the stormy path from life to death assemblages: the formation and

378 RESEARCH REPORTS

The Stormy Path from Life to Death Assemblages: TheFormation and Preservation of Mass Accumulations of

Fossil Sand Dollars

JAMES H. NEBELSICKInstitut und Museum fur Geologie und Palaontologie, Sigwartstrasse 10, D-72076 Tubingen, Germany

ANDREAS KROHInstitut fur Geologie und Palaontologie, Heinrichstrasse 26, A-8010 Graz, Austria

PALAIOS, 2002, V. 17, p. 378–393

Clypeasteroids can be very common in Recent, shallow wa-ter environments in a variety of biogeographic settings andrepresent important members of benthic invertebrate com-munities. Mass deposits of fossil clypeasteroids are alsocommon and characteristic of many Cenozoic shallow waterdeposits. Their distribution and formation, however, has re-ceived much less attention than molluscan counterparts, al-though fossil examples are found within all three of the cly-peasteroid suborders.

A comparison of two mass deposits of scutellid clypeaster-oids from the Miocene of the Mediterranean (Gebel Gharrasection, Eastern Desert, Egypt; Alahan Section, Mut Basin,Turkey) shows common features, but also significant differ-ences. Both were formed in high energy, coarse sandy, shore-face environments. The Gebel Gharra section consists of athick, multi-event accumulation with numerous sedimen-tary features dominated by complete and fragmented skele-tal remains of a single taxon (Parascutella). The accumu-lations in Alahan represents a single, thin, multi-taxon(Amphiope, Parascutella) deposit dominated by very wellpreserved, complete specimens. Both units are interpretedas proximal storm deposits based on the general sedimen-tary environment, clast relationships, and taphonomic fea-tures.

Four factors contributing to mass deposits of clypeaster-oid sea urchins in Cenozoic sediments include: (1) their gre-garious nature with very high density populations; (2) theirrelatively robust skeletal morphology; (3) the high transportcapacity of their flattened, low density skeletons; and (4)their habitat in shoreface environments which is conduciveto physical concentrations of skeletal material. The presenceof mass clypeasteroid accumulations is compared to otherechinoderm deposits and discussed within the context oftheir rapid evolution in the Cenozoic.

INTRODUCTION

Fossil accumulations offer numerous insights into thepaleontology and sedimentology of ancient communitiesand their depositional environments. These arise from theinvestigation and interpretation of (1) the ecological pa-rameters needed to sustain large populations of contrib-uting organisms, (2) taphonomic aspects dealing withskeletal morphologies, destructive agents, and time aver-

aging, (3) sedimentological processes leading to the con-centration of skeletal elements within specific deposition-al environments, and finally (4) changes in the make upand thickness of shell beds through time (see: Kidwell andBrenchley, 1994, 1996). Dense deposits of pelmatozoanechinoderms are common in Paleozoic and Mesozoic envi-ronments (review in Brett et al., 1997b). Reports of echi-noid accumulations are rare, despite the fact that they canbe common members of the macrobenthos in both Recentand fossil sediments.

In this study, mass accumulations of sand dollars are in-troduced, described, and compared. These clypeasteroidechinoids not only represent a novel body plan developedin the Cenozoic, but also thrive in shallow water, high en-ergy, mobile sediments, an environment in which echino-derm accumulations are restricted mostly to completelydisarticulated skeletons. The fact that sand dollars can bevery common in Recent environments allow detailed ac-tualistic comparisons and ecological interpretations to bemade. This paper aims to demonstrate the genesis andprevalence of mass accumulations of sand dollars. Theseare compared to other echinoid accumulations in the con-text of skeletal evolution and changing preservation po-tentials.

Sand Dollars

Sand dollars represent extremely flattened forms of ir-regular, clypeasteroid sea urchins (Durham, 1955, 1966;Mooi, 1989) that evolved in the late Paleocene from cassi-duloid ancestors (Smith, 2001). These taxa experienced anexplosive radiation in the Cenozoic (Smith, 1984; Mooi,1990) and show a wide geographic distribution, rangingfrom the tropics to temperate settings (Ghiold and Hoff-man, 1984, 1986). The changing paleogeographic distri-bution of clypeasteroids is well illustrated by their Mio-cene acme in the Mediterranean and Paratethys, as dem-onstrated by the presence of numerous nominal species ofClypeaster, Scutella, Parascutella, Amphiope, Fibularia,and Echinocyamus. The recent Mediterranean clypeaster-oid fauna is, in contrast, restricted to a single species of thelatter genus.

Sand dollars generally are restricted to shallow water,higher energy environments and include both depositfeeding, endobenthic forms, such as Echinarachnius,Echinodiscus, Encope, and Mellita (see Ebert and Dexter,1975; Mooi and Telford, 1982; Ellers and Telford, 1984;

MASS ACCUMULATIONS OF SAND DOLLARS 379

TABLE 1—Comparison of maximum densities of living sand dollars.

Species LocalityDepth

(m)

Maximumdensities

(m2)(J 5

juvenile,A 5 adult) References

Dendraster excentricus North Pacific, California and Mexico,bays

,12 45–153 Merril and Hobson, 1970

Dendraster excentricus North Pacific, California, tidal chan-nels


Dendraster excentricus North Pacific, California, protectedouter coast


Dendraster excentricus North Pacific, California, exposed out-er coast


Dendraster excentricus North Pacific, Puget Sound 0–1 86–629 Birkland and Chia, 1971Echinarachnius parma North Pacific, Siberian coast 50 100–400 Sokolova and Kusnetsov, 1960Echinarachnius parma Sable Island Bank, North Atlantic 50 185 Stanley and James, 1971Echinarachnius parma North Atlantic, Gulf of St. Lawrence 16–20 460–660 (J) Cabanac and Himmelman, 1994Echinarachnius parma North Atlantic, Gulf of St. Lawrence 2–20 25–80 (A) Cabanac and Himmelman, 1994Echinarachnius parma Atlantic, Passamaquoddy Bay, New

Brunswick0–4 215 Harold and Telford, 1982

Echinarachnius auritus Red Sea, Northern Bay of Safaga,Egypt

0–10 3 Nebelsick, 1999

Echinodiscus bisperforatus South Coast, South Africa 6 10 Bentley and Cockroft, 1995Encope grandis Northern Gulf of California, Mexico 0.4–1.3 .500 Ebert and Dexter, 1975Encope stokesi Pacific, Panama 0–5 65 Dexter, 1977Melitta grantii Northern Gulf of California, Mexico 0.4–1.3 56 Ebert and Dexter, 1975Melitta lata Puerto Rico 0–1 16 Kenk, 1944Melitta quinquiesperforata Beaufort Inlet, Atlantic Ocean, N. Car-

olina0–1 17 Weihe and Gray, 1968

Melitta quinquiesperforata Gulf of Mexico, Panama City, Florida 7–12 133–366 Salsman and Tolbert, 1965Melitta quinquiesperforata Gulf of Mexico, Pensacola Beach, Flori-

da0–1 .10 Moore, 1956

Melitta quinquiesperforata Gulf of Mexico, Tampa Bay, Florida 0–1 6–489 Lane and Lawrence, 1980Melitta quinquiesperforata South Atlantic, Rio Grande do Sul,

Brazil5 50 Borzone, 1993

Melitta quinquiesperforata South Atlantic, Parana Coast, Brazil 0–3 508 (J) Tavares and Borzone, 1998Melitta quinquiesperforata South Atlantic, Parana Coast, Brazil 0–3 690–749 (A) Tavares and Borzone, 1998Melitta quinquiesperforata Caribbean, west central coast, Vene-

zuela0.5–6 2570 (J) Penchaszadeh and Layrisse, 1985

Melitta quinquiesperforata Caribbean, west central coast, Vene-zuela

0.5–6 87 (A) Penchaszadeh and Layrisse, 1985


3 40–2370(J)

Penchaszadeh and Molinet, 1994


3 14–28 (A) Penchaszadeh and Molinet, 1994

Harold and Telford, 1990, and references in Table 1), andpartially exposed, suspension-feeding forms, such as Den-draster (Timko, 1976; Smith, 1981; Beadle, 1989). Theyalso can be very common in benthic habitats, can contrib-ute much to the total macrofaunal biomass, and controlbenthic community structure by intensively reworkingsurface sediments (Steimle, 1990). The fact that the ecolo-gy, population dynamics, facies distribution, and taphon-omy of sand dollars in Recent environments are wellknown allows direct actualistic comparisons to fossil rep-resentatives (Nebelsick, 1999).

Clypeasteroids belong to the most robust representa-tives of a phylum whose multi-plated skeletons are notcommonly destined for preservation as complete articulat-ed specimens (see Donovan, 1991). This robustness is dueprimarily to two features shared by most clypeasteroids:(1) interlocking plates strengthening plate boundaries,and (2) internal supports joining the oral and apical sides

of the flattened test. These features readily increase thepreservation potential of these echinoids (Seilacher, 1979;Nebelsick and Kampfer 1994; Brett et al., 1997b; Moffatand Bottjer, 1999; Nebelsick, 1999). Other morphologicalfeatures relevant to the present study are the extremeflattened form of the disc shaped skeleton, the presence ofa level lower surface and slightly doomed upper surface,and the presence or absence of lunules (see Durham, 1955,1966; Mooi, 1989 for detailed morphology).

Mass Accumulations and Mass Mortalities in RecentEchinoids

Two phenomena of importance for the interpretation offossil echinoid concentrations are well known from Recentenvironments: mass accumulations and mass mortalities.Very high densities of sand dollars have been reported forvarious sand dollar taxa (Table 1) reaching maximum val-

380 NEBELSICK & KROH

ues of over 1300/m2 for the suspension feeding Dendrasterexcentricus and over 500/m2 for the deposit feeding Mellitaquinquiesperforata. These densities only can be reached bymultiple stacking of individuals either on top of each otheror upright next to one another as is the case for Dendras-ter. These dense aggregations occur in shallow water, butspatial distributions and densities can be highly variable.In some cases, patchy distributions occur despite uniformsedimentary patterns (Birkeland and Chia, 1971); therealso can be clear differences in the distributions and den-sities of juveniles and adults (Cabanac and Himmelmann,1996). Anecdotal reports of other sand dollars aggrega-tions include Arachnoides placenta and Astriclypeus man-ni off the coast of China (Liao and Clark, 1995). Furthermass accumulations of Arachnoides also have been notedoff Australian beaches by Clark (1946). It is important tonote that the values in Table 1 represent maximal densi-ties as the recorded sand dollars also occur in much lowerdensities. Furthermore, not all Recent sand dollars havebeen recorded in mass accumulations (e.g., Echinodiscusauritus—Nebelsick, 1992; E. bisperforatus—Bentley andCockcroft, 1995).

Mass accumulations also are known from regular echi-noids especially in temperate environments where hugeaggregations of Strongylocentrotus franciscanus and S.droebachiensis decimate sea grass beds (e.g., Scheiblingand Stephenson, 1984). Mass mortalities of these echi-noids, as well as that of Lytechinus variegatus, are knownto leave the sediment surface littered with moribund anddead sea urchins (Pearse et al., 1977; Beddingfield andMcClintock, 1994; Scheibling and Hennigar, 1997). Thesephenomena also have been described for the deeply bur-rowing, irregular echinoid Schizaster canaliferus from theNorthern Adriatic Sea, which are forced to the sedimentsurface and subsequently killed by oxygen deficiency cri-ses (Stachowitsch, 1984). The taphonomic implications ofthe mass mortality events of Caribbean Diadema antillar-um (e.g., Lessios, 1988) have been studied in detail byGreenstein (1989). Storm events can produce up to deci-meter-thick strandline deposits of echinoids as reportedfor Echinocardium cordatum from the North Sea (Schafer,1962). Extensive reviews of mortality events in echino-derms have been provided by Pearse and Hines (1979) forthose caused by biotic factors, and by Lawrence (1996) forthose caused by abiotic factors.

The Fossil Record of Echinoderm Mass Accumulations

Most examples of echinoderm dominated depositionalenvironments are known for Paleozoic and Mesozoic pel-matozoans (Brett et al., 1997b). Disarticulated pelmato-zoans can be very common in widespread grainstone andpackstone deposits in carbonate environments. Thesethick and geographically extensive sheet like accumula-tions (5 regional encrinites—Ausich, 1997) dominated en-tire shelves from the Ordovician to the Jurassic. Mostmass accumulations of complete pelmatozoan skeletonsare associated with obrution deposits within storm influ-enced shelves, distal siliciclastic shelves, and carbonateramps (Brett et al., 1997b). Very well preserved echino-derm assemblages also are recorded from dysoxic to anoxicbasins (e.g., Seilacher et al. 1985).

Historical descriptions of fossil echinoids often are very

detailed with respect to morphology and locality, but thereis often little or no information as to the paleoecology orsedimentary environment from which the collections weremade, despite an extensive fossil record. The compilation(Table 2) of published records of mass accumulations ofclypeasteroids with reliable taxonomic and stratigraphicconstraints has been culled from cursory descriptions oreven from photographs, as is the case of Lower MioceneAstrodapsis antiselli and Upper Pliocene Dendraster gibb-si (Clark and Twitchell, 1915). Some mass accumulationsof clypeasteroids merit short descriptions. Kier (1957), forexample, described slabs of limestone almost entirely com-posed of closely packed Echinocyamus polymorpha fromthe Eocene of Somalia. The most detailed description of aclypeasteroid mass deposit, to date, is for Vaquerosellamerriami of the Buttonbeds, Lower Miocene Temblor For-mation (Grant and Hertlein, 1938; Moffat and Bottjer,1999). These occur in a sandstone matrix and make up to80% of the total sediment volume.

The deposits described here belong to a number of so-called ‘‘Scutella—sands’’ from the Miocene of the Mediter-ranean and Paratethys areas, that contain the extinctsand dollars Scutella, Parascutella, and Amphiope. Thesedeposits have not been well described, but differences inthickness, taxonomic composition, taphonomy, and sedi-mentation are apparent. Cottreau (1913) described abun-dant Parascutella paulensis occurring in thin layers, piledup on one another, all of them in normal life position. Ne-gretti (1984) mentioned horizons dominated by the largesand dollars Parascutella striatula associated with Am-phiope elliptica. These are accumulated on top of one an-other, but with only 20% considered in normal position.Another type of accumulation is shown by Parascutellahobarthi of the ‘‘Scutellensande’’ of the Lower AustrianLoibersdorf Formation (Kuhn, 1936; Nebelsick, 1999)which is common in coarse, micaceous sands. Skeletonsare found mostly in normal position, but do not form a co-herent shell bed as they are not piled on top of one anoth-er.

There are scattered reports for accumulations of com-plete echinoid test other than sand dollars. Hundreds ofthousands of exceptionally preserved Archaeocidaris fromthe Pennsylvanian Winchell Formation from Texas are re-ported from nearshore black shales (Schneider, 2001). An-other example is the Pliocene Yorktown Formation of Vir-ginia (Kier, 1972a) where remarkable well preserved reg-ular echinoids (Psammechinus philanthropus) are pre-served complete with lanterns, apical systems, spines, andeven pedicellariae. Mass accumulations of the very wellpreserved spatangoid Echinocardium orthonotum also oc-cur in this formation. The preservation of these echinoidsin a jumbled, haphazard manner in a nearshore environ-ment lead Kier (1972a) to conclude that they had not beenpreserved in life position but, instead, were caught up instorms currents and waves, and subsequently buried bysediment soon after death. Mass accumulations of Psam-mechinus (P. dubius) complete with spines and Echinocar-dium (E. leopolitanum) also have been recorded from theMiddle Miocene of Poland and the Ukraine (Radwanski,1973; Maczynska, 1988; Radwanski & Wysocka, 2001).Other examples of dense accumulations of very well pre-served echinoids include Albian spatangoids from Spain(Macraster cf. polygonus—Neraudeau and Breton, 1993),


TABLE 2—Mass accumulations of fossil clypeasteroids.

Species Locality Age Reference

Protoscutella mississippiensis Winona Fm., Clarke County, Missis-sippi

Lower Eocene Dockery, 1980

Echinocyamus polymorpha Anhydrite and Karkar Series, Soma-lia

Middle Eocene Kier, 1957

Periarchus lyelli Moody Branch Fm., Clarke C., Mis-sissippi

Upper Eocene Dockery, 1980

Periarchus lyelli ‘‘Scutella bed’’, eastern USA Upper Eocene Cooke, 1942‘‘Scutella’’ subtrigona Mera Fm., Rumania Lower Oligocene Koch, 1884–1887; Moisescu, 1972Parascutella subrotundiformis Colli Bereci, Vicentino, Italy Upper Oligocene Stefanini, 1911; Venzo, 1933Amphiope sp. Wadi Okheider, Eastern Desert,

EgyptLower Miocene this publication

Amphiope sp./Parascutella sp. Mut basin, Turkey Lower Miocene this publicationFibularia damensis ‘‘Button bed’’, Dam Fm., Saudi Ara-

biaLower Miocene Kier, 1972b

Parascutella deflersi Gebel Gharra Eastern Desert, Egypt Lower Miocene this publicationParascutella hobarthi Loibersdorf Fm., Mollase Zone, Aus-

triaLower Miocene Kuhn, 1936; Nebelsick, 1999

Parascutella paulensis Molasse of Vence, Maritime Alps,France

Lower Miocene Lambert, 1906

Parascutella paulensis near Saint Paul trois Chateaux,France

Lower Miocene Cottreau, 1913

Parascutella paulensis Montbrison-Fontbonau, Vaucluse,France

Lower Miocene Courville et al., 1988

Parascutella striatula Cote de la Nerthe, France Lower Miocene Negretti, 1984Scutella subrotunda Globigerina Limestone Fm., Malta Lower Miocene Challis, 1979; Boggild and Rose,

1984Vaquerosella merriami Buttonbed Sandstones, Central Cali-

forniaLower Miocene Grant and Hertlein, 1938; Moffat

and Bottjer, 1999Monophoraster duboisi Camacho Fm., Uruguay Upper Miocene Zinsmeister, 1980; Mooi et al., 2000Astrodapsis antiselli Santa Margarita and Neroly Fms.,

Calif.Upper Miocene Clark and Twitchell, 1915

Laganum depressum Ras Mahar, Hedjaz, Saudi Arabia Pliocene Dollfus and Roman, 1981Encope tamiamiensis Tamiani Fm., Florida Pliocene Oyen and Portell, 2001Melitta aclinensis Tamiani Fm., Florida Pliocene Oyen and Portell, 2001Dendraster gibbsii Jacalitos and Etchegoin Fms., Cali-

forniaUpper Pliocene Clark and Twitchell, 1915

cidaroids from the Danian of Denmark (Temnocidaris dan-ica, see Gravesen, 2001), and more camarodont regularechinoids (undescribed occurrences of Tripneustes sp.from the Miocene of France and Spain, Schizechinus fromAustria—unpublished data).

Disarticulated echinoid remains also can contribute toskeletal accumulations. Mass accumulations of robustechinoid spines can be so common as to characterize Tri-assic sediments (e.g., the Upper Triassic ‘‘Cidaris—Schi-chten’’ of the Northern Calcareous Alps—see Tollmann,1976). The Early Triassic Virgin Limestone Echinoid bedat Lost Cabin Springs, Nevada, represents a 3-meter-thickmass accumulation of cidaroid spines and is interpreted asa storm debris bed (Moffat and Bottjer, 1999). It consists ofa time averaged deposit of fresh and reworked allochthon-ous material transported from shallower into a deeper wa-ter environment.

GEOLOGICAL SETTING

Gebel Gharra Section, Eastern Desert, Egypt

The main focus of this study is a ca. 3.3-m-thick bed pre-serving very dense deposits of sand dollars near the baseof the Lower Miocene Gebel Gharra section, which lies inthe Eastern Desert of Egypt, near the city of Suez (Fig.

1A). The ca. 140-m-thick section represents a generaltransgression/regression sequence with shallow marinesiliciclastics in the lower section overlain by shallow ma-rine limestones (Fig. 2A). Planktonic foraminifera, calcar-eous nannoplankton, and miogypsinids indicate an UpperBurdigalian age (planktonic foraminifera zone M2-M4,nannoplankton zone NN4, and shallow benthic zoneSB25; Abdelghany and Piller, 1999; Mandic and Piller,2001). The section is probably identical to the famous fos-siliferous locality of Gebel Gineifa (5 Gebel Geneffe) visit-ed by Fuchs (1878, 1883) and Blanckenhorn (1901).

The Miocene sediments follow non-marine sediments ofOligocene age as inferred from the regional geological sit-uation (Said, 1990). The section begins with ca. 11 metersof fluvial/fluviomarine cross bedded sands and silts partlybioturbated with rare plant remains. Thereafter, the bed(Fig. 2A, bed 3) rich in the sand dollar Parascutella, whichis the subject of the present publication, is located strati-graphically. This bed is followed by sands (bed 4) rich in bi-valves and echinoids, and then poorly exposed sands andmarls (beds 5 and 6). A ca. 40 m thick siliciclastic dominat-ed sequence follows (beds 7–15) with conglomerates, bio-clastic sandstones with pectinid coquinas (see Mandic andPiller, 2001), larger foraminiferal rudstones, and marls.The top of the section consists of ca. 40 m of carbonates


FIGURE 1—Location maps and studied localities. (A) Egypt (Gebel Gharra section, Ft. Agroud Section). (B) Turkey (Alahan section).

(beds 16.1–30) with coralline algae, green algae, corals,pectinids, oysters, and larger foraminifera; this is followedby 8 m of siliciclastics (beds 31–36) and a ca. 14 meterthick carbonate sequence (beds 37–42).

This section is rich in sea urchins including 15 differenttaxa in at least 5 different echinoid assemblages, each ofwhich represents a distinct paleoenvironmental setting(Kroh and Nebelsick, 2001). The echinoid fauna was firstmentioned by Fischer (in Laurent, 1870) and is included inthe echinoid monograph of Fourtau (1920). Barthoux andDouville (1914) mention mass accumulations of scutellidsin the Eastern Desert of Egypt which may correspond tothe present locality, although this cannot be ascertaineddefinitely from the description found in the literature.

Thick beds containing mass deposits of sand dollars alsoare found within in the same sedimentary basin, includinga locality with similar Parascutella densities in the Ft.Agroud section (Fig. 1A) ca. 20 km to the SE of the sectiondescribed here. An additional thin bed preserving a massaccumulation of the lunulate astriclypid sand dollars Am-phiope sp. was found in the Wadi Okheider (Hagul-Sukh-na Basin) ca. 50 km S of Port Suez.

Alahan Section, Mut Basin, Turkey

The Mut Basin, situated in south-central Turkey (Fig.1B) consists of a transgressive system onlaping a relict to-pography formed during the Early Cenozoic. It includes awide range of different facies within a mixed terrigenous/

carbonate setting including isolated carbonate buildups(Bassant, 1999). The age of the sediments ranges fromNN4-NN6/7 (Burdigalian—Serravalian). Echinoids arecommon in a number of different facies including spatan-goids found within deeper water marls, whereas shallowwater habitats may be dominated by clypeasteroids. Themass deposit of sand dollars described herein originatesfrom the Alahan Formation (formally considered part ofthe Mut Formation in Gedik et al., 1979) that consists ofshallow water facies including sandy muds, sands, andcoarse-grained gravels, and conglomerates.

The Alahan profile is located 12 km NW of Mut (Fig.1B). The location and sedimentary log is described in de-tail by Bassant (1999). Numerous loose blocks containinga plan view of the mass accumulation were traced back toa single bed within the section (meter 60 of log 7.2 in Bas-sant, 1999). Sand dollars and other echinoids also occur inother beds, though not in such large numbers. These ma-rine sands have been interpreted as shoreface depositswithin prograding siliciclastic cycles in an internal plat-form setting. The sediments are dated using nannoplank-ton to the NN4 Zone, which extends from the Upper Bur-digalian to the basal Langhian.

METHODS

In Gebel Gharra, a full scale section of the Parascutella-rich bed (Fig. 2B) was made by tracing the outlines of com-ponents on transparent plastic sheets. Orientation data


FIGURE 2—Gebel Gharra section. (A) Lithologic log. (B) Detailed sec-tion of mass scutellid accumulation.

for 50 specimens of each echinoid-rich layer were mea-sured using a geological compass and analyzed using theprogram StereoNett. Results include dip direction and an-gle, as well as pole axis distribution and contour plots, instereonet projections (Fig. 3). Morphometric analysis wasconducted on 112 complete Parascutella individuals (Figs.4, 5) collected across the locality (Figs. 6, 7). Taphonomicdata were collected in the field by analyzing the contentsof a ca. 5 kg large bulk sample which was assessed by cal-culating the weight percentage of the different compo-nents preserving the specific taphonomic features in ques-tion (see Nebelsick, 1992). The mass deposit within theAlahan profile also was documented by tracings on plasticsheets (Fig. 8). Numerous large blocks containing the sanddollar bed allowed a plan view of the deposit (Fig. 9). Taph-onomic characterization was conducted in the field andfrom collected material. The collected echinoid material isdeposited in the Natural History Museum, Vienna.

RESULTS

Gebel Gharra Section, Eastern Desert, Egypt

The ca. 3.3 m thick fossiliferous interval (Fig. 2B) is acoarse grained, moderately sorted calcareous sandstone.It has an erosive base that cuts deeply into underlying finesand and silt of fluvio-marine origin (Abdelghany and Pill-er, 1999). Above the base, reworked sandstone cobbles,clay clasts, and shell fragments (Kuphus, Ostrea) arefound. As a whole, the bed is very fossiliferous, includingthe mass accumulation of complete and fragmented sanddollars belonging to the genus Parascutella. Although pre-sent throughout, these echinoids are concentrated con-spicuously in three distinct layers (Fig. 2B), each varyingin thickness and stratigraphic position. The first twosand-dollar beds can be traced over the entire length of theoutcrop, which extends laterally for 300 meters.

The fauna is dominated by the sand dollar Parascutelladeflersi (Fig. 4). Other echinoids also occur, including an-other sand dollar Amphiope bioculata, as well as Clypeas-ter acclivis and Echinolampas ampla. Bivalves occur insubordinate numbers, and consist of oysters and pectinidsas well as calcified siphonal tubes of the terredid bivalveKuphus. Encrusting oysters, serpulids, bryozoans, andbarnacles occur on the echinoid tests and bivalves.

The dominant Parascutella species has a typical thin,subcircular test (Fig. 4). The oral surface is flat whereasthe apical surface with the characteristic petalodium isslightly domed. The oral surface shows distinct, shallow,polyfurcating food grooves with primary and secondarybranching, a centrally positioned peristome, and submar-ginal periproct in the posterior interambulacrum. The in-ternal morphology of the test is characterized by a densemeshwork of internal supports. The length of the testranges from 57.1 to 97.1 mm, with a mean of 75.1 mm. Thehigh correlation between length and width (Fig. 5) sug-gests that they belong to a single species.

Parascutella specimens show a wide range of preserva-tion. All specimens are denuded, lacking spines. Fragmen-tation is common, resulting in pie-shaped fragments thatconsist of numerous articulated plates. Fragmentation oc-curs along plate boundaries and often follows the perradi-al row of pores from the poriferous zones of the petals.


FIGURE 3—Directional data for dip measurements of sand dollars within layers 1–3. (A, E, I) Dip direction in 128 classes, circle perimeterrepresents 10 measurements. (B, F, J) Dip angle in 108 classes. (C, G, K) Pole axis distribution (stereo net, equal area projection, lowerhemisphere). (D, H, L) Pole axis contour plots (stereo net, equal area projection, lower hemisphere).

Outside the petals, fragmentation follows the perradialsutures of the ambulacra. Twenty one percent of the ana-lyzed bulk sample material are encrusted mostly by ser-pulids and oysters. Rare evidence of bioerosion consists ofoblong holes, up 5 mm long and up to 6 mm deep. The testsshow various orientations, and occur with both the oraland apical side facing downwards. Post-depositional taph-onomic features include in situ fragmentation of completespecimens (Fig. 7A), syntaxial overgrowths, and grain im-pressions in the skeleton. The surfaces of exposed speci-mens often are covered by an oxidized crust typical for de-sert environments.

Detailed Description of Parascutella Layers

The lowermost scutellid-layer is 25-to-30 cm thick andlies about 150 cm above the base of the interval (Fig. 2B).Its base and top are well defined. The base shows an irreg-ular undulation of up to 40 cm whereas the thickness re-mains more or less constant across the outcrop (Fig. 6B).The layer is characterized by the very common occurrenceof complete tests and large fragments of Parascutella thatare very densely packed, forming a component supportedechinoid breccia. Specimens are orientated both in life po-sition (oral side down) and upside down. The specimens

commonly show encrustation by serpulids and oysters. Nu-merous sedimentary features are present. In some cases,clear imbrication is present (Fig. 7B); in others, a jumbledpile of individuals has resulted from echinoids jamming upagainst upright specimens (Fig. 7A). Additionally, ‘‘tent-like’’ structures formed by a number of opposing steeply in-clined to vertically positioned specimens occur. Anothercharacteristic feature are ‘‘couplets’’ consisting of two spec-imens pressed together along their flat oral surfaces. Com-pass measurements show a predominant orientationaround 60% (Fig. 3A) with most of the specimens inclinedatan angle of thirty to forty degrees (Fig. 3B). Pole orienta-tions (Fig. 3C) and the contour plot (Fig. 3D) show a corre-sponding density maximum at 2408/608 with a second den-sity high at 118/788, corresponding to a flatter plane at 1698dip direction and 128 dip angle. Dip angles steeper than 708show N/S orientations. Sixty four percent of the measuredspecimens show orientations with the mouth facing down-ward.

The second (middle) scutellid-layer is similar to the first.It lies ca. 2.1 m above the base of the sandstone and variesin thickness between 30 and 50 cm (Figs. 2B, 6A). Its basealso shows an undulating relief. The base and top of thislayer, however, do not seem to be as sharply defined asthose of layer 1. In some places, this layer clearly splits into


FIGURE 4—Parascutella deflersi. (A) Aboral (dorsal) view. (B) oral (ventral) view. (C) lateral view.

FIGURE 5—Scatter diagram of the test length vs. test width of 112specimens of Parascutella deflersi.

FIGURE 6—Details of the scutellid layers at section Gebel Gharra.(A) Scutellid layer 2 showing an undulating relief and lateral splittinginto two sublayers. (B) Scutellid layer 1 showing the difference in echi-noid density between the background sedimentation (upper half) andthe echinoid accumulation (lower half). Scale bar equals 2 cm.

separate units (Fig. 6A), which amalgamate laterally. Theechinoid tests are packed densely and in contact with eachother, as in the first layer. Similar patterns of imbricationand stacking also can be observed. Measurements of dip di-rection and angle indicate a dominant orientation towards808N with a subordinate vector in the opposite direction.Most specimens are inclined at an angle of 10–308 (see Fig.3E, F). Pole orientations and the contour plot (Fig. 3G, H)reflect the shallow dips with a maximum density of pole ori-entations at the center of the stereo plot. A second densityhigh, however, shows similar concentration of pole orienta-


FIGURE 7—Details of the scutellid layers at section Gebel Gharra.(A) Scutellid layer 2 showing stacking and in situ fragmentation. (B)Scutellid layer 1 showing imbrication. Scale bar equals 2 cm.

FIGURE 9—Plan view of a slab from the scutellid mass accumulationat Alahan section showing both right side up and upside down spec-imens of the sand dollars Amphiope and Parascutella.

FIGURE 8—Cross section of the scutellid mass accumulation at Alahan section (oblique lines indicate minor faults present throughout thebed).

tions as in Level 1 reflecting the steeper dips towards 808N.The stereo plot shows that the subordinate dip direction to-wards 2608N is, in fact, restricted to shallow dipping speci-mens. As in Layer 1, 64% of the measured specimens showorientations with the mouth facing in a downward direc-tion.

The third, uppermost Parascutella—layer lies ca. 3 mabove the base of the bed and has a thickness of 20 to 30 cmacross the exposure (Fig. 2B). This layer does not have anundulatory base and is poorly defined at the base and top.It also differs in that the echinoids are packed less densely,rarely touching one another and less uniformly distributed.Furthermore, most of the echinoids are inclined slightly(between 10–208) with very few steeply inclined specimens(Fig. 3I, J). A clearly bi-directional N-S orientation is indi-cated by the measurements. The pole orientations and con-tour plot (Fig. 3K, L) underline the predominance of flat ly-ing specimens. Imbricated echinoids, jamming, and tent-like structures, as seen in layers 1 and 2, are not present.Interestingly, only 40% of the specimens show orientationswith the mouth facing downwards.

The sediments between the layers show lower densitiesof sand dollars, although they are still common (Fig. 2B).The taphonomic signatures of individual specimens do notdiffer from those of the three layers described above. Bothcomplete and fragmented specimens are present. Surface

preservation is similar including well preserved and abrad-ed forms, as well as both bare and encrusted specimens.

Alahan Section, Mut Basin, Turkey

The bed consists of a multi-taxon coquina co-dominatedby the sand dollars Amphiope (26 recognized isolated spec-imens ranging from 6 to 10.4 cm in length) and Parascu-tella (24 specimens from 6 to 9 cm in length). Both sanddollars are similar in size and morphology, but Amphiopepossesses two subrounded, closed lunules in the posteriorinterambulacra. Subsidiary echinoids are large Clypeaster(N 5 1), the cassiduloid Echinolampas (N 5 2), as well asthe small, thin shelled spatangoid Agassizia (N 5 6). Cha-otically arranged molds of bivalves and gastropods arealso present, but leached. The matrix consists of a poorlysorted, coarse, variegated sandstone, including heterolith-ic terrigenous components and small fragmented bio-clasts.

The cross section of the echinoid bed shows numeroussand dollars concentrated within a 20-cm-thick layer (Fig.8). The sand dollars mostly lie subparallel to bedding,some at angles of ca. 308. They lie in contact with one an-other and can be piled on top of each other, often bundledwithin a few centimeters. The plan view (Fig. 9) showsthem forming a dense pavement. Orientation in both lifeposition (oral surface facing down) as well as upside down(oral surface facing up) are present. The specimens are de-nuded totally of spines. The surface and ambitus are verywell preserved, evidence for abrasion is rare. Fragmenta-tion is also rare as most specimens are complete. Evidence


for encrustation is restricted to a single small oyster on anAmphiope test.

DISCUSSION

Interpretation of General Depositional Environment

The sedimentary record of both mass deposits suggestsa high energy, coarse sandy, shoreface environment. Thisis consistent with the ecology of Recent and fossil sand dol-lars as deposit feeding, shallow burrowers in such envi-ronments (see Nebelsick, 1999). The shoreface environ-ments occupied by these echinoids are favorable to physi-cal concentrations through various processes, such astransport, winnowing, and amalgamation of skeletal ma-terial (see Kidwell and Brenchley, 1996). The presence ofthe Gebel Gharra accumulation at the base of a transgres-sion is conducive to its preservation. The continued deep-ening of the nearshore sedimentary environment duringthe transgression would have prevented these featuresfrom being cannibalized by successive events.

Comparison of the sedimentologic and taphonomic fea-tures of the two localities reveals both similarities and dif-ferences in the assemblage characteristics. Both mass ac-cumulations occur within sedimentary sequences wheresand dollars are common. Both consist of laterally exten-sive beds, containing completely denuded adult speci-mens. The Gebel Gharra section consists of a thick, multi-event accumulation dominated by complete and fragment-ed examples of a single taxon (Parascutella), in which thesand dollars are heavily abraded and encrusted. Sedimen-tary features of the mass accumulations are common im-brication and specimen jamming. The deposit in Alahan,in contrast, represents a single, thin, multi-taxon depositdominated by very well preserved, complete specimens;some indication of imbrication is present, other sedimen-tary and taphonomic features are rare.

Sand Dollars as Sediment Clasts

The significance of the low density echinoderm stereomand resulting high hydrodynamic mobility for the forma-tion of concentrated deposits has been discussed in thecontext of Paleozoic encrinites (e.g., Ausich, 1997, Brett etal., 1997b). It is important, while considering possibletransport mechanisms, to consider the hydrodynamicproperties of the sand dollar clasts with their thin shapes,flat oral surfaces, and slightly doomed apical surfaces.These hydrodynamic properties have been examined indetailed investigations on current flow and feeding forDendraster (O’Neill, 1978). In addition, the effect of liftand drag has been studied on non-lunulate sand dollars(Telford and Harold, 1982).

The low specific weight of sand dollar skeletons meansthat they can be transported readily by water currents.Lower transport velocities result in specimens orientedparallel to the bedding plane. Increased water velocitiescombined with the additional lift due to the cross-sectionalshape (similar to an airfoil) implies a high probability ofchaotic transport leading to irregular orientation of speci-mens. Reduction in transport velocities will lead to orien-tation of entrained specimens. This seems to be reiteratedby the characteristic orientation of the fossil sand dollars

in the study areas. They are arranged, in part, chaotically,sometimes showing clear imbrication, but often orienta-tions are more or less parallel. The observed imbricationand stacking phenomenon are indicative of high transportvelocities. The observed Parascutella-‘‘twins’’ with facingoral surfaces may represent hydrodynamic stable struc-tures, but this has to be tested in flume experiments.

Both lunulate (Amphiope) and non-lunulate sand dol-lars (Parascutella) are present in these investigated massdeposits either dominating the assemblage or present assubsidiary components. The lunules of sand dollars alsohave been tested experimentally for their hydrodynamicproperties by Telford (1981, 1983) who showed that theirpresence can force the sand dollar onto the substrate.There is a critical velocity, however, at which the sand dol-lars become dislodged (e.g., 43 cm s21 for a 94 mm longMellita quinquiesperforata; Telford, 1983). The fact thatthe presence of lunules did not play a significant role inthe inclusion of sand dollars as sediment clasts within theinvestigated mass deposits suggests that higher transportvelocities were present that played a significant role in thedistribution of components. Detailed flume experimentswith sand dollars tests are, however, needed to furthercharacterize the nature of hydraulic transport of theseclasts.

GENERIC INTERPRETATION OF MASSACCUMULATION

The polymodal fabrics within the studied beds, especial-ly in the Gebel Gharra section, suggest that several sedi-mentary processes acted to produce the observed mass ac-cumulations and associated sediments. The genesis of fos-siliferous deposits can be analyzed with respect to eventvs. background sedimentation. Event sedimentation isrepresented by at least some of the high density scutellidlayers, background sedimentation by the sediments be-tween echinoid accumulations. Differences in taphonomicsignature and sedimentologic features of the high densitylayers point to the presence of different genetic processes.Four different possible mechanisms for producing the ob-served mass accumulations are discussed: (1) in situ win-nowing, (2) storm induced supratidal accumulation, (3) ob-rution, and (4) proximal tempestites. These depositionalmodes certainly are not exclusive; for example, proximaltempestites can lead to obrution deposits (Brett et al.,1997a). The internal complexity of some of the beds may,in fact, indicate multi-process origins for these accumula-tions.

Background sedimentation is represented by the sedi-ment between the high density layers in the Gebel Gharrasection. The preservation of individual specimens does notdiffer from those within the enriched layers. Both com-plete and fragmented, well preserved, as well as abradedand encrusted, specimens occur. These sediments repre-sent higher energy, coarse sandy, shoreface environmentswith high sand dollar densities as commonly found in oth-er Cenozoic shallow water environments. After death, theskeletons were exposed on the sediment surface and sub-jected to fragmentation, abrasion, encrustation, and bio-erosion. Concentration processes and events led to theiramalgamation within distinct beds as discussed below.


Reworked, in situ Accumulations

The third Parascutella layer of the Gebel Gharra sectionis interpreted to represent an in situ accumulation of sanddollar skeletons. The higher densities of sea urchins with-in this layer, as opposed to densities found in intervalsrepresentative of the background sedimentation, mayhave resulted from either higher population densities orincreased wave generated turbulence and winnowing,leading to an enrichment of the relatively large echinoidskeletons. The specimens in this bed are mostly complete,only slightly tilted, and do not form a dense aggregation ofsand dollars. Bi-directional dip deposits of slightly in-clined specimens suggest wave reworking which wouldalso lead to a high number of overturned specimens. Al-though winnowing probably occurred in this high energyenvironment, it is not believed that this layer representslong term effects of sedimentary reworking and conden-sation that resulted in hiatal concentrations (Fursich,1978), or reworked fossils associated with erosional trun-cation (5 lag concentrations; Kidwell, 1991). The compo-nents are not restricted to very highly resistant bioclasts,nor is their any indication of firmgrounds or hardgrounds.Biostratigraphic condensation also has not occurred. Fur-thermore, the observed components do not show differenttaphonomic signatures than those in the background sed-iments and first and second scutellid layers.

This interpreted depositional environment is similar tothe one reconstructed for the Miocene Buttonbed Sand-stones of central California (Moffat and Bottjer, 1999) orthe shingled coquinas depicted from the Miocene of Franceby Seilacher (1979). In these localities, sand dollars arepreserved shifted along the flat oral surface leading to adense, imbricated accumulation of dead shells.

Storm-Induced Supratidal Event Accumulations

Monospecific, storm-induced supratidal beach depositshave been reported for molluscs (Boyajian and Thayer,1995) and echinoids (Schafer, 1962). The mass transportonto the shore of the irregular, infaunal echinoid Echino-cardium during a storm, as reported in Schafer (1962), re-sulted in a 5 km long, 8 m wide, and 30 cm deep accumu-lation of complete, well preserved skeletons. These were,however, quickly reduced to very small fragments throughwave activity, such that none of the original deposit waspreserved after a few days to weeks. Although they werecertainly more robust than thin shelled spatangoids, it isstill unlikely that even Parascutella or Amphiope skele-tons would survive the repeated wave agitation of a shore-line environment. This, together with the sedimentologi-cal evidence, excludes supratidal accumulation as the de-positional environment for either locality.

Obrution Deposits

Obrution deposits represent rapid burial of organismskilled by mass mortality events leading to exceptionalpreservation of completely articulated multi-element skel-etons (Seilacher et al., 1985; Brett and Seilacher, 1991;Brett et al., 1997a). Obrution deposits containing echino-derms are most common in low energy environments,slightly below normal storm wave base which is deeper

than the interpreted sedimentary environment of thestudied sections (Brett and Seilacher, 1991; Brett et al.,1997a). The complete smothering of a very dense livingpopulation (Table 1) potentially could produce the ob-served high densities of fossil sand dollars. The investigat-ed deposits do not, however, represent a census assem-blage (see Kidwell and Bosence, 1991). The sudden obru-tion of a living or recently dead population would lead tothe burial of very well preserved specimens, which is cer-tainly not the case in layers 1 and 2 of the Gebel GharraSection. These horizons contain a time averaged assem-blage of denuded specimens which are often highly frag-mented and heavily encrusted. The Alahan deposit (Figs.8, 9) is a more likely candidate for obrution as it representsa relatively thin accumulation of very well preserved spec-imens; but, articulated spines are again not present. Fur-thermore, the uncharacteristic dense mixture of differentechinoid genera suggests that Alahan does not represent aprimary assemblage. In fact, no described in situ pre-served mass accumulations as documented in the Recent(Table 1) have been reported yet from the fossil record al-though other examples require detailed study to deter-mine this condition.

Proximal Storm Deposits

The first and the second scutellid layers of the GebelGharra section represent event condensations by proxi-mal storm deposits based on bed morphology, clast rela-tionships, and taphonomy. High energy transport is sup-ported by: (1) the erosive, irregularly scoured base whichundulates over a height of 40 cm; (2) the persistency of in-ternal layer morphology across large distances (at least300 m); (3) directional transport as indicated by steep dipdirections; and (4) clast relationships with densely packedimbricated-and-jammed sand dollar skeletons. In theserespects, these layers are similar to other storm events re-sulting in shelly coquinites containing parautochthonousdebris and non-horizontal, oblique and edgewise compo-nent orientations (Brett et al., 1997a). Aguirre et al. (1996)also report the presence of channelized bases, amalgam-ated beds, and densely packed, very common fossils con-sisting of mostly fragmented and abraded disarticulatedvalves in Lower Pliocene storm accumulations of pectinidbivalves.

The tent-like structures consisting of opposed steeplyinclined to vertically positioned sand dollars are interpret-ed as escape structures (fugichnia) which can be commonin storm deposits (see Pemberton and MacEachern, 1993).This structure results from burrowing animals trappedbeneath the stacked sand dollar shells, pushing them-selves through the echinoid layer tilting the specimens oneither side of the burrow. Similar perpendicular reorien-tation of biogenic clasts by bioturbators also is reported byAguirre et al. (1996).

Post-mortem deposition of a large number of individualsis indicated by the presence of encrusted specimens. En-crustation of infaunal, irregular echinoids most certainlyoccurs after death and subsequent exposure on the sedi-ment surface. Encrustation can be common on both Re-cent and fossil examples of infaunal echinoids (compareNebelsick et al., 1997). The presence of both encrusted andnon-encrusted shells demonstrates that time averaged ac-


cumulation of shells are included within the deposit. Rel-atively short transport distances are indicated by the pres-ence of reworked parautochthonous shells of local prove-nance, as sand dollars are also common in the sedimentsbetween the echinoid rich layers.

The internally complex layer development with amal-gamated horizons in Layer 1 and 2 of Gebel Gharra dem-onstrates that these layers may represent multiple events.The differences in dip orientation between the first andsecond layer may be indicative of wave reworking of thesecond layer, the top of which is characterized by a higherrelative amount of fragmented skeletons.

The mass accumulation within the Alahan section alsois interpreted as a storm deposit. This is based on the un-dulating base of the bed, the variable orientations of com-ponents, presence of imbrication, and accumulation of dif-ferent species within the same bed.

COMPARISON TO OTHER ECHINODERM DEPOSITS

The described mass accumulations of articulated echi-noid skeletons present a radical change from previousPhanerozoic echinoderm deposits. This pertains not onlyto the echinoderm clasts, but also to the depositional en-vironment. Clypeasteroids represent the main contribu-tors to echinoderm shell beds in the Cenozoic. In this role,they replace pelmatozoans which dominate encrinitesfrom the Ordovician to the Jurassic (Ausich, 1997). Thistaxonomic shift reflects not only the general withdrawal ofstalked pelmatozoans from shallow water environments,but also the radiation of clypeasteroids into a new settingfor echinoderms—the shoreface environment. Echino-derm remains are generally rare in Paleozoic and Meso-zoic shoreface environments, with only a few occurrencesof articulated specimens in distinct bedding planes attri-buted to sudden burial by redeposited sands (e.g., in Or-dovician and Devonian Starfish beds—Goldring and Ste-phenson, 1972). Although disarticulated pelmatozoansare obviously very common in encrinites (Ausich, 1997),articulated crinoid ossicles again only occur on rare bed-ding planes and in scattered accumulations.

The lack of echinoderm remains in Pre-Cenozoic, shore-face environments has a two fold origin. Firstly, as report-ed by Brett et al. (1997b), the shoreface environment is un-suitable for colonization by a majority of echinoderm dueto a high concentration of suspension load, an unstablesubstrate, and abrasive nature of the environment. Sec-ondly, many of the echinoderms that do inhabit higher en-ergy environments lack rigidly sutured tests and normallywill not survive post-mortem transport and reworking ascomplete skeletons. The evolution of clypeasteroids in theCenozoic broke this trend as they not only flourish in shor-eface environments, but also possess relatively robustskeletons that can survive a high degree of transport andabrasion.

The evolution of clypeasteroids decidedly increases theoccurrence of echinoid shell beds in the rock record. Thetests of sand dollars with their relatively thick skeletons,plate locking, and internal supports have a higher chanceof preservation than those of regular echinoids and otherirregular echinoids. Thus, clypeasteroids are includedmore commonly in shell beds than other echinoids. Thelack of records for regular echinoids with shingled (echin-

othuroids) or abutting plates (diadematoids) is not sur-prising considering their poor preservation potential. Thishas been shown by the lack of a taphonomic spike in thesedimentary record following the Diadema mass mortalityevent in the Caribbean (Greenstein, 1989). The exception-al preservation of Archaeocidaris (Schneider, 2001) showsthat weakly sutured echinoids also can be included inmass accumulations. The prevalence of cidaroid spines inthe rock record (Greenstein, 1992) and their inclusion inmass accumulations (Moffat and Bottjer, 1999) clearly re-flects the higher stability of their robust spines comparedto that of the test.

There may be a parallel, less dramatic rise in the pres-ence of regular echinoid mass accumulations during theCenozoic. This reflects the concurrent rise of camarodontechinoids (including Psammechinus, Schizechinus, andTripneustes) that are more resistant to disarticulationthan other regulars (Kidwell and Baumiller, 1989; Green-stein, 1991). Exceptionally preserved spatangoids also canoccur in mass accumulations despite their thin shells, asshown by Echinocardium accumulations from the York-town Formation (Kier, 1972a) and Middle Miocene fromthe Ukraine (Radwanski & Wysocka, 2001). It is, however,not clear to what extent these represent laterally exten-sive, continuous shell beds. In general, more data areneeded on the taphonomy and sedimentary environmentof these accumulation to allow for detailed comparisons.

MASS ACCUMULATIONS AND THE EVOLUTION OFCLYPEASTEROIDS

Clypeasteroids are one of the youngest major clades ofmarine invertebrates to have evolved. Their rapid expan-sion is attributed to highly successful colonization of high-er energy shallow water environments due to their novelconstructional morphology and feeding strategies (Sei-lacher, 1979; Smith, 1984). The occurrence of mass accu-mulations of clypeasteroids roughly mirrors the evolutionand geographic expansion of this clade. The Miocene acmeof scutellid sand dollars in the Mediterranean region, forexample, is reflected readily in the prevalence of knownmass accumulations (Table 2). The geographic distribu-tion of these accumulations follows that of the known dis-tribution of fossil and Recent clypeasteroids (Ghiold &Hoffman, 1984, 1986). All three suborders of the Clypeas-teroida (Clypeasterina, Laganina and Scutellina) contrib-ute to dense accumulations. These are not constrained bythe size and morphology of the clypeasteroids, with clastsranging from the minute Fibularia and Echinocyamus(smaller than 1 cm in size) to larger sand dollars (exceed-ing 10 cm in diameter). Both lunate and non-lunate formsare included, and are not restricted to extremely flattenedsand dollar morphologies as pea shaped (e.g., Fibularia)and button shaped (e.g., Vaquerosella) forms also are in-cluded. Although different processes are suggested fortheir origin, the accumulations seem to be restricted to theshallow water, coarse sandy environments in which mod-ern clypeasteroids thrive. Given the widespread taxonom-ic and geographic occurrence of these mass accumulations(Table 2), it is expected that more undescribed examples ofother clypeasteroid species should be encountered.

This study documents that echinoid concentrations canbuild significant shell beds. They are comparable to Ce-


nozoic mollusc accumulations as far as taxonomic diversi-ty (with monotaxic and polytaxic assemblages), thickness(up to 1.5 m for single beds—Moffat and Bottjer, 1999),and complexity (including composite, polymodal origins)are concerned. Kidwell and Brenchley (1994, 1996) postu-lated that an increase in rates of bioerosion, durophagouspredation, and bioirrigation during the Mesozoic led to anincrease of taphonomic pressures and, thus, influencedthe preservation of components within bivalve-dominatedshellbeds. The response to this pressure led to an increaseof hardpart durability, larger body size, infaunal habit,and cheaper and faster growing organic microstructures.Increased predation pressures are believed to drive theevolution of irregular sea urchins to deeper and more effi-cient burrowing techniques, as well as increasing spineprotection on the apical surface (McNamara, 1994). Thefact that significant echinoid shell beds first developed inthe Cenozoic is linked primarily to the evolution of cly-peasteroid echinoids with their novel architecture. Thisnot only allowed echinoids to colonized shoreface environ-ments. It also, as a side affect, insured that their skeletonshad an increased chance of survival in these shallow wa-ter, higher energy environments.

SUMMARY

(1) Clypeasteroids, especially sand dollars, form verylarge standing populations in shallow water environ-ments. These are known from various Recent taxa andfrom different geographic settings. Although in situ pres-ervation of such high density populations as fossil shellbeds has yet to be described, their presence in modern set-tings demonstrates that ample skeletal material can bepresent for their inclusion as clasts in mass deposits.

(2) The clypeasteroids possess a relatively stable, robustskeletal morphology, and represent the most likely echi-noderm to be preserved as ‘‘complete’’ skeletons in near-shore settings. Clypeasteroids possess a number of fea-tures suited to the higher energy habitats, including inter-nal supports and plate interconnections. Their character-istic flattened shape, combined with the low densityechinoderm stereom and resulting light weight, lead to ahigh transport capacity of their skeletons. This shape isconducive to imbrication allowing a very tight stacking ofindividuals. It also results in aligned orientations allowingdip measurements to be made and interpreted.

(3) The shoreface environments occupied by these echi-noids are favorable to physical concentrations throughvarious processes, such as transport, winnowing, andamalgamation of skeletal material. Variations in the pres-ence and magnitude of these processes can, however, leadto a variety of depositional characteristics ranging fromthin, single event deposits to thick, multi-event concentra-tions. Although sharing many sedimentogic and tapho-nomic features, these can differ strongly as to thickness,lateral distribution, clast orientation, density of deposits,and preservation of components.

(4) The evolution of Clypeasteroids dramatically in-creased the presence of echinoderm skeletal deposits inshoreface environments. Mass accumulations of sand dol-lars also represent the main contributors to echinodermshell beds in the Cenozoic. The distribution of clypeaster-oid accumulations in the Cenozoic roughly follows the ex-

pansion and distribution of this clade from the Eocene on-wards. Examples are found in all three clypeasteroid sub-orders. They reflect the acme of clypeasteroid develop-ment in the Miocene of the Mediterranean area. Theirpresence approximately matches that of their past andpresent biogeographic distribution.

ACKNOWLEDGEMENTS

This project is supported by the German Science Foun-dation (DFG): project No. NE537/5–1 to James H. Nebel-sick (University of Tubingen) and STE 857/1–1 to Fritz F.Steininger (Senckenberg Museum, Frankfurt), and theAustrian Science Foundation: project No. P11886-GEO toWerner E. Piller (University of Graz). We thank OsmanAbdelgany, Johann Hohenegger, Oleg Mandic, WernerPiller, Michael Rasser, Hannes Reisinger, Jurgen Schlafand Frithjof Schuster. We thank two anonymous review-ers for helpful comments.

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