right valve; D. Isognomon sp. (GMI 1709), internal mould of left valve; E. Tellinidae? indet. (GMI 1712), internal mould of left valve; F�G.Laternulidae sp. (GMI 1609), internal mould of articulated shell (left valve) in external and dorsal views; H. Barbatia? sp. (GMI 1714), internal mould of right valve; I. Arca? sp. (GMI1610), internal mould of right valve; J. Isognomon sp. (GMI 1606), internal mould of left valve; K. Arcomya sp. (GMI 1608), internal mould of left valve.

auricles well differentiated, with anterior one being slightlysmaller; no gapes; outer surface with radial ribs and concentricgrowth lines.

32. Ctenoides sp. (Fig. 7I)

Medium sized (49 mm in height); outline oval, height exceedingwidth; beak acuminate and anterior margin straight; outer surfacewith concentric growth lines. Only a single internal mould avail-able; no dentition seen.

33. Limatula semisulcata (Nilsson, 1827) (Fig. 7D)

Small sized (up to26mminheight); outline oval, heightexceedingwidth; strongly inflated, hinge edentulous, auricles small and sub-equal; no gapes; outer surfacewith fine radial ribs, most conspicuoustowards valve centre and absent from posterior and anterior ends.

34. Limea sp. (Fig. 7N)

Small sized (up to 30 mm in height); outline suborbicular orovate, height exceeding width; hinge margin relatively short;hinge with series of short denticles on either side; auricles welldifferentiated and appearing larger than in L. ovata (see above);no gapes; outer surface with radial ribs; crenulated shell margin.

35. Pseudolimea sp. (Fig. 7M)

Small sized (13 mm in height); outline obliquely ovate toorbicular; gibbous; beak near centre of moderately long cardinalarea, ligament pit broad; outer surface with numerous concentricgrowth lines and radial ribs. Only a single valve available.

Gryphaeidae36. Vultogryphaea laciniata (Nilsson, 1827) (Fig. 4AeD)

Medium sized (up to 45 mm in height); outline rounded to oval;left valve cup shaped, outer surface with concentric growth linesand few strong radial ribs with tendency to form hollow spines;attachment area small; right valve as thin lid, slightly concave, withconcentric growth lines; shell typically exogyrine and coiled,resulting in narrow ligamental and subligamental areas; commis-sural shelf well-developed, with chomata dorsally.

Species here interpreted to have been cemented during juvenilestage and later becoming free-lying, as based on very smallattachment area, morphological differences between valves andrelatively thin-shelled and concave left valve.

37. Gryphaeostrea canaliculata (J. Sowerby, 1813) (Figs. 4EeF, UeVand Fig. 5EeF)

Medium sized (up to 42 mm in height); outline oval, heightexceeding width; left valve cup shaped, with small opisthogyratebeak; right valve forming thin lid, slightly convex or concave andmuch smaller than left valve, leavingwide freemargin on left valve;ligament area triangular; first opisthogyrate and later straight;resilifer broader than bourrelets; commissural shelf well-developed, in some cases with subligamental gutter without cho-mata; outer surface of both valves with concentric growth lines,becoming squamate at flanks; attachment area large.

38. Pycnodonte vesicularis (Lamarck, 1806) (Fig. 4IeL, ReT)

Medium sized (up to 75 mm in height); outline round toobliquely oval; right valve forming weakly convex or concave lid;

ligament area normally short, straight and triangular, with bour-relets and resilifer of near-equal size; hinge line usually straight andshort; commissural shelf well-developed with chomata restrictedto small, commonly fairly deep, gutters on both sides of hinge;outer surface with growth lines near commissure; shell withvesicular structure.

39. Amphidonte haliotoideum (J. Sowerby, 1813) (Fig. 4OeQ)

Medium sized (up to 70 mm in height); outline oval to commashaped; commissural shelf well-developed in both valves, withchomata along entire margin; left valve deep, with a spirally curvedumbonal region; right valve flattened convex or concave witha curved umbonal region; outer surface with smooth growth lines;attachment area relatively large, in some cases, whole left valvecemented to substrate.

40. Ceratostreon sp. (Fig. 4G, M and N)

Medium sized (up to 41 mm in height); outline elongate tooval; left valve cup shaped with umbonal region forming largerend; commissural shelf well-developed with chomata dorsally;outer surface with smooth growth lines, plus keel rising intoprominent spines; attachment area relatively large; only leftvalves available.

41. Exogyra? sp. (Fig. 5I and J)

Medium sized (38 mm in height); outline elongate, oval; valvecup shaped with umbonal region forming larger end; twisted beak;ligament area triangular, resilifer broader than bourrelets;commissural shelf poorly developed; no chomata visible; outersurface with smooth growth lines plus a keel rising from beak tohalfway down shell. Only a single right valve available.

42. Hyotissa semiplana (J. de C. Sowerby, 1825) (Fig. 4H)

Large sized (up to 90 mm in height); outline variably lobate;right valve flat to slightly convex, with well-developed commis-sural shelf, chomata present along entire shelf; adductor musclescar reniform, with both ends well rounded. Only right valvesavailable.

Ostreidae43. Acutostrea incurva (Nilsson, 1827) (Fig. 5D, M and N)

Medium sized (up to 80 mm in height); outline variable, butmostly elongate, straight or, more commonly, curved; ligamentarea in left valve acuminate, high triangular, resilifer broader thanbourrelets; commissural shelf with rounded subligamental gutterwith chomata, and both gutters and chomata well-developedfrom hinge to adductor muscle area, but effacing ventrally; outersurface of both valves with concentric undulating growth lines,becoming squamate in last stages of growth; attachment areanormally small.

44. Rastellum diluvianum (Linnaeus, 1767) (Fig. 5AeC and G)

Large sized (up to 160 mm in height); outline elongate, occa-sionally crescentic to triangular-crescentic; shells with crescenticoutline have convex anterior and concave posterior flanks, tendingto be vertical to commissural plane; chomata present along entirecommissure; adductor muscle scars large and subtriangular tocomma shaped; ligament area broad and straight to opisthogyrate;outer surface of both valves with numerous branching costae,

turning into strongly plicate shell margin, forming an interlocking,acutely angled, zigzag commissure; posterior shell margin notalways plicate; attachment area relatively large, commonly occu-pying entire posterior flank.

Trigoniidae45. ‘Trigonia’ sp. (Fig. 6E, I and J)

Internal moulds and external casts only; of small size (up to27 mm in height); outline triangular to suboval; marginal carinaprominent; two symmetrical, striated teeth visible in mould;outer surface with concentric ribs on anterior part, crossed byradial ribs in median portion of shell; posterior part with radialribs.

Cardiidae46. Cardiinae indet. (Fig. 6AeD)

Internal moulds only; of small size (up to 27 mm in height);outline subtrigonal to suboval; both valves evenly inflated; umboprosogyrate, turned forwards; posterior margin obliquely trun-cated; pallial line visible along entire margin; no ornament ordentition seen.

47. Granocardium? sp. (Fig. 6K and L)

Internal mould only; one specimen available, of medium size(40 mm in height); outline subtrigonal to suboval; both valvesevenly inflated, posterior margin obliquely truncated; umbo pro-sogyrate, beak relatively straight; outer surface with radial ribs andconcentric growth lines; no dentition seen.

Tellinidae48. Tellina? sp. (Fig. 5H)

Internal moulds only; small sized (up to 14 mm in height);outline elongate, suboval; umbo positioned almost centrally, beaksorthogyrous; siphonal gape posteriorly; outer surface withconcentric growth lines; no dentition seen.

49. Tellinidae? indet. (Fig. 9E)

Internal moulds only; of medium size (up to 30 mm in height);outline elongate, suboval; umbo positioned almost centrally,siphonal gape posteriorly; outer surface with concentric growthlines; no dentition seen.

Arcticidae50. Arctica sp. (Fig. 6Q, T and U)

Internal moulds only; of medium size (up to 48 mm in height);outline subtrigonal to suboval; umbo prosogyrate, both valvesevenly inflated; posterior margin obliquely truncated; pallial linevisible along entire margin; hinge of heterodont type; no ornamentpreserved.

Icanotiidae51. Icanotia grosseplicata (Lundgren, 1894) (Fig. 8U)

Internal moulds only; of medium size (length up to 70 mm);outline elongate; both valves equally weakly inflated, umbo posi-tioned anteriorly; siphonal gape posteriorly; anterior end muchnarrower than posterior; outer surface with concentric growthlines and five prominent radial ribs posteriorly; no dentition seen.

Hiatellidae52. Panopaea regularis (d’Orbigny, 1849) (Fig. 9A)

Internalmouldsonly;of large size (up to68mminheight); outlineelongate, suboval; umbo positioned almost centrally; siphonal gapeposteriorly; pallial line visible along entire margin, with wide pallialsinus posteriorly; no dentition or ornament preserved.

Radiolitidae53. Biradiolites suecicus (Lundgren, 1870) (Fig. 8S and T)

Medium sized (up to 40 mm in height); attached valve conical,free valve forming lid; attached valve straight or arched, with noteeth and posterior ligament; outer surface with longitudinalornament and growth lines.

Pholadomyidae54. Goniomya sp. (Fig. 9C)

Internal moulds only; of medium size (length up to 56 mm);outline elongate to suboval; both valves equally weakly inflated,umbo positioned anteriorly; siphonal gape posteriorly; beaksalmost orthogyrous; outer surface with concentric growth lines; nodentition seen.

55. Arcomya sp. (Fig. 9K)

Internal moulds only; of medium size (length up to 75 mm);outline elongate; both valves equally weakly inflated, umbo posi-tioned anteriorly; siphonal gape posteriorly; pallial line visiblealong entire margin; no dentition or ornament preserved.

Laternulidae56. Laternulidae sp. (Fig. 9F and G)

Internal moulds only; of large size (up to 44 mm in height);outline elongate, oval; umbo positioned almost centrally; siphonalgape posteriorly; deep holes, positioned dorso-posteriorly probablyreflect internal umbonal plate; pallial line visible along entiremargin, wide pallial sinus posteriorly; no dentition or ornamentseen.

Poromyidae57. Liopistha sp. (Fig. 6O)

Internal moulds only; of small size (up to 28 mm in height);outline triangular to suboval; anterior end shorter than posterior;outer surface with coarse radial ribs, except on dorsal part ofposterior slope; no dentition seen.

Indeterminate58. GMI 1715 (Fig. 7J)

Internal mould only; only a single specimen known, of mediumsize (length 42 mm); outline elongate; valve inflated, umbo posi-tioned anteriorly; siphonal gape posteriorly; pallial line visiblealong entire valve margin; no dentition or ornament seen.

59. GMI 2602 (Fig. 5K and L)

Internalmould only; only a single specimen known, of small size(14 mm in height); outline subcircular; valve weakly inflated, umbopositioned anteriorly; pallial line visible along entire valve margin;no dentition or ornament seen.

60. GMI 3501 (Fig. 8A)

Internal mould only; only a single valve known, of medium size(46 mm in height); outline drop shaped; valve cup shaped, withportions of auricles preserved; outer surface with rounded radialribs and riblets, broad intercostal intervals with radial lines andwell-developed, undulating concentric growth lines. Clearly a pec-tinid, but generically and specifically indeterminate.

61. GMI 3701 (Fig. 8M)

Internal mould only; only a single specimen known, of smallsize (length 29 mm); outline mytiliform; beak more strongly

Table 3The bivalve fauna from four Late Cretaceous rocky shore localities, and their modes of life aIvö Klack, C¼ cemented, By ¼ byssally attached, Bo ¼ boring, I¼ infaunal, F ¼ free-lying (1991; �Zítt, 1992; Johnson and Hayes, 1993 and �Zítt et al., 1997).

Species Germany Cenomanian Mexico Camp

Opis bicornis IGastrochaena ostreae BoLima reichenbachi ByPecten laminosus ByPecten acuminatus ByPecten elongatus ByNeithea quinquecostata # FNeithea digitalis ?Spondylus striatus CSpondylus hystrix CLopha carinata CRastellum diluvianum # CPycnodonte vesicularis # COstrea? cf. hippopodium #Exogyra lateralis CExogyra sigmoidalis CAmphidonte haliotoideum # CExogyra reticulataHyotissa semiplanaOstrea operculataGryphaeostrea cf. canaliculataSpondylus omaliiOstrea sp. # C?Amphidonte sp. CLyriochlamys cf. traskii BySpondylus sp. # C?Atreta n. sp. CCoralliochama orcutti COyster sp. AAtreta? sp. 1Atreta? sp. 2Spondylus sp. #Arca sp.Cucullaea sp.Modiolus sp. #Pteriidae sp.Isognomon sp. #Inoceramus sp.Oxytomidae sp.Chlamys sp. #Camptonectes sp. #Regalilima cf. marlburiensisOstreidae sp.Trigonia aff. marwickiPterotrigonia waitangiensisPsammobiidae sp.Bivalvia sp.Total number of species 16 6Total number of species in guilds I ¼ 1

Bo ¼ 1By ¼ 4F ¼ 1C ¼ 8? ¼ 1

By ¼ 1C ¼ 5

acuminate than inMytilus (see above); valve inflated; ventral shellmargin straight and flattened; no dentition or ornament seen.

5. Discussion

The bivalves constitute the most diverse and abundant faunalgroup which inhabited the rocky shore at Ivö Klack, being repre-sented by just over 60 species. This diversity is high in comparisonwith other Late Cretaceous rocky shore bivalve assemblages fromGermany, the Czech Republic, New Zealand and Mexico (Table 3).The late Cenomanian e early Turonian fauna from the CzechRepublic was studied with respect to cementing species found onboulders; fifteen taxa have been recorded (Nekvasilová and �Zítt,

s interpreted here. All species present are suspension feeders. # indicate presence atbased on Pietzsch, 1962; Crampton, 1988; Nekvasilová and �Zítt, 1988; Lescinsky et al.,

anianeMaastrichtian Czech Republic LateCenomanianeEarly Turonian

New ZealandSantonian

CCCCCCC

ByByByByByByByByByByCIII?

15 15C ¼ 14? ¼ 1

I ¼ 3By ¼ 10C ¼ 1? ¼ 1

Fig. 11. Pie-diagram showing the distribution of 60 bivalve species from Ivö Klackamongst the various guilds.

1988; �Zítt, 1992; �Zítt and Nekvasilová, 1996; �Zítt et al., 1997)(Table 3). This number slightly exceeds that for Ivö Klack (14cementing species), suggesting that the entire rocky shore bivalvefauna in the Czech Republic may have been as diverse as at IvöKlack. The Cenomanian rocky shore bivalves from Germany(Pietzsch, 1962) constitute the most diverse Cretaceous assemblagedescribed to date outside Ivö Klack; it comprises 16 species, hereassigned to five guilds, namely the cemented epifaunal, the epi-byssate, the infaunal, the free-lying and the boring guild (Table 3).The CampanianeMaastrichtian rocky shore fauna from Mexico isrepresented by one byssate and five cementing species, indicatingthat only species directly associated with the rocky shore weredescribed, similar to the Czech fauna (Lescinsky et al., 1991;Johnson and Hayes, 1993). A Santonian fauna from New Zealand(Crampton, 1988) includes 15 species of bivalve which are hereassigned to the cemented epifaunal, the epibyssate and the infaunalguild. This guild structure indicates that both the subtidal, intertidaland the adjacent sedimentary seafloor environments wereincluded in that study. However, that particular fauna is from theSouthern Hemisphere and has no species in common with any ofthe other Late Cretaceous rocky shore faunas. Comparisons maythus be difficult, although the diversity would be expected to besimilar at similar latitudes and in comparable climates. All theCretaceous faunas are less diverse in comparison to Ivö Klack as faras number of species and number of guilds are concerned. TheGerman and Czech faunas, roughly 10 myr older, lived at similarlatitudes and in comparable climatic regimes within the sameepeiric sea as that described from Ivö Klack. Furthermore, thediversity of cementing species from the Czech Republic is similar tothat at Ivö Klack, which indicates that at least these two Europeanrocky shores may have hosted a comparable species diversity asrecorded from Ivö Klack. Thus, it is most likely that the lowerdiversity of other Late Cretaceous faunas can be explained bytaphonomic processes, such as mechanical destruction, or byincomplete taxonomic assessment of taxa not directly cemented tothe rocky shore.

The species diversity among modern rocky shore bivalve faunasfrom different latitudes is low in comparison to the Ivö Klackassemblages, even when infaunal species inhabiting associatedsediment are included. The highest bivalve diversity is found atOtranto in the southern Adriatic Sea (Italy), where 33 subtidalspecies have been counted (Fig. 10) (Terlizzi et al., 2003). The highdiversity at Ivö Klack may represent the cumulative effect ofgenerations of bivalves accumulated in the associated 22e24 mthick carbonate succession along the rocky shore. The accumulationof many generations of bivalves will lead to a higher total diversitycompared to any snapshots of faunas from similar modern settings,but, on the other hand, large taphonomic losses of aragoniticspecies and fragile, thin-shelled species have had a negative impact

Fig. 10. The bivalve fauna from four modern rocky shore l

on the diversity at Ivö Klack. The fauna is interpreted as comprisinga within-habitat, time-averaged assemblages (sensu Walker andBambach, 1971). According to Kidwell (1998, 2002), the relativeabundance of within-habitat, time-averaged assemblages is slightlyhigher than their modern counterparts. The much higher bivalvediversity at Ivö Klack is, however, interpreted as representinga genuine picture.

The cementing and epibyssate guilds at Ivö Klack are particu-larly important and comprise 61 per cent of all species recognised(Fig. 11). The guild structure provides a source for interpretation ofthe former environment along the rocky shore and is a way totackle palaeoecological aspects, independent of the number ofspecimens of each species preserved and, thereby, independent ofpreservational and collection biases. The high number of species inthe attached guilds can probably be ascribed to the turbulentconditions along the steep coast, where it was advantageous tobe attached firmly to the substrate. Cementation provides thestrongest attachment and all of the commoner species found atIvö Klack belong to this guild, indicating relatively strong turbu-lence along the shore (Table 1). The high number of species in theshallow infaunal guild demonstrates that life just below the sed-imentewater interface was attractive, with good water circulationand oxygenation, but lower energy levels, precluding erosion of

ocalities and their positional relation to the substrate.

Fig. 12. Reconstruction of the bivalve fauna from Ivö Klack in life positions on and along the rocky shore.

shells or uprooting and destruction of the commonly thin-shelledvalves. The water depth must thus have been at least a fewmetres, so as not to be affected by the relatively strong turbulencealong the shore. The distribution of the bivalves amongst the guildsis similar to other Late Cretaceous bivalve faunas, with the majorityof species belonging to the cementing and bysally attached guilds(Table 3). The Late Cretaceous guild structure is similar to thatknown for modern rocky shores where the intertidal zones areincluded in the various studies (Fig. 10). Only the Italian bivalvecommunity differs with a predominance of infaunal species,probably because only bivalves from the subtidal zonewere studiedand the most turbulent environment excluded. The similaritybetween the Late Cretaceous and modern guild structures placesthe bivalve fauna from Ivö Klack as a within-habitat, time-averagedassemblage and thereby documents it as part of a true rocky shorecommunity.

The high species diversity and the wide variety of bivalve shellmorphologies document an environment along the rocky shorewith a variety of habitats occupied by species with differentattachment strategies. The preference of bivalves for differenthabitats is based on their resistance to turbulence and on compar-ison with modern relatives (Fig. 12; Table 2). This palaeoecologicalsubdivision is, like the guild structure, independent of preservationand collection biases. It is assumed that species which preferred

high-energy conditions were also able to live in lower-energysettings, whereas the opposite can be ruled out. Habitat 3, therelatively protected environment, is thus occupied by the mostdiverse fauna, because species which lived in habitats 4 and 5 werealso able to cope with habitat 3, following the above assumption.Some species interpreted to have been present in a certain habitatdue to their capability to resist wave energy may actually not havebeen present, due to biological factors such as competition for spaceand species interactions. The preferred habitats of bivalves havebeen interpreted in order to obtain an overview of their distributionalong the rocky shore. This shows that bivalves were all able toinhabit the rocky shore environment and thus were not subse-quently swept in from adjacent environments, which is supportedby the well-preserved fossils and the mainly angular nature of shellfragments, indicative of limited or no transport (Surlyk andSørensen, 2010).

A well-developed epifaunal zonation, comprising three zones, ispreserved on gneiss boulders and hummocks (Surlyk andChristensen, 1974). The highest zone is dominated by S. labiatus,commonly the sole fossil found in this zone, which occupies theuppermost part of the vertical sides of boulders and hummocks. Thenext lower zone is dominated by A. haliotoideum and A. stobaei; thisrepresents themore or less vertical sides of boulders. Oysters are ableto live and feed in suspension-rich waters due to their self-cleaning

Fig. 14. Right valve of Amphidonte haliotoideum encrusted by a large number ofconspecific juveniles.

mechanism. The closer the vertical wall was to the seafloor, themoresediment was brought into suspension by waves. Many spondylidsarealso foundinthiszone, indicating thatsediment insuspensionwasnot the causal zonation factor between the two upper zones. Thedifference in bivalve assemblages of the two highest zones probablyreflects variable tolerance to turbulence and competition for space.The lowermost zone is dominated by serpulids and occupies theoverhanging lowerpartof theboulders,whereenergy levelswere lowand light poor (Surlyk and Christensen, 1974; Sørensen and Surlyk,2010). The absence of photonegative bivalves in the lowest zonemaybedue to attachment strategies.Most photonegativebivalves arenestlersandfissuredwellerswhichnormallyareattachedbyabyssus;this means that they will not be preserved in situ.

Oysters are the commonest invertebrates at Ivö Klack; theyplayed a significant ecological role in this environment. Many of theoysters have a high density of cementing and boring fauna whichmade them important for the overall biodiversity by producingsmall-scale habitats for encrusting and boring fauna (Fig. 13). Theirstationary shells formed a stable substrate for small epifaunalspecies and many of these probably benefitted from the strongfeeding currents of the oysters (Sørensen and Surlyk, 2010). Theoysters probably reduced competition for space by occupyingdifferent substrates, and the two dominant oysters, R. diluvianumand A. haliotoideum had different life positions. The medium-sizedA. haliotoideum is found encrusting hummocks and boulders by thewhole of the left valve, in close proximity with conspecific indi-viduals and even onto them (Fig. 14). The large-sized, thick-shelledR. diluvianum is commonly found grown together and notencrusting boulders (Fig. 15). Their heavy shells and relatively smallattachment scars would probably make it impossible for them to beattached to boulders where they would have been sitting at analmost right angle to the substrate in the turbulent environment.They are thus interpreted as having constructed oyster banks onthe seafloor immediately adjacent to the rocky shoreline. Thepreference for different substrates would have minimised compe-tition for space, as high densities suggest.

Rudistid bivalves were widely distributed during the LateCretaceous and Biradiolites suecicus from Ivö Klack represents theirmost northerly occurrence known, together with Canadian (Sas-katchewan) records (Caldwell and Evans, 1963). These radiolitidsare small sized, less than 30 mm in diameter, compared to taxa

Fig. 13. Right valve of Rastellum diluvianum, sh

from the Maastrichtian of Peru which reached diameters up to600 mm, probably as a result of symbiosis with zooxanthellae(Philip and Jaillard, 2004). The small size and paucity of rudistids atIvö Klack is probably due to the northern location where symbiosiswith zooxanthellae may have been reduced or altogether absent,due to lower water temperatures and illumination. Zooxanthellate,non-reef forming corals are known from Ivö Klack, indicating thatthe average water temperature must have been at least 18 �C(Sørensen and Surlyk, 2011). It is impossible to state whether or notthe rudistids lived in symbiosis with algae, but both zooxanthellatecorals and rudistids are rare in comparison to tropical settings,indicating that the cooler waters were not favourable for these taxaor their potential symbionts.

The byssate bivalve genera Mytilus and Barbatia are bothsuccessful on rocky shores today; their record extends back to theLate Jurassic and Triassic, respectively. The earliest known occur-rence ofMytilus in associationwith ancient rocky shores is from thelate Oligocene of Oregon and that of Barbatia from the Maas-trichtian of Alabama (Miller and Orr, 1988; Bryan, 1992; Johnsonand Baarli, 1999). Their presence on the early Campanian rocky

owing a high density of encrusting fauna.

Fig. 15. Nine valves of Rastellum diluvianum cemented together, illustrated in two views. The oysters grew upon each other and constructed banks along the rocky coast. Theircommon growth perpendicular to the substrate and their relatively small attachment areas can be seen on the specimens in the upper part of the figure. Living orientation of thespecimens cannot be inferred. LV ¼ left valve; RV ¼ right valve; ? ¼ unidentified valve.

shore at Ivö Klack thus documents the earliest example of bothMytilus and Barbatia in association with such an environment.

The extent to which the bivalves described here are represen-tative of the original fauna is difficult to estimate, since thearagonite-shelled taxa are preserved as internal/external mouldsonly and are mainly represented by few specimens of each species.This indicates that the taphonomic loss of this group is high. Theratio between aragonitic and calcitic bivalve species (i.e, 19 vs 37),indicates a reasonable preservation potential for aragonitic species,since it has been shown that the majority of bivalve species fromIvö Klack were attached and epifaunal in the turbulent environ-ment and most epifaunal species are calcitic (Heinberg, 1999;Hautmann, 2006). Thus, a higher number of calcitic species is tobe expected in all cases. The preserved bivalve fauna is accordinglyconsidered to be representative of the bivalve community living onthe rocky shore where the intertidal, the subtidal and the associ-ated skeletal sands are included in this environment.

6. Conclusions

� The rocky shore bivalve fauna from Ivö Klack is mainly wellpreserved and, with just over 60 species on record, it is morediverse than any other Cretaceous and modern rocky shorefaunas described to date. The high diversity most likely isa genuine reflection of the fauna living along the shore at anytime and can only in part be explained by the cumulative effectof generations of bivalves; taphonomic loss would probablyhave equalised this.

� Seven guilds are recognised; 59 species are referred to these.The cementing and epibyssate guilds comprise the largestnumber of species due to the advantage of being firmlyattached to the substrate in a turbulent, high-energy environ-ment. The high number of species in the shallow infaunal guild

indicates good water circulation but rather limited energylevels, precluding erosion, uprooting and destruction of thecommonly thin-shelled valves. This distribution of bivalvespecies between the guilds is similar to what has beendescribed for other Late Cretaceous rocky shore communitiesand what is known from modern rocky shore, with a predom-inance of attached species.

� Five habitats are recognised, representing different hydrody-namic conditions and light penetration (illumination levels).The distribution of bivalves between the habitats shows that allspecies were able to live in the rocky shore environment whichincluded the intertidal, the subtidal and the associated loosesediments, and thus were not subsequently washed in fromnearby settings. This is supported by the well-preserved fossilsand the mainly angular shell fragments which are indicative oflimited or no transport.

� Three zones of cementing fauna are recognised on large gneissboulders and hummocks. The distribution of encrustingbivalves is interpreted to be the result of competition for spaceand tolerance to water turbulence.

� Oysters were the commonest invertebrates along the shore andare important for biodiversity as they offered abundant, small-scale habitats for encrusting and boring fauna and thusincrease both the number of niches and faunal diversity. Someof the species appear to have formed oyster banks along therocky shore, whereas others encrusted the large boulders,thereby minimising competition for space.

� The small rudistid B. suecicus is, together with forms fromSaskatchewan (Canada), the most northerly known LateCretaceous occurrence of these bivalves. The small size andpaucity of rudistids indicate that the cooler water temperature,compared to tropical settings, was not favourable for them ortheir potential symbionts.

� The successful rocky shore bivalve genera Mytilus and Barbatiahad their earliest known occurrence on rocky shores at IvöKlack.

� The bivalves thus formed the most important invertebrategroup living along the late early Campanian rocky shore at IvöKlack in terms of diversity, density and biomass. This contrastswithmodern rocky shores which are dominated by gastropods,but this may be an artefact since the gastropod fauna at IvöKlack has suffered significant taphonomic losses directly linkedto their aragonitic shell mineralogy.

Acknowledgements

This study was funded by the Danish National Research Counciland the Carlsberg Foundation. Claus Heinberg is thanked forconstructive comments on an early manuscript version.

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