in late cretaceous rudists from central italy: radiolites...

185Bollettino della Società Paleontologica Italiana, 44 (3), 2005, 185-192. Modena, 30 novembre 2005

ISSN 0375-7633

INTRODUCTION

In Late Cretaceous times, Rudist bivalves flourishedin carbonate platform environments, reaching their peakdiversity with the extraordinary diffusion of taxabelonging to the Radiolitidae and Hippuritidae families.

They colonized diverse environments of theMediterranean Tethys becoming significant organicbuilders and sediment constituents of neritic carbonates(Philip, 2003). Well-exposed successions yielding rudistbivalves have been already studied extensively in centralItaly (e.g. Chiocchini & Mancinelli, 1977; Mariotti,

New data on the relationship between shape and palaeoenvironmentin Late Cretaceous Rudists from Central Italy:

Radiolites and Distefanella (Radiolitidae)

Riccardo CESTARI

R. Cestari (present address), ENI E&P, Unità Geografica Italia, Via del Marchesato 13, I-48023 Marina di Ravenna (RA), Italy;[email protected]

KEY WORDS - Bivalves, Rudists, Radiolitidae, Palaeoenvironment, Late Cretaceous, Central Italy.

ABSTRACT - Analyses on the shell shape and structure of some rudist bivalves belonging to the Radiolitidae family have beenperformed on specimens from carbonate successions cropping out in central Italy and from Museums’ collections.

Radiolites trigeri (Coquand) and R. darìo (Catullo) have a conical to slender cilindrical right valve provided with a flat and littledeveloped left valve, the shell structure may have a well developed cellular network. These species are mainly found in Late Turonian-Santonian mud-supported carbonates of inner platform and ramp settings with medium to low hydrodynamic regime.

Distefanella salmojraghii Parona, D. bassanii Parona, D. douvillei Parona and D. guiscardii Parona have an extremely elongateright valve provided with a cupular and well developed left valve, the shell is extremely thin and made of compact calcite. They are mainlyfound in Santonian grain-supported bioclastic limestones of platform margin settings with medium to high hydrodynamic conditions.

The occurrence of these radiolitid species shows an asymmetric geographic distribution, caused by a complex physiography of thecarbonate platforms in the Mediterranean Tethys during the Turonian-Santonian (Late Cretaceous) when the East-West drivenCircumglobal Tethys Current favoured the diffusion of bioclastic Distefanella facies in the successions today facing the Adriatic side ofthe Apennine chain. On the other hand, Radiolites assemblages set in mud-supported limestone were widespread in the successionstoday facing the Tyrrhenian Sea. This asymmetric distribution should be assumed as reminiscence of a windward-leeward control oncarbonate platform growth.

RIASSUNTO - [Forma del guscio e struttura dei bivalvi Rudiste nel Cretaceo Superiore: Radiolites e Distefanella (Radiolitidae) inItalia centrale] - Nel Cretaceo Superiore le Rudiste, bivalvi appartenenti all’ordine Hippuritoidea, colonizzarono gran parte degliambienti di piattaforma carbonatica raggiungendo la loro massima diversità con specie appartenenti alle famiglie Radiolitidae eHippuritidae. In questo lavoro, vengono prese in considerazione l’organizzazione, l’assetto morfostrutturale del guscio e la distribuzionedi alcune specie appartenenti alle radiolitidi che danno utili informazioni riguardo l’ambiente di sedimentazione delle piattaformecarbonatiche che si svilupparono nell’area mediterranea della Tetide nel Turoniano-Santoniano (Cretaceo Superiore). In particolaresono stati riesaminati alcuni dati già pubblicati sull’area dell’Appennino centrale (Italia centrale), integrati con osservazioni originalinelle successioni di M. Rotondo e M. Orsello-Inghiottitoio nei Monti d’Ocre, le serie di Trevi e Fiuggi nei M.ti Simbruini, nell’area diSonnino nei M.ti Ausoni, a S. Polo Matese nei M.ti del Matese e nella Montagna della Majella.

In generale, le radiolitidi sono caratterizzate da una valva destra con forma da conica a cilindrica e valva sinistra solitamente pocosviluppata ed appiattita. La struttura del guscio è tipicamente cellulare, costruita dalla sovrapposizione ritmica di laminae e muri.

Le specie Radiolites trigeri (Coquand) e R. darìo (Catullo) sono caratterizzate da forma da conica allungata a cilindrica e valvasinistra appiattita e poco sviluppata, con struttura del guscio da prevalentemente cellulare nelle forme a minore tasso di accrescimentoa prevalentemente compatta in quelle ad elevato tasso di crescita. Esse risultano essere ben rappresentate nelle successioni fangosostenutea idrodinamismo medio-basso di piattaforma interna e di rampa del Turoniano-Santoniano.

Tra le distefanelle, le specie Distefanella salmojraghii Parona, D. bassanii Parona, D. douvillei Parona e D. guiscardii Parona sonocaratterizzate da una valva destra estremamente allungata, guscio molto sottile composto da calcite compatta e da una valva sinistracupuliforme ben sviluppata. Esse sono ben rappresentate nelle successioni prevalentemente granosostenute ad idrodinamismo medio-alto di margine di piattaforma del Santoniano.

Considerando il medesimo intervallo cronostratigrafico, le specie di radiolitidi esaminate risultano avere una distribuzione geograficaasimmetrica: le distefanelle sono ben rappresentate nelle successioni affioranti nei M.ti d’Ocre, nella Majella e nei M.ti del Matesementre Radiolites trigeri e R. darìo sono ben rappresentate nelle dorsali carbonatiche più ad occidente, quali i M.ti Simbruini e i M.tiAusoni. La distribuzione geografica asimmetrica di queste rudiste viene messa in relazione al complesso assetto fisiografico dellepiattaforme carbonatiche nell’area mediterranea della Tetide nel corso del Turoniano-Santoniano. In questo intervallo temporale lacrescita dei sistemi carbonatici era condizionata dalla corrente paleooceanica tetisiana che, spingendosi da est verso ovest, favoriva losviluppo di biofacies a rudiste (come la Distefanella) bioclastiche e granosostenute nelle successioni della catena appenninica che oggiguardano verso l’Adriatico. Verso la parte occidentale, invece, sono diffuse le associazioni oligotipiche a Radiolites (come Radiolitestrigeri e R. darìo) in successioni fangosostenute. Questa distribuzione asimmetrica deve essere attribuita al controllo sopravento esottovento che ha condizionato la crescita delle piattaforme carbonatiche nell’area italiana.

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186 Bollettino della Società Paleontologica Italiana, 44 (3), 2005

1982; Accordi & Carbone, 1988; Damiani et al., 1992;Pons & Sirna, 1992; Chiocchini et al., 1994; Simone etal., 2003).

This study focuses on some slender taxa belongingto Radiolites and Distefanella of the Radiolitidae familyreported from the Turonian-Santonian (LateCretaceous) carbonate successions cropping out incentral Apennines (central Italy) (Fig. 1). These taxahave been taken into account among the others as theyshow similar general shell features but differentrelationship with the sedimentary substrate. Radiolitestrigeri (Coquand) and R. darìo (Catullo) are mainlyfound in mud-supported limestone of inner platformand ramp environments while Distefanella salmojraghiiParona, D. bassanii Parona, D. douvillei Parona andD. guiscardii Parona are found in grain-supportedsediments of outer platform settings.

The aim is therefore to give an interpretation of themain factors that shaped the shell architecture of theseradiolitids and that favoured their asymmetricdistribution on carbonate platforms, eventually underthe influence of the Circumglobal Tethys Current(Hotinsky & Toggweiler, 2003).

MATERIALS

Assemblages yielding Radiolites and Distefanellahave been considered from carbonate successionscropping out in the Ocre Mts. (Rotondo Mt. and M.Orsello-Inghiottitoio sections in Chiocchini et al., 1994;Damiani et al., 1992) (Fig. 1a), in the Simbruini Mts.(Trevi and Fiuggi sections in Cestari et. al. 1992) (Fig.1b), in the Ausoni Mts. (Sonnino and Amaseno area;Chiocchini & Mancinelli, 1977; Damiani et al., 1992)(Fig. 1c), in the Matese Mt. (at S. Polo Matese andPiano di Corte in Accordi et al., 1990a) (Fig. 1d) and inthe Majella Mt. (Accordi et al., 1990b; Stössel, 1999;Stössel & Bernoulli, 2000) (Fig. 1e). The inferred ageof these species is Late Turonian-Santonian

(Accordiella conica-Rotorbinella scarsellai Zone andNezzazatinella picardi-Nummoloculina heimi pars Zonein Chiocchini et al., 1994). Analyses have also beenperformed by studying specimens housed in museumcollections (Bassani Collection housed at the CentroMusei delle Scienze Naturali di Napoli, Parona Collectionat the University of Turin, rudist collections at “LaSapienza” University of Rome and at the Museum ofGeology and Palaeontology of the University of Padua.Figured samples are housed at the Departamento deGeologia, Universidad Autònoma de Barcelona (labelPUAB).

Fig. 1 - Studied localities in central Italy. a)Ocre Mts. (Rotondo Mt. and M. Orsello-Inghiottitoio successions); b) Simbruini Mts.(Trevi and Fiuggi); c) Ausoni Mts. (Sonninoand Amaseno); d) Matese Mt. (S. PoloMatese); e) Majella Mt.

Fig. 2 - Main shell features of a radiolitid bivalve. The right valveis conical to cylindrical while the left valve is usually opercular(modified from Cestari & Sartorio, 1995).

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187R. Cestari - Occurrence of Radiolites and Distefanella (Hippuritoidea) in central Italy

SHELL SHAPE AND STRUCTURE

Over the last years, rudist palaeontology focusedon the relationship between morphology and substrate(Gili et al., 1995; Sanders & Pons, 1999) also basingon the comparison with other fossil groups (Seilacher,1998; Johnson et al., 2002). In particular, radiolitids’unusual morphology is the product of a peculiarconstructional pattern of the shell that could generateextremely varied shapes from conical to more or lesselongate cylindrical, characterized by a well developedright valve and usually by a smaller and operculiformleft valve (Fig. 2).

Shell structure is composed by a thin aragonitic innerlayer and by a thick outer low-Mg calcite layer, thislast often with cellular network (Fig. 3) formed duringthe vertical growth of laminae (or lamellae) delimitatinggrowth rhythms, separated by muri (Fig. 4). Thecomplex laminae stacking and their orientation duringvertical growth give rise to the cellular structure (Pons& Vicens, 2005). The genus Radiolites is marked by aright valve with conical to cylindrical shape,predominant cellular structure in the outer shell layerand with a left valve little developed. Among the nearly150 species and subspecies assigned to the genus(Sánchez, 1981; Steuber T., www.ruhr-uni-bochum.de/sediment/rudinet, A palaeontological database of RudistBivalves (Mollusca: Hippuritoidea, Gray 1848 - accessedmay 2005)), two species well represented in the studyarea have been taken into account: Radiolites trigeri(Coquand) and R. darìo (Catullo), with this last onegrouping great part of “Gorjanovicia” taxa (Cestari,1992).

In these species, the shape of the left valve is flatand provided with a weak cardinal apparatus that isinterpreted to be related to muddy substrate withmedium to low hydrodynamism (Cestari & Pons, 2005).

In the Trevi and Fiuggi successions (SimbruiniMts.), Radiolites trigeri (Coquand) (Radiolites sp. 1in Cestari et al., 1992) is found in beds formed byscattered specimens or organized in bouquets or smallclusters, set in mud-supported limestone with otherradiolitids e.g. Durania arnaudi (Choffat), Sauvagesia

sp. and a fossil assemblage indicative of the LateTuronian. Above in the same succession, Radiolitesdarìo (Catullo) is found in bouquets and clusters madeof elongate specimens in mainly mud-supportedlimestone with Bournonia sp. and Sauvagesia sp. anda microfossil assemblage of Santonian age.

Near Amaseno and Sonnino villages and in theadjacent area (Ausoni Mts.), slender radiolitids withlittle developed, flat opercular left valve belonging toRadiolites trigeri (Coquand) show well developedvertical growth rates marked in the ornamentation by“lames externes” (Pl. 1, figs. 1-2). They form smallbioconstructions made of bouquets and small clustersof specimens (Pl. 1, fig. 3) with shells made of cellularand compact calcite outer shell layer (Pl. 1, figs. 4-6),of Late Turonian age. In the same area conical tocylindric-shaped radiolitids (Pl. 2, fig. 1) belonging toRadiolites darìo (Catullo), spread on inner platformenvironments during the Coniacian-Santonian throughdifferent ecomorphotypes. Here, oligotypic andmonotypic assemblages occur in mud-supported, partlydolomitized, limestone. R. darìo is found in bouquetsand clusters often made of slender specimens (Pl. 2,figs. 2-3); low growth rates produced conicalspecimens dominated by cellular shell structure whilehigher growth rates favoured conically shapedspecimens with thin compact-dominated outer shelllayer. These species are usually represented bymonotypical to oligotypical communities in low tomoderate energy mud-supported deposits, withmoderate to high sedimentation rate, mainly in innerplatform to ramp settings.

Twenty-five species are currently assigned to thegenus Distefanella (Sánchez, 1981; Steuber, 2005),among these four species instituted by Parona (1900,1911) at S. Polo Matese village in the Matese Mts. havebeen considered: Distefanella salmojraghii Parona, D.bassanii Parona, D. douvillei Parona and D. guiscardiiParona. The left valve is cupuliform and provided witha well developed cardinal apparatus that is related to a

Fig. 3 - Radiolites sp. transverse section of the right valve. Theouter shell layer shows the cellular network (a) and a welldeveloped ligament ridge (b). Simbruini Mts. Base width = 2 cm.PUAB-74.402.

Fig. 4 - Radiolites sp. radial section of the right valve showingthe superimposition of stacked laminae (L1, L2, Ln outlinedlines) and muri (M1, M2, Mn) delimitating growth rhythms(GR1, GR2, GRn). Simbruini Mts. Base width = 0.5 cm. PUAB-74.403.

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substrate with medium to high hydrodynamism andsignificant bioclastic input (Cestari & Pons, 2005) (Pl.2, fig. 5). The right valve has an extremely elongatecylindric shape and a shell structure made of a verythin outer layer with compact calcite. Their elongateshape, the thin shell and the rapid growth favoured totheir diffusion in grain-supported and unstable bottomswith high sedimentation rates. In the type locality ofthe type species of the genus the age is Late Turonian-early Campanian (Accordi et al., 1990a; Carannante etal., 1993) or limited to the Santonian for the lack ofradiolitids with canaliculate upper valves (Cestari &Sartorio, 1995). Distefanella generally occurs inmoderate to high energy grain-supported deposits, withhigh sedimentation rate, in platform margin to distalramp environment. Distefanella is also reported inScaglia type pelagic deposits of northern Italy (Cestari& Trevisani, 2005).

In the Inghiottitoio succession (Ocre Mts.),Distefanella beds are found in grain-supportedlimestone intercalated to mud-supported facies bearingradiolitid assemblages composed of Biradiolitesangulosus (d’Orbigny), Radiolites sp., Sauvagesia sp.among others (Pl. 2, fig. 4). These beds have beenobserved in a condensed succession few tens metersthick, following the Cenomanian emersion. Thedepositional settings is referred to shelf marginbioclastic deposits of Late Turonian-Coniacian age(Damiani et al., 1992).

At San Polo Matese these taxa form oligo- andmonotypic banks and are in assemblages withBiradiolites canaliculatus (Pejovic), Sauvagesiatenuicostata Polšak, Plagioptychus paradoxus(Matheron), Milovanovicia sp., Jerinella sp. amongothers. These taxa flourished in marginal areas ofcarbonate platform (barrier banks in Accordi et al.,1990a).

In the Majella Mt., well exposed rudist assemblageswith Distefanella are reported (Accordi et al., 1990b;Stössel, 1999; Stössel & Bernoulli, 2000) along withother taxa such as Radiolites, Praeradiolites,Plagioptychus, Hippurites, Vaccinites, and Hippuritella.Distefanella form nearly oligotypic assemblages thatcan laterally extend for hundred meters. Assemblagesare set in medium to coarse bioclastic limestone (Pl.2, fig. 6).

DISCUSSION

Great part of the shells of the considered taxa weredeeply inserted in soft carbonate sediment and theycan be considered as sediment stickers (Gili et al., 1995)of semi-infauna. They could have a slender right valvewith the left valve flat and little developed as in Radiolitesor cupuliform and well developed as in Distefanella,well suited for different water regimes. The inferredrapid vertical shell accretion appears as particularlysuited for distancing the commissure and its relatedbiological activity from the substrate in a valve almostcompletely inserted in soft sediment, under highsedimentation rate conditions (Fig. 5). The differentshell shape of the left valve has to be related to the

EXPLANATION OF PLATE 1

Figs. 1-6 - Radiolites trigeri (Coquand).1 - Lateral view of the right valve, radial bands are partially broken due to compaction. Cyclic vertical shell growth reflected

in the external ornamentation. Late Turonian. Amaseno (Ausoni Mts.). Sample PUAB-74.397.2 - Dorsal lateral view of a curved right valve indicating unstable substrate. Clionid sponge borings can be observed. Late

Turonian. Amaseno (Ausoni Mts.). Sample PUAB-74.398.3 - Oblique sections of partly recrystallized right valve. A bouquet made of young specimens is attached (left). Late

Turonian. Amaseno (Ausoni Mts.). Polished slab. Base width is 4 cm. Sample PUAB-74.399.4 - Transverse section of the right valve of two specimens with well preserved cellular network. The ligament ridge can be

observed in both of them. Late Turonian. Amaseno (Ausoni Mts.). Polished slab. Base width is 6 cm. Sample PUAB-74.400.

5 - Detail of the shell structure made of compact calcite layers passing into cellular network. Amaseno (Ausoni Mts.).Polished slab. Base width is 0.7 cm. Sample PUAB-74.400a.

6 - Detail of the outer shell layer with cellular network. Late Turonian. Amaseno (Ausoni Mts.). Polished slab. Base widthis 1.2 cm. Sample PUAB-74.400a.

Fig. 5 - Sketch of an elongate Radiolites, provided with a flat leftvalve, set in mud-supported sediment (on the left) and of aDistefanella, with a robust left valve, inserted in grain-supporteddeposit (on the right). LV = left valve; RV = right valve.

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189Pl. 1R. Cestari - Occurrence of Radiolites and Distefanella (Hippuritoidea) in central Italy

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different sedimentary environments: Radiolites couldcolonize mud-supported bottoms with medium to lowhydrodynamic regime of inner platform and rampsettings while Distefanella flourished in grain-supporteddeposits with medium to high hydrodynamic conditionswhere common winnowing and sediment reworkingby waves and currents were a typical feature of shelfmargin settings.

As stated by Cestari & Pons (2005), the generalgrowth and distribution of rudist biofacies should havebeen driven, among other factors, by the atmosphericand oceanic patterns.

During great part of the Cretaceous, theCircumglobal Tethys Current flowed westwardfavouring the diffusion of bioclastic Distefanellabiofacies (among others) in the carbonate successionstoday cropping out along the eastern (or Adriatic) sideof the Apennine chain. On the other hand, rudistbiofacies with Radiolites trigeri (Coquand) or Radiolitesdarìo (Catullo), are commonly set as oligotypicalassemblages in mud-supported limestone that arewidespread in the carbonate successions today facingthe western (or Tyrrhenian) side. Apart the complexand intense geodynamic history, this general asymmetricdistribution of Rudist biofacies can be assumed asreminiscence of a windward-leeward control oncarbonate platform growth.

CONCLUSIONS

The analysis of some rudist bivalves belonging tothe Radiolitidae occurring in central Italy carbonatesuccessions and the examination of museum rudistcollections indicate that shell shape and structure ofthese radiolitids are intimately linked to the sedimentaryenvironment. Radiolites trigeri (Coquand) and R. darìo(Catullo), with this last one grouping great part of“Gorjanovicia” taxa, are reported in Late Turonian-Santonian mud-supported carbonates of inner platformand ramp settings, pertaining to isolate carbonateplatform of the central Mediterranean area during theLate Turonian-Santonian times.

Four species of Distefanella - i.e. D. salmojraghiiParona, D. bassanii Parona, D. douvillei Parona, andD. guiscardii Parona - have been taken into account;these are reported in mainly Santonian grain-supportedbioclastic limestone of shelf margin settings withmedium to high hydrodynamic regime. It has to be notedthat the geographic distribution of these taxa isasymmetric, with the Distefanella species wellrepresented in carbonate successions cropping out inthe Ocre Mts., Matese Mt. and Majella Mt., while theother two Radiolites species are well represented inmuddy deposits of the western carbonate successionsas in the Simbruini Mts. and in the Ausoni Mts.

This distribution has to be referred to the complexphysiography of the carbonate platforms in theMediterranean Tethys during the Turonian-Santonian(Late Cretaceous) that favoured the spreading of certaintaxa in the different carbonate platform environments.The diffusion of bioclastic rudist biofacies, such asthe Distefanella biofacies, in the carbonate successionstoday cropping out along the eastern (or Adriatic) sideof the Apennine chain should have to be related to theatmospheric and oceanic patterns linked to theCircumglobal Tethys Current that, during great part ofthe Late Cretaceous, flowed westward. On the otherhand, rudist biofacies with Radiolites trigeri or R. darìo,are commonly set as oligotypical assemblages in mud-supported deposits that are widespread in the carbonatesuccessions today facing the western (or Tyrrhenian)side.

Apart the complex and intense geodynamic history,this general asymmetric distribution of rudist biofaciescan be assumed as reminiscence of a windward-leewardcontrol on carbonate platform growth.

ACKNOWLEDGEMENTS

Many thanks to Prof. R. Matteucci of the “La Sapienza”University of Rome and Prof. J.M. Pons of the UniversitatAutònoma de Barcelona, who kindly followed and improvedthis work. I am also indebted to Prof. J. Pignatti for his review ofa draft of this paper. Review by Prof. A. Laviano and suggestionsby the Board of the Bollettino della Società Paleontologica Italianaare gratefully acknowledged. Thanks are due to Dott.ssa I. Del

EXPLANATION OF PLATE 2

Figs. 1-3 - Radiolites darìo (Catullo).1 - Lateral view of two right valves with conical shape. Sonnino, Ausoni Mts. Base 7 cm. “La Sapienza “ University

collection.2 - Shell concentration in mud-supported limestone. Sonnino area, Ausoni Mts. Hammer on the left as size reference.3 - Slender specimen with multigeniculate growth in soft sediment. Sonnino area, Ausoni Mts. Pen in the upper right corner

as size reference.

Figs. 4-6 - Specimens of the genus Distefanella.4 - D. douvillei Parona. Transverse section of the right valve set in bioclastic grainstone made of fragments of other

radiolitids. Inghiottitoio succession, Ocre Mts. Base width is 10 cm.5 - D. salmojraghii Parona side view of the holotype with both the valves. Note the left (upper) valve with cupular shape.

Bassani Collection, CMSN-17006.6 - Distefanella thicket made of tens slender specimens in bioclastic limestone, also with multigeniculate growth indicating

soft bottoms (e.g. specimens on the right upper corner). Focalone Mt., Majella Mt.

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191Pl. 2R. Cestari - Occurrence of Radiolites and Distefanella (Hippuritoidea) in central Italy

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Re of the Centro Musei delle Scienze Naturali di Napoli for thefacilities given in studying the Bassani collection.

Work presented at the final meeting of the COFIN Project2002-2004 “La risposta delle piattaforme carbonatiche allevariazioni climatiche: esempi dal Triassico e dal Terziariomediterraneo”, held in Modena on 8 July 2005.

This work is dedicated to the memory of my father Generoso.

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Manuscript received 01 September 2005Revised manuscript accepted 21 October 2005

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