the muddy bottom of lake pannon ^ a challenge for dreissenid settlement (late … · 2010-12-03 ·...

The muddy bottom of Lake Pannon ^ a challenge fordreissenid settlement (Late Miocene; Bivalvia)

Mathias Harzhauser a;�, Oleg Mandic b

a Naturhistorisches Museum Wien, Geologisch^Pala«ontologische Abteilung, Burgring 7, A-1014 Wien, Austriab Institut fu«r Pala«ontologie, Universita«t Wien, Geozentrum, AlthanstraMe 14, A-1090 Wien, Austria

Received 17 October 2002; received in revised form 22 July 2003; accepted 11 November 2003

Abstract

The Late Miocene dreissenids of Lake Pannon ^ a long existing eastern-central European lake ^ bear witness totwo very different modes of life, which allowed these bivalves to successfully settle in what appears to be aninhospitable environment. One guild is restricted to the genera Mytilopsis Conrad, 1858 and Sinucongeria Lo‹renthey,1894. These representatives are extreme r-strategists which tend to produce temporal boom-and-bust populationsduring ‘time windows’ of improved ecological conditions. These opportunities are interpreted here to correlate withcyclic shifts in the epilimnion/hypolimnion relation and the corresponding oxygenation of bottom waters, eventswhich are probably triggered by low-frequency orbital forcing. The second guild is composed solely of CongeriaPartsch, 1836. The paradox of the proliferation of a single species, characterised by extreme size and weight, in amuddy, oxygen-deficient lake environment avoided by nearly all other molluscs points to a ‘hidden’ mechanism notaccessible to other species. We tentatively link this mechanism with chemosymbiosis. Functional morphology ininterplay with a highly sulphidic, over-saturated interstitial water support an interpretation of the large Congeriasubglobosa as a successful ‘H2S pump’ model.> 2003 Elsevier B.V. All rights reserved.

Keywords: Dreissenidae; molluscs; settlement strategies; chemosymbiosis; Lake Pannon; sequence stratigraphy

1. Introduction

Lake Pannon formed at about 11.5 Ma at theSarmatian^Pannonian boundary in place of therelic Paratethys Sea (Ro«gl, 1998). The vast brack-ish to freshwater lake covered the Pannonian Ba-sin complex ^ a Neogene back-arc basin type of

eastern-central Europe (Royden and Horva¤th,1988) encircled by the Alps, the Carpathians andthe Dinarids. The aquatic mollusc fauna of thelong existing Lake Pannon is characterised by ahigh percentage of endemisms and conspicuousradiations (Mu«ller et al., 1999). The developmentof the fauna was controlled by the gradual fresh-ening of the water body as well as by geodynamicprocesses, resulting in profound changes in lakegeometry (Magyar et al., 1999a). Aside from thehighly evolved lymnocardiids (Cardiidae, Bival-via), especially the manifold dreissenids display a

0031-0182 / 03 / $ ^ see front matter > 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0031-0182(03)00735-1

* Corresponding author. Tel. : +43-1-52177576;Fax: +43-1-52177459.

E-mail addresses: [email protected](M. Harzhauser), [email protected] (O. Mandic).

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Palaeogeography, Palaeoclimatology, Palaeoecology 204 (2004) 331^352

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remarkable evolution. The study of this group,composed of the endemic fossil genera Congeria,Sinucongeria and Dreissenomya, and the extantMytilopsis and Dreissena, reaches back to theearly 19th century, starting with Partsch (1836).Since then, numerous systematic contributionshave enriched our knowledge on the Central andEastern European dreissenid lineages. Palaeoeco-logical studies, however, such as those of Korpa¤s-Ho¤di (1983), Szilaj et al. (1999), Mezo« et al.(1999) or Vrsaljko (1999), are comparatively rareand strongly focus on synecological aspects. Inthis paper strategies and mechanisms are emphas-ised which allow byssally attached, more or lesshardground-dependent dreissenids to settle thepelitic substratum of Lake Pannon. These settle-ment strategies seem to be correlated at least

partly with lake level £uctuations which causedphases of oxygen-de¢cient bottom conditionsand changing sediment loads. These extreme andshort-lived environmental changes favoured op-portunistic species, which are typical within thedreissenids. K-strategists, however, could hardlycompete within this unfriendly environment with-out developing special adaptations.

2. Dating and lithology

In the Vienna Basin the investigated mass oc-currences are con¢ned to the Middle PannonianMytilopsis czjzeki Zone and the Lymnocardiumschedelianum Subzone according to the currentbiozonation introduced by Magyar et al. (1999b)

Fig. 1. Correlation of the Lake Pannon stratigraphy (biozone calibration to the absolute time and magnetostratigraphic scales ac-cording to Magyar et al., 1999b) with the standard chronostratigraphic, biostratigraphic and magnetostratigraphic zonations (cali-brations of MN-Zones according to Steininger et al., 1996, of magnetostratigraphic chrons and M/NN-zones according to Berg-gren et al., 1995). The shaded area indicates the time slice during which the herein described dreissenid assemblages proliferated.

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M. Harzhauser, O. Mandic / Palaeogeography, Palaeoclimatology, Palaeoecology 204 (2004) 331^352332

(Fig. 1) (Spiniferites paradoxus Biochron sensuMagyar et al., 1999a). This interval, characterisedby the co-occurrence of Congeria subglobosaPartsch, 1836 and Melanopsis vindobonensis Fuchsin Fuchs and Karrer, 1870, was originally intro-duced as Vienna Basin ‘Zone E’ by Papp (1951a).Currently, pelitic sediment of this zone is well ex-posed in the Vienna Basin at the section Henners-dorf, which served as base for this study (Fig. 2).The magnetostratigraphic dating of the sectionHennersdorf allowed its correlation with thechron C5n (Magyar et al., 1999b). The corre-sponding sequence is characterised throughoutthe Vienna Basin by predominantly pelitic sedi-mentation, re£ected in thick clay deposits whichwere exploited in numerous brickyards along thewestern margin of the basin. Sandy intercalationsare rare and seem to be restricted to the basalparts. Today, only the pit Hennersdorf close toVo«sendorf 10 km south of Vienna (Austria) is stillaccessible, whereas all others are ¢lled by ground-water. One of the already abandoned pits at Vo«-sendorf was intensively studied by Papp and The-nius (1954), who described more than 150 taxa ofvertebrates and invertebrates. The development at

Hennersdorf corresponds fully to that describedby Papp and Thenius (1954) from Vo«sendorf.The succession illustrated in Fig. 3 starts with acurrently unexposed 2-m-thick green clay layerwithout macrofossils ; this is followed by up to1 m sand with a diverse but allochthonous faunaincluding numerous molluscs and vertebrate re-mains. Upwards follow a few metres of blue-grey clay with in situ clusters of Congeria subglo-bosa Partsch, 1836 characterising the present bot-tom of the opencast pit at Hennersdorf. The baseof the next unit is marked by a remarkable 4-cm-thick bivalve coquina, termed herein He 4; it isdominated by disarticulated shells of Mytilopsisczjzeki (Ho«rnes, 1867) and Congeria zsigmondyiHalavats, 1883. This marker bed underlies anabout 8-m-thick green-blue-grey clay and siltyclay unit intercalated by numerous thin layersmade up of masses of predominantly articulatedSinucongeria primiformis (Papp, 1951b) and scat-tered, disarticulated Lymnocardium schedelianum(Partsch, 1831) shells. The top of the Hennersdorfsection is represented by about 7^8 m yellow-greyclay and silt which is practically barren of macro-fossils except for a marker coquina bed (He 1) in

Fig. 2. Geographic position of the studied section Hennersdorf in the Vienna Basin (Austria).

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M. Harzhauser, O. Mandic / Palaeogeography, Palaeoclimatology, Palaeoecology 204 (2004) 331^352 333

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its basal part. The He 1 bed is characterised byabundant Mytilopsis spathulata (Partsch, 1836),Melanopsis vindobonensis and scattered Congeriasubglobosa followed by various other melanop-sids, dreissenids and lymnocardiids.

3. Palaeobiogeography

Although we deal here mainly with assemblagesof the Vienna Basin, the remarkable dreissenidmass occurrences are not con¢ned to this area,as indicated in Fig. 4. A similar mass occurrenceof Mytilopsis czjzeki was documented by Ja¤mboret al. (1985) from the Dra¤va formation in theBalaton area (its shell outline is reminiscent of

Sinucongeria primiformis and the two are oftenlumped together in the literature). There, the spe-cies is also extremely frequent in silty o¡shoredeposits. The shells are still articulated and anappreciable transport from shallower settingscan be excluded.

Korpa¤s-Ho¤di (1983, 1985) also reported accu-mulations of Mytilopsis czjzeki from several local-ities in Hungary. She considered them as a con-stituent of the ‘Congeria czjzeki^Paradacna abichi’association and of the ‘Congeria banatica’ associ-ation contributed additionally by Congeria zsig-mondyi. As in the Vienna Basin, this assemblageis typically found in grey homogeneous clay andsilt with traces of pyrite. The assemblage was in-terpreted as evidence for pliohaline, nutrient-rich,

Fig. 4. Palaeogeographic map of Lake Pannon during its Middle Pannonian maximum extension (‘Zone E’), modi¢ed after Mag-yar et al., 1999a, showing the distribution of dreissenid assemblages described in the present study.

Fig. 3. Log of the section Hennersdorf close to Vienna/Austria in the Vienna Basin, re£ecting a predominately pelitic develop-ment. The succession can be traced currently over more than 800 m distance and is an excellent reference section for several al-ready abandoned clay pits on the uplifted tectonic blocks throughout the Vienna Basin.

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aphytal environments of the open lake at waterdepths between 15 and 60 m. From the northwest-ern Serbian Kolubara Basin, Stevanovic (1985a)reports mass occurrences of Mytilopsis czjzeki ac-companied by abundant Congeria zsigmondyi ;they stem from clays underlying sands bearingthe index fossil Lymnocardium schedelianum. Si-nucongeria primiformis is apparently also commonin the basin. A quite similar occurrence ^ alsowith Lymnocardium schedelianum and in peliticsediment ^ is documented by Skerlj (1985) fromSlovenia. The typical Middle Pannonian (‘Ser-bien’) occurrence near Karagaca (SSE Belgrade)was originally documented by Pavlovic (1928).The faunal assemblages redescribed by Stevanovic(1985c) with Congeria subglobosa, C. zsigmondyi,Mytilopsis spathulata, M. czjzeki and also Sinu-congeria? lithodomiformis highly resemble thestudied one from Hennersdorf. Moreover, thecorresponding assemblages are present in the ad-jacent Kreka Basin of NW Bosnia (Stevanovic,1985b) as well as in the Eastern Slovakian Dan-ube Basin (Fordinal, 1997). Finally, Marinescu(1985) cites a fauna of Congeria subglobosa, Con-geria zsigmondyi and Mytilopsis czjzeki from‘Zone E’ of the Romanian part of the Pannonianbasin complex.

These assemblages are best and fully developedalong the western and southern coasts of LakePannon (Fig. 4) ; the gap in the eastern part ofthe lake is probably related to a lack of availableinformation. This indicates that the assemblageshad a wide distribution throughout Lake Pannon.Thereby, Congeria subglobosa and Congeria zsig-mondyi formed loose populations contrasting withthe mass occurrences of the typical r-strategistsMytilopsis czjzeki and Sinucongeria primiformis.The mechanism which allowed these species re-peatedly to settle the muddy bottom of the lakeduring the Middle Pannonian transgression inshort-lived populations a¡ected the entire lakeecosystem. This phenomenon cannot be explainedby local conditions of the Vienna Basin.

4. Taxonomy and palaeoecology

The synoptic systematic presentations by Jeke-

lius (1944) and Papp (1953) continue to serve asthe foundation stones of our knowledge on themollusc fauna of early Lake Pannon. More re-cently, the classi¢cation of the family Dreisseni-dae has been improved by Nuttall (1990), whointroduced two subfamilies, i.e. Dreisseninae andDreissenomyinae. The taxonomic concept appliedin the present study follows that revision, if notnoted otherwise. A comprehensive discussion ofthe herein mentioned taxonomic units ^ includingsynonymy, type species and taxonomic contents ^is presented by Nuttall (1990).

4.1. Dreisseninae

The Dreisseninae comprise, in contrast to theDreissenomyinae, typically byssate endobionts.They are represented by two modern genera, i.e.Mytilopsis and Dreissena. These are highly e¡ec-tive ¢lter feeders that consume various kinds ofseston. Both are documented to pass a plankto-trophic larval stage (Morton, 1969; Nuttall,1990). Dreisseninae originated during Eocenetimes, probably from brackish Corbiculidae (Nut-tall, 1990).

4.1.1. MytilopsisThe genus Mytilopsis comprises the earliest rep-

resentatives of Dreisseninae. It did not undergomajor radiation until Miocene times. The initialimportant morphological diversi¢cation started inthe Early Miocene (Kochansky-Devide and Slis-kovic, 1978, 1981). The centre of origin was in theparalic to lacustrine system established on theperidinaric land forming the geographical barrierbetween the Paratethyan and Mediterranean re-gions (Ro«gl, 1998).

The subsequent, even more spectacular, radia-tion began in the Upper Miocene with the forma-tion of Lake Pannon, which became established atthe site of the ¢nally isolated Central ParatethysSea (Mu«ller et al., 1999). Despite apparent home-omorphic relationships between several taxa ofboth independent radiation events, it seems thatthe second Dreissenidae radiation again originat-ed from a primitive Mytilopsis (Kochansky-De-vide and Sliskovic, 1978; Nuttall, 1990). Hence,Mytilopsis gave rise in the Late Miocene not only

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to modern Dreissena but also to the extinct LakePannon endemics Congeria and Sinucongeria.

The euryhaline Mytilopsis, in contrast to Dreis-sena, seems to comprise primarily brackish watertaxa. Hence, the modern representatives inhabit-ing mangrove swamps of Western Africa toleratesalinities of up to 25x (Archambault-Guezou,1976). According to ostracod data for the LakePannon Congeria czjzeki Zone, all herein dis-cussed species could settle waters of 3^15x (Ko-va¤c et al., 1998).

4.1.2. DreissenaLike the primitive mytiliform Mytilopsis, Dreis-

sena is a byssate epibiont and typical r-strategistthat massively colonises various primary and sec-ondary hardgrounds. It developed from Mytilop-sis by complete reduction of the apophysis, a pro-tuberance of the septum acting as byssal/pedalretractor catchment (Nuttall, 1990).

In contrast to Mytilopsis, Dreissena is a typicalinhabitant of fresh water riverine and lacustrinesettings. Due to their role as natural hazards,Dreissena polymorpha (Pallas) and the quaggamussel Dreissena bugensis (Andrusov) are excep-tionally well studied. These freshwater species tol-erate only slightly brackish conditions. In theirnatural habitats they prefer salinities from 0.2^3.5x (Orlova et al., 1998, Liakhnovich et al.,1994), whereby some populations may toleratelevels up to 6.0x. Only the Caspian Sea repre-sentative Dreissena rostriformis (Deshayes) is ap-parently able to tolerate enhanced salinities,although values above 14x are lethal (Archam-bault-Guezou, 1976).

4.1.3. CongeriaCongeria is restricted by Nuttall (1990) to semi-

epifaunal, byssate sediment recliners. The MiddlePannonian Congeria subglobosa represent its typespecies. The present study will demonstrate that itis a typical K-strategist. Nuttall (1990) was the¢rst to point out the strange anomaly of the Con-geria species within the dreissenid stock, re£ectedby unusually large and heavy shells ; this feature isapparently restricted to the Lake Pannon repre-sentatives. The present study discusses this phe-nomenon in relation to chemosymbiosis.

All Congeria but especially its type species arecharacterised by an extremely thickened shell,which is a highly surprising feature for a mudrecliner in an apparently oxygen- and nutrient-de-pleted environment. The most reliable explanationmight be found in some kind of symbiosis withphoto- or chemosynthetic bacteria providing theenergy needed for such intensive mineralisation.Perhaps the installation of a symbiotic interactionwill turn out to be the primary factor that led tothe o¡shoot of the genus Congeria.

4.2. Dreissenomyinae

Dreissenomyinae are an endemic Paratethyangroup of typically infaunal representatives thatbecame extinct during Pliocene time. They arean Upper Miocene o¡shoot of primitive Dreisse-ninae, apparently related with the modioliformgroup including Mytilopsis amygdaloides and M.czjzeki (Papp, 1951b; Marinescu, 1977; Nuttall,1990). The Middle Pannonian Sinucongeria primi-formis stock represents its earliest representatives.The Dreissenomyinae from the Maeotian of theDacian Basin have been recently shown to be notolder than the Late Pannonian (Papaionopol etal., 1995; Magyar et al., 1999a).

The subfamily comprises only two genera, i.e.Sinucongeria and Dreissenomya, since Carinato-congeria Stevanovic, 1990 with its type speciesCongeria digitifera Andrusov, 1897 is apparentlynot related (Vrsaljko and Sremac, 1999; Mu«ller etal., 1999). They di¡er from typical Dreisseninaeby a more or less well-developed dorsal shell mar-gin. Moreover, a unique feature for Dreisseno-myinae is the development of an infaunal modeof life of many species of Sinucongeria and Dreis-senomya (Marinescu, 1977).

Dreissenomya and Sinucongeria, considered assubgenera of Dreissenomya by Marinescu (1977)and Nuttall (1990) are treated as genera in thepresent study. Both taxa are not only clearly mor-phologically de¢ned, but developed adaptationsto di¡erent modes of life. This treatment is fullyconsistent with the generic a⁄liation in other bi-valve families (e.g. Pectinidae by Waller, 1991)and avoids the virtual monotypy state for thesubfamily Dreissenomyinae.

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4.2.1. SinucongeriaThe shell habitus as well as an integripalliate to

slightly sinupalliate mantle scar in earliest Sinu-congeria still coincides with modioliform Dreisse-ninae. Apparently, the ¢rst important morpholog-ical change in Sinucongeria appeared in the LatePannonian with the rise of typical sinupalliate andinfaunal forms. Accordingly, a shallow infaunalway of life which is characteristic of more highlyevolved Sinucongeria did not develop before theLate Pannonian. Indeed, the present study pro-vides clear evidence that the Middle PannonianSinucongeria primiformis was an epibiontic r-stratigist that was ecologically still close to prim-itive dreissenids.

4.2.2. DreissenomyaDreissenomya displays reduction of a character-

istic dreissenid feature: the septum that is a ledgespanning the anterior interior shell margins, serv-ing for anterior adductor and pedal/byssal retrac-tor attachment. It is deeply sinupalliate alreadywithin the earliest representatives (Marinescu,1977; Nevesskaya et al., 1993), pointing clearlyto a deep infaunal way of life. It is stronger lat-erally compressed, less dorsally pointed and prin-cipally more thin-shelled than Sinucongeria.

Dreissenomya is most probably an o¡shoot ofa primitive Sinucongeria, although the timing ofits initiation is obscure. The presence of its ear-liest representatives (Sinucongeria zujovici and

Fig. 5. A ‘H2S pump’ candidate ^ Congeria subglobosa from the Pannonian stage type area (Vo«sendorf environ, ‘Zone E’, MiddlePannonian). (1) Ventral view of an articulated specimen from Hennersdorf included into a pyrite concretion; longitudinal axis(l.a.) = 81 mm. (2) Articulated specimen from the clay-pit S Leobersdorf; l.a. = 81 mm, ventral view showing the byssal/pedal gap(compare text). (3) Left valve from Vo«sendorf; l.a. = 62 mm; (a) exterior view. (4) Right valve from Vo«sendorf; l.a. = 70 mm;(a) interior view showing the central longitudinal ridge (compare text); (b) view perpendicular to posterior adductor insertior.(5) Right valve from Vo«sendorf showing extremely thickened ventral area; l.a. = 68 mm.

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S. unionides) in the Middle Pannonian sedimentsis not con¢rmed reliably (Marinescu, 1977; Steva-novic, 1990).

5. Dreissenid assemblages

All herein studied bivalves are, at least as juve-niles, byssally attached epibionts. The o¡shore en-vironment of Lake Pannon, which is dominatedby muddy bottoms of clay and silt devoid of peb-bles and clasts, is a poor base for byssate bivalves.Nevertheless, the mass occurrences document theability of some species to compete for the sparselyavailable hardsubstrata. The gregarious occupa-tion cannot be related to the utilisation of mi-cro-habitats, which were quite rare due to the uni-

form bottom conditions of the vast lake. Fourtypes of epifaunal settlement strategies can bedocumented from the o¡shore clays of the MiddlePannonian. These can be de¢ned as mixed grega-rious (1 type), monospeci¢c gregarious (2 types),and monospeci¢c patchy (1 type) settlement. Twospecies represent the monospeci¢c patchy settle-ment type: Congeria subglobosa and Mytilopsisspathulata. Both managed to form small clustersor nests on the muddy bottom but applied quitedi¡erent strategies.

5.1. Congeria subglobosa ^ monospeci¢c patchyassemblages

In the basal part of the clay pit, the large-sized,robust Congeria subglobosa (Fig. 5) is frequently

Fig. 6. A ‘legacy hunter’ ^ Mytilopsis spathulata from the Pannonian stage type area (Vo«sendorf environ, ‘Zone E’, Middle Pan-nonian). (1) Left valve from Vo«sendorf; l.a. = 42.8 mm; (a) exterior view; (b) interior view showing the large adductor scar andelongated septum with posterodorsally discharged apophysis. (2) Articulated specimen from Leopoldsdorf; l.a. = 28 mm; (a) leftexterior view; (b) posterior view showing the siphonal gap; (c) dorsal view; (d) ventral view showing the byssal/pedal gap. (3^5)Mytilopsis spathulata colonies in shells of Congeria subglobosa from Hennersdorf: (3) 74 mm; (4) 76 mm; (5) 80 mm. Note thevariability in the number of individuals and the di¡erent stages of growth due to the struggle for limited settling space.

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found in situ with valves articulated and the bys-sal/pedal gap pointing downward. It usuallyforms small, monospeci¢c clusters of few speci-mens (3^5 individuals ; Fig. 13b). Althoughclosely adjoining, they usually do not touch eachother, but are several cm apart. The distributionof these accumulations shows no obvious pattern.However, the specimens are usually fully grownand thus these accumulations are interpreted assmall populations which achieved the adult stage.Catastrophic collapses of multi-generation popu-lations, including juveniles, are completely missingin these layers.

5.2. Mytilopsis spathulata ^ monospeci¢c patchyassemblage

Mytilopsis spathulata, a medium-sized, elon-gated bivalve (Fig. 6) was a byssally attached epi-biont which, in contrast to Congeria subglobosa,was unable to adapt to soft bottom conditions

throughout its life. It thus depended on hard bot-tom elements on the muddy substratum, speci¢-cally the valves of Congeria subglobosa. In thelowermost part of the investigated section, valvesof the large Congeria in convex-side-down posi-tion are often settled by clumps of 7^35 Mytilop-sis spathulata. The densely stacked pattern of thecolony is still preserved in situ (Fig. 6, 3^5). Theattachment is restricted to the inner side of theshell.

5.3. Sinucongeria primiformis ^ monospeci¢cgregarious assemblage

This assemblage type (Fig. 7) is typical in themonotonous pelitic unit above the He 4 layer anddeveloped at least 18 times within a 6-m-thickunit. The small-sized, elongated Sinucongeria pri-miformis forms a moderately dense pavement ofonly partly contacting individuals (Figs. 8 and13a). Most specimens are articulated; fragmenta-

Fig. 7. A boom-and-bust strategist ^ Sinucongeria primiformis from the Vo«sendorf environ; ‘Zone E’, Middle Pannonian. (1) Leftvalve from Vo«sendorf representing the holotype (Coll. NHM Vienna); (a) exterior view; (b) interior view; (c) ventral view;(d) dorsal view; (e) detail showing ligamental and septal area ^ note the small apophysis included into the septal area; (f) anteri-or view. (2) Articulated specimen from Hennersdorf; l.a. = 23 mm; (a) right exterior view; (b) ventral view; (c) dorsal view; (d)anterior view.

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tion is low. Only few other mollusc species co-occur in this assemblage, such as Lymnocardiumschedelianum and Lymnocardium brunnense.

5.4. Congeria zsigmondyi/Mytilopsis czjzeki ^mixed gregarious assemblage

This assemblage (Fig. 9) is restricted at Hen-nersdorf to a single, widespread and very charac-teristic layer (He 4 in Fig. 3). It forms a conspic-uous boundary in the pelitic sediment and istherefore often outcropped despite the highly in-dustrialised exploitation method. In detail thisonly 4-cm-thick layer reveals a multi-phase histo-ry (Fig. 10). It overlays monotonous darkgreen-grey clay with the already described Congeria sub-globosa clusters. The lower boundary is indicatedby an indistinct relief formed by a coquina of few-mm-broad fragments of thin-shelled bivalves.Lymnocardiid fragments and unidenti¢able thin-shelled dreissenids constitute this shell bed. Theunderlying 3^5 cm clay displays distinct bioturba-tion of shell hash-¢lled burrows. Towards the topthis dense, component-supported coquina be-comes slightly looser and also bears well-pre-

served but disarticulated shells of Mytilopsis czjze-ki £oating within the shell hash. Theapproximately 1-cm-thick uppermost part is char-acterised by the occurrence of numerous disarticu-lated, unfractured shells of Congeria zsigmondyi.Congeria ¢rmocarinata is also common but lessfrequent; subordinate Mytilopsis czjzeki co-oc-curs. This coquina ceases abruptly and is overlainby 8 cm of darkgreen clay with few scatteredspecimens of Congeria ¢rmocarinata and Congeriazsigmondyi £oating in the pelitic sediment. Thesespecimens are usually almost articulated (bothvalves are found connected or very close to eachother) but are never in life position. Upsectionfollows the ¢rst layer of Sinucongeria primiformis,indicating the beginning of another cycle.

6. Discussion

6.1. Strategies of settlement

6.1.1. Congeria subglobosa ^ the advantage of the‘H2S pump’

The mode of life of Congeria subglobosa has

Fig. 8. Distribution pattern of valves of Sinucongeria primiformis in a discrete bed based on ¢eld data at Hennersdorf (arrowspoint in umbonal direction, white circles indicate shells in convex-side-down position). The distribution re£ects random orienta-tion of the valves in both assemblages; no preference for a convex-side-up position can be stated.

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Fig. 9. Representatives of the ‘Mytilopsis czjzeki/Congeria zsigmondyi’ gregarious assemblage from the Middle Pannonian (‘ZoneE’) of the southern Vienna Basin. (1) C. zsigmondyi accumulation from Mannersdorf a.d. Leitha; l.a. = 57 mm, orthogonal view.(2) Right valve of C. zsigmondyi from Regelsbrunn a.d. Donau; l.a. = 32.5 mm; (a) exterior view; (b) interior view; (c) ventralview; (d) dorsal view. (3) Right valve of M. czjzeki from Regelsbrunn a.d. Donau; l.a. = 19 mm; (a) exterior view; (b) interiorview; (c) detail of the septal area showing the spoon-like apophysis beneath the septum; (d) dorsal view; (e) anterior view;(f) ventral view.

Fig. 10. Detail of layer He 4 documenting a multi-phased origin of the coquina. See text for discussion.

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been interpreted quite di¡erently in the literature.Morton (1970) suggested a byssally attached, in-faunal mode of life. Papp (1985) illustrated Con-geria subglobosa as a byssally attached epifaunalspecies lying on the sediment surface. Finally,Nuttall (1990) reconstructed a byssally attachedsemi-infaunal mode of life, with short exhalantand inhalant siphos positioned on the posterodor-sal shell margin. However, considering the softbody morphology of modern dreissenids and theposition of posterior muscle and mantle catch-ments in Congeria subglobosa, a much more reli-able interpretation is a position of the inhalantsipho along the posteroventral margin and a po-sition of the exhalant sipho at the most posteriorpart of the posterodorsal margin (Fig. 11). Suchsiphonal positions exclude an infaunal way of life.The byssal/pedal gap is well developed, thus theshell was always open toward the bottom sedi-ment, forming a considerable loop of up to 22mm length and up to 11 mm width. Such a huge‘pseudo-byssal gap’ would have meant excessivebyssal attachment and is in harsh contrast to amuddy sediment without pebbles, sand grains orsecondary hardgrounds. This discrepancy clearlyindicates that this derived byssal gap has to be putin the perspective of another functional morphol-ogy.

The anterodorsal shell area in Congeria subglo-bosa is distinctly broadened and slightly concave,with its maximum concavity culminating in the

byssal gap. This area of the shell is commonlyextremely thickened in adult individuals, reminis-cent of some kind of ‘anchor mechanism’ provid-ing the animal with stability by the low positionof the balance point. This enhanced the weight,and the enlarged surface area of the shell part incontact with the sediment surface signi¢cantly im-proved the stable position. The species was obvi-ously excellently adapted for sediment reclining.Thus, after the initial byssally attached juvenilestage, the attachment became inconsequentialfor the large, heavy adults. A semi-infaunal modeof life might have arisen due to sedimentation.This resulted in a partly buried stage by passivecoverage with suspended material and could havebeen promoted by passive sinking of heavy shellsinto the water-saturated bottom sediment. Bothmechanisms would also explain the typical, articu-lated preservation of many Congeria subglobosanests.

The preservation mode and the pelitic sedimentsuggest very calm conditions on the lake bottomduring formation of Congeria clusters. Rarely, in-faunal lymnocardiids co-occur, but all other epi-faunal elements are absent. Note that the Myti-lopsis spathulata colonies in Congeria subglobosashells, as described below, are absent in the layerswith in situ Congeria subglobosa. Moreover, theshells are often enclosed by pyrite concretions,indicating oxygen-depleted environments, whichfavoured the accumulation of sulphides in thesediment. Despite this rather unfriendly environ-ment, the high number of heavy, full-grown indi-viduals shows that Congeria subglobosa £ourishedand was optimally adapted to this kind of habitat.The hypertrophied shell points to an adequateamount of available energy that was used for min-eralisation. Moreover, it appears paradoxical thatthe development of hypertrophied, heavy shellsdisagrees with ‘the light-weight paradigm of amud £oater that faces the danger of sinking intoa soupy sediment’ (Savazzi, 1985; Seilacher,1990). Ancestors of the species within the ‘subglo-bosae lineage’ sensu Papp (1985) display prereq-uisites concerning the overall shape but are dis-tinctly more fragile. Therefore, the increase inshell thickness seems to be an adaptive featureof Congeria subglobosa coinciding with a switch

Fig. 11. Reconstruction of the anatomy and mode of life ofCongeria subglobosa. The internal organisation is modi¢edafter Yonge and Campbell (1968) for the Recent Dreissenapolymorpha.

PALAEO 3260 11-2-04


in habitat preference from agitated nearshore tocalm o¡shore settings.

Hypertrophied bivalve shells are commonly ex-plained as a result of symbiotic interaction withchemo- or photosynthetic micro-organisms (Sei-lacher, 1990). Since photosynthetic zooxanthellatealgae have been discovered in the mantle marginof the giant tridacnids, similar processes have alsobeen suspected for hypertrophied Mesozoic shal-low-water bivalves like rudists and megalodonts.Congeria highly coincides with the shell form ofsome megalodonts, but this homeomorphy can bebetter explained by their adaptations to sedimentreclining than by the same environmental require-ments. Indeed, based on the sediment and biofa-cies analysis at Hennersdorf, Congeria was habi-tuated to environments with low light content. Asymbiosis with photosynthetic organisms is there-fore unlikely. In contrast, by utilising reduced sul-phur as an energy source the chemosynthetic sym-biontic bacteria present in the tissue of manybivalve groups highly bene¢t from reduced, sul-phide-rich sediment and dissolved sulphide frominterstitial water (Taylor and Glover, 2000). Suchsymbiosis can distinctly in£uence bivalve growth,as shown by growth experiments with lucinids(Taylor and Glover, 2000) or by examples of thegiant deep-sea representatives Calyptogena andBathymodiolus (Gage and Tyler, 1991). Evidenceis also available that extinct inoceramids ^ repre-senting the largest bivalves ever known, with amaximal shell-length of 178 cm ^ harboured che-mosynthetic symbionts (MacLeod and Hoppe,1992, 1993).

The conspicuous interior morphology of Con-geria subglobosa could be a result of body reor-ganisation characteristic of chemosymbiotic bi-valves (Taylor and Glover, 2000). The mostimportant problem in such groups is to separatethe sulphide-rich water in¢ltrating from the sedi-ment from the oxygen-bearing water inhaled bythe sipho which is used for normal respiration(Reid, 1998). Hence, the anteroventral thickenedportion of the shell, which coincides with the po-sition of the hypothetically thickened, symbiont-bearing ctenidia, is clearly separated from theposterodorsal shell region bearing the inhalant si-pho. A distinct antimarginal ridge marks their

boundary. Moreover, as the anteroventral gapcan hardly be explained by the presence of a thickbyssus bunch, a more reliable explanation wouldbe its function for accessing the sulphide-rich in-terstitial water from the bottom sediment. Theapparent load of the heavy, widened anteroven-tral shell part onto the sediment could e¡ect theisostatic rise of such water into the shell cavity.The foot might have served as a regulator for thecontact with the sul¢dic water, which would havebeen harmful if admitted into the shell interior inuncontrolled amounts. A corresponding processhas been termed by Seilacher (1990) the ‘H2Spump’ model.

Savazzi (2001) has emphasised the di⁄culties ofrecognising chemosymbiosis in fossil bivalves. Heconcluded that no evidence for chemosymbiosis isavailable except for a speci¢c ichnofossil, pro-duced by the probing bivalve foot (e.g. Zuschinet al., 2001). Nevertheless, Savazzi (2001) pre-dicted that careful analysis might reveal instancesof chemosymbiosis among fossil bivalves. Indeed,a more or less synchronous study by Taylor andGlover (2000) presented such data. The latter au-thors investigated Recent and fossil lucinids andconcluded that ‘the most convincing evidence forthe presence of chemosymbiosis may come fromthe morphology of the shell and preserved lifepositions...’. The chemosymbiont-bearing lucinidspossess a so-called mantle gill organ, that is apallial blood vessel, impressed into the shell inte-rior and striking along the interior side of themantle impression. A corresponding line ofclaw-like impressions is also visible in the inves-tigated specimens of Congeria subglobosa. Note,however, that in contrast to lucinids the inhalantsipho in Recent dreissenids is placed posteriorly.Therefore, among other discussed morphologicalfeatures especially the large size suspects an addi-tional source of energy aside from suspensionfeeding enabling such massive shells mineralisa-tion.

In conclusion, Congeria subglobosa ^ in con-trast to other herein described dreissenids ^ showsall features of a typical, highly adapted K-strate-gist which managed to settle otherwise poorly in-habited lake environments. A hypothetical butunproven explanation for the success of Congeria

PALAEO 3260 11-2-04


subglobosa in settling muddy and oxygen-depletedbottoms of Lake Pannon might be found in che-mosymbiosis.

6.1.2. Mytilopsis spathulata ^ a ‘legacy hunter’pro¢ting from the deceased

Within each Mytilopsis spathulata clump, adultand subadult specimens co-occur, although alsocolonies of more or less equal-sized specimensmay occur. The settlement was thus multi-phasedrather than a single event. Rarely, juveniles arefound in these clumps within the gaping valvesof adult Mytilopsis spathulata. This late stage ofcolonisation documents the decline of settlementspace and indicates the ¢nal phase before thebreakdown of the spatially restricted colony. Col-onisation was clearly limited only by space, whilstfood resources and sedimentation were not thecontrolling factors. Mytilopsis spathulata is neverfound attached to the external surface of Congeriasubglobosa or any other molluscs. This might beexplained by the preference of the juveniles tosettle in protected cavities as o¡ered by the shellsof Congeria subglobosa and rarely by apertures ofthe gastropod Melanopsis.

Although this type of colonisation is con¢nedto occurrences of valves of Congeria subglobosa,there is no interference of competition betweenthese species. Other shells, e.g. lymnocardiids orconspeci¢c specimens, probably did not o¡er thestability of the heavy and deeply concave Conge-ria subglobosa shells. Nonetheless, Mytilopsis spa-thulata is never associated with in situ Congeriasubglobosa nests and seems to have avoided thisslightly deeper, less agitated and oxygen-depletedhabitat. The ecological range of Congeria subglo-bosa was clearly broader than that of Mytilopsisspathulata, which did not penetrate as far o¡-shore as C. subglobosa. This settlement of Myti-lopsis spathulata is unknown from Congeria zsig-mondyi and Congeria ¢rmocarianta shells. Overall,Congeria species, in contrast to Mytilopsis, seemto have been adapted to rather low oxygenationand thus £ourished during the rapid transgressiondocumented in the lower part of the TST of thesection Hennersdorf. During the probably evenless aerated conditions of the early HST at thetop of the section, Congeria subglobosa is the

only species which is rarely found in life positionabove the He 1 layer.

6.1.3. Sinucongeria primiformis ^ boom and bustThe analysis of the size distribution within the

shell pavements strongly suggests that most indi-viduals represent fully grown specimens, whereasjuveniles are very rare (Fig. 12). This fact and thenegligible thickness indicate that the shell bedsdocument single populations which reached theadult stage before their ¢nal collapse. This inter-pretation predicts only sporadically suitable con-ditions for settlement, because longer periods offavourable conditions would result in the forma-tion of thick shell beds contributed by severalpopulations. Therefore, the pavements are consid-ered to represent more or less census assemblagesas de¢ned by Kidwell and Bosence (1991). Censusassemblages, however, might be expected to pro-duce polymodal size frequency curves due to themixture of two or more age groups if more thanone population is represented (Tanabe and Ari-mura, 1987). The unimodal size distribution ob-served in the described assemblage seems to con-tradict this prediction. An explanation of thisdiscrepancy might be that these dreissenids wereextreme r-strategists ^ ‘explosive opportunists’sensu Levinton (1970) ^ which formed boom-and-bust populations (Kidwell, 1991) with low ju-venile mortality.

Sinucongeria primiformis and Mytilopsis czjzekimost likely fed on planktonic algae and zooplank-ton. The breakdown of populations after the sud-den blooms might also be related to declining

Fig. 12. Size distribution of Sinucongeria primiformis. Theshells display a unimodal size distribution which rangesaround the usual adult size of the species.

PALAEO 3260 11-2-04


PALAEO 3260 11-2-04


food resources. The excess of adults within thepavements might thus be explained by competi-tion of already established animals with their lar-vae, hindering larval settlement on living colonies.As reported by Strayer et al. (1996), adult Dreis-sena polymorpha populations can outcompetetheir planktonic larvae for limited food resources.The lack of population refreshment by juvenileswould ultimately result in a population decline.There is no sedimentological feedback withinthese pavements which points to accelerations orslowing of the sedimentation rates. Therefore, thepreservation of the articulated shells is hardly ex-plained by rapid burial but might rather be corre-lated with low oxygenation and low energy con-ditions in the hypolimnion of Lake Pannon.Indeed, shell cavities are commonly incrustedwith pyrite or marcasite, pointing to anoxia as areliable cause both for their sudden death as wellas for the missing displacement through bioturba-tion.

6.1.4. Congeria zsigmondyi/Mytilopsis czjzeki ^shell hash as lucky chance

The sedimentary succession reveals that the twodreissenid species did not initially settle themuddy substrate but attached to already depos-ited, densely packed shell fragments. These mighteither result from a thin-shelled, more or less au-tochthonous dreissenid pioneer species or mighthave been transported to the basin by wave en-ergy and currents. The latter interpretation seemsunlikely based on the taxonomic composition ofthe coquina. Generally, all transported, allochtho-nous coquinas which derive from nearshore hab-itats of Lake Pannon yield abundant gastropodssuch as Melanopsis vindobonensis and Melanopsispygmaea and bivalves such as Mytilopsis balato-nica, Mytilopsis spathulata and scattered unionids.These assemblages also repeatedly occur at Hen-nersdorf and Vo«sendorf in silty^sandy lenses and

bear nothing in common with the herein discussedlayer. Hence, we interpret the basal coquina ofshell hash as a within-habitat, time-averaged as-semblage. The poor sorting of the shell hash alsocontradicts an interpretation as a distal tempest-ite.

The high fragmentation rate might be linked tobioturbation (Kidwell and Bosence, 1991); addi-tionally, an exposure over a long period of timewould have supported the decline of shell stabilityof the thin-shelled valves (Zuschin and Stanton,2001).

Up to 85% of the thick-shelled valves in theupper part of the layer He 4 are deposited withthe convex side down (see also Fig. 10). This po-sition contradicts a sedimentation under currentin£uence, which would rather result in the morestable convex-side-up position. In contrast, thegaping valves might be expected after death tokeel over due to the balance point of the shells.

6.2. Sequence stratigraphy and dreissenidsettlement

Another close-by, equivalent development isknown from Hodon|¤n (Czech Republic) in thenorthern Vienna Basin. Jiricek (1985) describes athreefold succession: the lower unit (‘E1’) com-prises thin lignites in the base, followed by 11 mclay with Lymnocardium schedelianum and 5 mblue-green marl. The second unit (‘E2’) consistsof 3.5 m yellowish marl and is poor in macrofos-sils. The top of the succession is formed by sandwith coquinas consisting of Mytilopsis spathulata,Congeria subglobosa and Unio atavus. Kova¤c et al.(1998) include these units in their reconstructionof the Pannonian sequence stratigraphy of theVienna Basin. According to that study, the basalpart of the succession represents a transgressivesystems tract (TST) which passes into the lowaerated conditions of a highstand systems tract

Fig. 13. Interpretation of sedimentary cycles at the section Hennersdorf based on the conspicuous shifts and redundancies in thedreissenid assemblages. No mixing of the assemblage types occurs, indicating a straightforward development which might best belinked with lake level rise during the Middle Pannonian. The phase of Sinucongeria pavement development seems to comprisefour cycles, which are characterised by a dense succession of pavements in the initial part of the cycle and a gradual decrease ofpavements towards the top of each cycle.

PALAEO 3260 11-2-04


(HST) within the second unit. In basinal deposits,this sea-level rise during ‘Zone E’ culminates indino£agellate blooms and the deposition of claysenriched in Spiniferites close to the maximum£ooding surface. Finally, the top unit of Hodon|¤n(‘E3’) is correlated with a lowstand systems tract(LST).

The section Hennersdorf may best be correlatedwith the two basal units of Hodon|¤n. The occur-rence of the Melanopsis^Congeria shell bed in thevery top of the section, followed by macrofossil-poor clay, agrees well with the basal HST of thesecond unit at Hodon|¤n. In contrast, the sandytopunit of Hodon|¤n (‘E3’ ; correlated with aLST) could not be detected in the current out-crop, which is truncated by Pleistocene gravel.

The remarkable distribution pattern of dreis-senid bivalves on the muddy bottom of LakePannon during the Late Miocene is obviouslytriggered by the repeated but short-lived establish-ment of conditions favouring the sometimes evengregarious settlement by larvae. Nevertheless, thepresented settlement types hardly co-occur, butrather seem to prove the ability of each speciesto settle very distinct habitats that are otherwiseavoided by most other taxa. A rough successionof the Mytilopsis spathulatasCongeria subglobo-sasMytilopsis czjzeki/Congeria zsigmondyisSi-nucongeria primiformis assemblages can be docu-mented as illustrated in Fig. 13. This succession isclearly linked to the rise of the lake level during aTST.

The initial transgressive impulse, documentedin the basal-most part of the succession, led tothe formation of thick coquinas and lignites insandy sediment. The subsequent rapid transgres-sion caused poorly oxidised conditions on the lakebottom coinciding with the sedimentation of darkclay. Most probably a well-developed hypolim-nion established at that time. This nutrient- andoxygen-poor, barely agitated environment wassuccessfully settled only by very few lymnocar-diids and by Congeria subglobosa, which probablyutilised energy by chemosymbiosis.

A decrease in sediment input could account forthe development of the shell hash in the basal partof the marker layer He 4 (Fig. 13c). This processmight be quite similar to that described by Abbott

(1997) as ‘Mid-Cycle Condensed Shellbed’ (MCS).These shell beds represent parautochthonous con-centrations of shells and fragments during phasesof reduced rates of terrigenous sediment accumu-lation; articulated bivalves are common, mosttaxa are preserved in a mud-rich matrix. Abbott(1997) recorded such MCS, which do not corre-spond to the maximum £ooding surface, fromupper transgressive systems tracts. Consequently,the following unit of clay with the typical Sinu-congeria primiformis pavements is correlated withthe upper TST. The spatial distribution of thosepavements reveals a conspicuous pattern whichseems to comprise four cycles of ‘pavement gath-ering’. Each cycle starts with a comparatively nar-row-spaced succession of 2^3 pavements and dis-plays a decrease in abundance towards the top.The next cycle is indicated with the onset of thenext group of pavements. The four cycles bear 7,5, 5, and 3 pavements (Fig. 13) and thus show aslight decrease in number upwards. These settle-ment phases seem to be linked to £uctuations inproductivity and short-termed mixing of the hy-polimnion. Sedimentation, however, did not sig-ni¢cantly change during these episodes.

The top of the fourth cycle is poorly de¢ned;the change of sediment colour from green-greytowards yellow-grey might indicate a boundarybut could also result from post-Miocene oxidisa-tion of the sediment. The top unit, however, dif-fers markedly in being nearly fossil free. Only fewCongeria subglobosa settled the lower part of thisunit. Oxygen-depleted bottom waters might havehampered any settlement by molluscs during thedeposition of the pelite ^ an interpretation which¢ts well to the description of Kova¤c et al. (1998)concerning the low-aerated conditions during theHST at Hodon|¤n. Hence, the thick shellbed (He 1)in the base of that unit at Hennersdorf might be aproper candidate for the maximum £ooding sur-face (MFS).

6.3. Orbital forcing as a trigger for dreissenidsettlement?

This sequence stratigraphical approach allows avery careful estimation of the absolute duration ofthe formation of the various units and cycles. The

PALAEO 3260 11-2-04


duration of the Lymnocardium schedelianum Sub-zone is estimated to span approximately 1 my byMagyar et al., 1999b. About 0.8 my contribute tothe more or less equivalent ‘zone E’ according toKova¤c et al. (1998). As discussed above, the para-sequences preserved along the western margin ofthe Vienna Basin do not span the whole sequence,but represent the TST and only a part of theHST. Taking into account the asymmetry be-tween the TST and the HST as indicated by Ko-va¤c et al. (1998), less than half of the time of thesequence can be attributed to the preserved units.Hence, a very rough estimation predicts a sedi-ment accumulation of approximately 0.05 mm/yrfor the 20-m-thick unit of Hennersdorf and Vo«-sendorf during 0.4 my (for the compacted sedi-ment!). This average sedimentation rate is quitereasonable compared to data from Lake Baikal,from where Colman et al. (1995) estimated V5.1cm/ka.

Consequently, the duration for cycles b and cof the Sinucongeria pavement-bearing unit (Fig.13) ranges about 28 ka and ca. 38 ka for cyclea. Despite the cautious nature of this estimation,the cycles could well be linked to high-frequencyorbital forcing such as precession (21 kyr) orobliquity (41 kyr). Actually, the sedimentarycycles of the Late Miocene of the Pannonian Ba-sin have already been linked to climatically drivenrelative lake-level changes in the Milankovitchfrequency band by Juha¤sz et al. (1997, 1999).Based on numerical cycle analysis and magneto-stratigraphy of numerous boreholes of the Panno-nian Basin in Hungary, Juha¤sz et al. (1997)deduced three types of cycles in Pannonian sedi-ments, i.e. V19-ka, V50-ka and V400-ka cycles.Later, Juha¤sz et al. (1999) re¢ned the data andpredicted 19-ka, 71-ka and 370-ka cycles whichthey related to precession, obliquity, and the lon-ger period of eccentricity, respectively.

Thus, the settlement of the lake bottom by Si-nucongeria primiformis did not occur randomly butfollowed periodic modi¢cations that allowed theestablishment of short-lived populations. Slightclimatic changes related with these cycles mighthave caused cooler and/or more humid phaseswhich favoured a repeated mixing of the hypo-limnion with the epilimnion. The gradual decrease

of humidity and/or the slight increase in surfacewater temperature during each cycle led to the re-establishment of rather stable, low-aerated oreven oxygen-depleted hypolimnic conditions re-£ected in the scarceness of dreissenid pavements.

7. Conclusions

The described dreissenid palaeocommunitiesformed during the Pannonian ‘Zone E’ sensuPapp (1951a) or the Spiniferites paradoxus Bio-chron sensu Magyar et al. (1999a). At that timeLake Pannon attained its maximum areal extent,which can be roughly estimated at about 280 000km2. The rapid transgression seems to coincidewith the establishment of oxygen-de¢cient muddybottoms which provoked the development of es-pecially adapted settlement strategies.

The family Dreissenidae successfully met thechallenge by quite di¡erent modes, which all re-sulted in the massive distribution throughoutLake Pannon. The genera Mytilopsis and Sinucon-geria turned out to be typical r-strategists whichmanaged to settle even rather harsh habitats dur-ing short-lived events of improved ecological con-ditions. Thin shellbeds of Sinucongeria can betraced in contemporaneous outcrops across morethan 800 m distance and seem to have coveredenormous areas of the Vienna Basin, revealingthem to be even more gregarious than most extantaquatic pest species such as Dreissena polymorphaor Dreissena bugensis.

The succession pattern of these boom-and-bustoccurrences within the section Hennersdorf indi-cates that their formation was triggered by somekind of cyclicity. This cyclicity may be correlatedbest with low-frequency orbital forcing, whichcaused periodic changes in the epilimnion/hypo-limnion relation in the o¡shore zone of the Vien-na Basin. The climatic back-lash within each cyclewas answered by the formation of a more stablehypolimnion with oxygen-depleted bottom condi-tions, which hindered Mytilopsis and Sinucongeriafrom settlement.

The second, more sophisticated, strategy is rep-resented by Congeria subglobosa ^ the largest mol-lusc of Lake Pannon at that time. Both morphol-

PALAEO 3260 11-2-04


ogy and environmental parameters strongly sug-gest that this strange bivalve utilised energy fromchemosymbiosis. Its extreme byssal gap morphol-ogy and the interior shell morphology point to ahighly adapted mode of life as a biological ‘H2Spump’. This made the animal less dependent onenvironmental shifts than other dreissenids andwould explain its wider spatial distribution.

Acknowledgements

This work was supported by the FWF-projectsP-13745 Bio and P-15724. This paper pro¢tedgreatly from comments by M. Zuschin (Universityof Vienna), W.E. Piller (University of Graz), M.Stachowitsch (University of Vienna), and R.Roetzel (Geological Survey Vienna). Many thanksare due to the reviewers (E. Savazzi, H. Sanders)for their helpful suggestions and to F. Topka(Natural History Museum Vienna) for prepara-tion and help during ¢eldwork.

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