Abstract

Three mass accumulations of sea urchins from the Miocene of Sardinia show a number of taphonomic features which set them apart from previously described echinoid assemblages from the Cenozoic in which they represent: (1) monotypic assemblages; (2) include very well-preserved remains of either regular or spatangoid echinoids; and (3) originate in deeper water environments. These accumulations are compared using a detailed sedimentological and taphonomic analysis including preservational fabrics, taphonomic signatures, size frequency distributions, density of occurrences and preferred orientations. The possible role of gregarious behaviour contributing to mass occurrences and the specific sedimentary events leading to the excellent preservation are discussed. The interpreted depositional environment of all three deposits is that of a storm-dominated, siliciclastic shelf environment. A phymosomatid assemblage represents rapid burial through obrution of a highly dense, freshly dead community. A Brissopsis-dominated spatangoid assemblage represents a mixed accumulation of parautochthonous and transported skeletons. The third assemblage consisting of regular echinoid spines and rare tests represents a composite tempestite. Differences in the depositional environments are related to their position along onshore–offshore gradient with the first two beds originated in a deeper setting than that of the spine accumulation. This study shows that the preservation of assemblages containing complete regular echinoids and spatangoids is higher in deeper water settings than in shallow water environments.

Keywords

  1. Mass occurrences
  2. miocene
  3. regular echinoids
  4. Sardinia
  5. sedimentology
  6. spatangoids
  7. taphonomy
Unlike clypeasteroid echinoids, which can frequently form mass deposits in shoreface environments and are relatively common in Oligo-Miocene sedimentary sequences (Nebelsick & Kroh ; Belaústegui et al. ; Mancosu & Nebelsick ), mass accumulations of other echinoids are rare in the fossil record. The detailed analysis of such accumulations is thus of interest with respect to taphonomic pathways and depositional environments along shelf gradients.
Sporadic mass deposits of regular echinoid remains have been reported from various clades and time periods. Aggregations of thousands of exceptionally well-preserved Archaeocidaris occur in nearshore, fine-grained sediments of the Pennsylvanian Winchell Formation of Texas (Schneider ; Schneider et al. ). Further dense accumulations include cidaroids (Temnocidaris danica) from the Paleocene of Denmark (Gravesen ), the pedinoid Diademopsis helvetica from the Early Jurassic of southern Germany (Bloos ) and very well-preserved camarodont Psammechinus philanthropus from the Pliocene Yorktown Formation of Virginia (Kier ) and Psammechinus dubius from the Middle Miocene of Poland (Radwański & Wysocka ). More recently, Jagt & Deckers described a dense fossil assemblage of the phymosomatid Gautheria from the Meerssen Member of the Maastricht Formation that preserved lanterns, peristomal membranes, apical disc plating and the spine canopy. Disarticulated regular echinoid remains can also contribute to skeletal accumulations as shown by a remarkable monospecific mass accumulation of regular echinoid spines from the Lower Triassic Virgin Limestone of Nevada (Moffat & Bottjer ).
Spatangoids also occur in mass accumulations as shown, for example, within an Albian assemblage of very well-preserved Macraster cf. polygonus from Normandy (Néraudeau & Breton ). The chaotic deposition of very well-preserved Echinocardium orthonotum from the Pliocene Yorktown Formation of Virginia was attributed by Kier to specimens being caught up in storms currents and subsequently being buried in a nearshore environment. Radwański & Wysocka proposed a similar origin of rapid burial related to storm events in a sub-littoral environment for the mass accumulation of Echinocardium leopolitanum from the Middle Miocene of the Ukraine, where the echinoids are preserved with their complete spine canopy.
This study compares three mass accumulations of echinoids from the Miocene of Sardinia that differ markedly from most previously described sea urchin assemblages with respect to depositional environment and taphonomy. Two of these mass accumulations occur in the Lower Miocene sediments of the Marmilla Formation. The first occurrence consists of a dense accumulation of a new phymosomatid echinoid that will be described elsewhere. Another assemblage consists solely of the spatangoid echinoid Brissopsis, found only four centimetres below the above-mentioned occurrence. The third mass accumulation mainly consists of the spines of an unidentified regular echinoid from the Upper Burdigalian (Lower Miocene) to Langhian (Middle Miocene) Gesturi Marls. The juxtaposition of such remains with such seemingly different preservation potentials can provide important insights into the following: (1) the dynamics and origin of echinoid mass accumulations; (2) the palaeoecology and sedimentology of shelf depositional environments; and (3) our understanding of preservational bias of the echinoid fossil record.

Gregarious behaviour and mass mortalities of echinoids

Both high population densities as a result of gregarious behaviour and mass mortalities can potentially be important in generating fossil echinoid concentrations. Gregarious behaviour has been reported for a number of extant regular echinoids such as Diadema, Centrostephanus, Strongylocentrotus, Lytechinus and Echinometra (e.g. Randall et al. ; Bauer ; Sammarco ; Andrew & Underwood ; Greenstein ; Forcucci ; Peterson et al. ; Andrew & Byrne ; Miller et al. ). Centrostephanus rodgersii, for examples, can form dense aggregation of more than 60 individuals per square metre (ind/m2). Moore et al., Camp et al. and Peterson et al. reported aggregations of Lytechinus variegatus with density ranging from 43 to 600 ind/m2. High densities of extant regular echinoids are mainly interpreted as the result of spawning events or predator avoidance strategies (Bauer ; Tegner & Levin ; Pennington ; Levitan ; Lessios ; Miller et al. ).
High-density populations have also been documented for extant spatangoids such as Echinocardium and Brissopsis. Field studies on echinoids Echinocardium cordatum show that this spatangoid can inhabit both littoral and offshore environment burrows in depths from few centimetres to about 20-cm deep in sandy and silty sediments. Ursin and Buchanan documented populations of E. cordatum from the North Sea coasts occurring offshore at depth of 30–40 m, dispersed in large discrete patches at maximum densities of 40 ind/m2. Higher densities of Echinocardium (up of 200 ind/m2) are reported from Seto Inland Sea (Japan) (Nakamura ) and from the Belgian continental shelf (Degraer et al. ).
In the Mediterranean Sea, the spatangoid echinoid Brissopsis lyrifera, a predominantly infaunal deposit feeder which inhabits soft muddy substrates (Hollertz & Duchêne ; Hollertz ) frequently occurs between 60- and 200-m depth and becomes very abundant in the deeper bathyal zone (Tortonese ; Néraudeau et al. ) where dense monospecific populations are present (Féral et al. ). Typical densities vary between 5 and 60 ind/m2 (Ursin ). Similar observations were made in the North Sea where highly dense associations of B. lyrifera and the ophiuroid Amphiura chiajei at depths ranging from 50 to 200 m (Hollertz et al. ).
Mass mortalities have been documented for both regular echinoids such as L. variegatus, Diadema antillarum, Strongylocentrotus droebachiensis, S. purpuratus and spatangoids, such as E. cordatum and Schizaster canaliferus (Schäfer, ; Glynn ; Miller & Colodey ; Stachowitsch ; Greenstein ; Beddingfield & McClintock ; Junqueira et al. ; Scheibling et al. ; Hendler ). Echinoid communities can be decimated by various biotic and abiotic factors including disease, hypoxia, abrupt salinity and temperature changes and storm events, which can dislodge echinoids and wash them up onto the beach (Schäfer, ; Pearse & Hines ; Ebelin et al. ; Riedel et al. ; Scheibling & Lauzon-Guay ; Hendler ).

Taphonomic processes affecting echinoids

A number of studies on the preservation potential of echinoid tests and spines have shown the importance of skeletal architectures, soft part connective tissues, life habits and the complex interplay of various environmental processes (e.g. Smith ; Allison ; Kidwell & Baumiller ; Greenstein, ; Nebelsick & Kampfer ; Nebelsick ; Moffat & Bottjer ; Banno ; Smith & Rader ). Experimental taphonomic investigations on regular echinoids conducted by Kidwell & Baumiller and Greenstein reveal that reworking can induce rapid disarticulation of spines and precludes the preservation of fragile complete tests. The investigated echinoids show a similar sequence of skeletal disarticulation ranging from intact to completely disarticulated coronas although there are differences in the timing of events depending on skeletal architectures and temperature conditions. Donovan & Gordon, through a semi-quantitative analysis of the fossil record of regular echinoid taxa from the Pliocene and Pleistocene of the Caribbean Region, demonstrated a good agreement with the taphonomic experiments mentioned above. Regular echinoids frequently suffer lethal and non-lethal damage and breakage that can be related to impact and predation. Ebert showed that echinoid damage caused by impact from rolling cobbles and against rocks is a common feature in the shallow water high-energy environments where the echinoids can be dislodged, particularly during storms.
Few experimental studies of spatangoid decay have been conducted under both natural and laboratory conditions. Nebelsick et al. reported the biostratinomic history of the spatangoid Schizaster canaliferous during mass mortality events under oxygen deficiency conditions in the sub-littoral, low energy, environment of northern Adriatic. Infaunal echinoids emerged from the sediments and rapidly died. Spine were lost within 4 days after death, while denuded tests remained as a whole for months to years retaining the same orientation with the aboral side facing upwards forming the basis for encrustation and colonisation by multi-species clumps (Nebelsick et al. ; Nebelsick ). Similar mass movements of benthic infauna, including B. lyrifera, to the sediment surface have been recorded in the North Sea when low values of oxygen (c. 2 ml/l) were reached (Dyer et al. ). Further reports exemplifying the sensitivity to hypoxia of B. lyrifera have been made by Diaz & Rosenberg and Rosenberg et al. Laboratory studies conducted by Banno on Schizaster lacunosus from Japan documented decay processes at different depths and temperatures showing increasingly rapid spine loss with increasing temperatures (four weeks at 20°C, five days at 30°C), while lower water temperatures drastically inhibit disarticulation with the spines remaining attached for more than 10 months.

Geological framework

The Oligo-Miocene sedimentary succession of Sardinia is located within the NNW–SSE orientated Sardinian Basin, a tectonic trough that extends from the Gulf of Sassari in the northwest to Cagliari in the south (Fig. A). Three main sedimentary cycles are recognised within the Sardinian Basin, the origin of which is subject to debate (Cherchi & Montandert ; Assorgia et al. ; Lecca et al. ; Funedda et al. ; Carmignani et al. ). The sediments of the basin show a wide range of both terrestrial and marine facies and can be highly fossiliferous including the echinoid mass accumulations described in this paper.
Fig. 1. A, distribution of Lower-Middle Miocene sedimentary rock in Sardinia. B, simplified geological map of the Villanovaforru area. C, simplified geological map of the Ussana area. The asterisk indicates the sites from which fossil echinoids were collected.
The phymosomatid echinoid and spatangoid assemblages originate from the Gennas locality (N 39°37′19″ – E 8°52′34″) located 1 km south of Villanovaforru, a small village in the Marmilla region, western-central Sardinia (Fig. A, B) and are found within the Marmilla Formation. The third regular echinoid accumulation occurs within the Gesturi Marls c. 1.5 km south of the village of Ussana (N 39° 22′15″ – E 9° 5′37″), southern Sardinia (Fig. A, C).
Both the first and the second Oligo-Miocene sedimentary cycles are present in the Marmilla region and Ussana area (Fig. ). The sedimentary sequence starts with the Oligocene to Aquitanian Ussana Formation consisting of polygenic conglomerates, breccias, sandstones and siltstones. Facies range from continental to transitional and littoral environments (Pecorini & Pomesano Cherchi ; Cherchi et al. ). The Ussana Formation is followed by the littoral siliciclastic facies of the Upper Oligocene to Burdigalian Nurallao Formation, originally named the ‘Gesturi Sandstone’ (Cherchi ). This formation is sub-divided into two members: (1) the basal Duidduru Conglomerate Member formed by polygenic heterometric conglomerates with interbedded sandstones assigned to a deltaic environment (Sowerbutts & Underhill ); and (2) the Serra Longa Sandstones Member which includes sandy-conglomerate alternations followed by fossiliferous sandstones and sandy limestones representing a shoreface environment.
Fig. 2. Stratigraphy of the first and the second Oligo-Miocene sedimentary cycles recognised in the central and southern Sardinia.
The Nurallao Formation is partially heteropic with and followed by the Villagreca Limestones and the mainly fine-grained sediments belonging to the Ales Marls and Marmilla formations (Cherchi, ; Assorgia et al. ). The Villagreca Limestones represent shallow, high energy, carbonate ramp and are dated to the Chattian to Early Burdigalian. The Ales Marls consist of grey marls with sporadic-interbedded thin turbiditic sandstones and are considered to represent bathyal deposits. The upper part of these deposits has been ascribed to the Aquitanian based on micro-fossils (Cherchi et al. ).
The Ales Marls are followed by the Marmilla Formation (Cherchi ) from which the first two echinoid assemblages described here originate. This formation is a volcano-sedimentary sequence that consists predominantly of fine sandstones, siltstones and marls with sporadically interbedded coarse sandstones, lavas and ignimbrites. The facies are interpreted to represent littoral to epibathyal environments. The Marmilla Formation is micro-palaeontologically dated as Late Aquitanian to Early Burdigalian (Cherchi ; Pomesano Cherchi ; Cherchi ; Fornaciari & Rio ; Barca et al. ).
The Marmilla Formation, which ends the first sedimentary cycle (Assorgia et al. ; Barca et al. ), is unconformably followed by the Gesturi Marls (Cherchi ), which belong to second Miocene sedimentary cycle. This informal unit mainly consists of a monotonous hemipelagic fine-grained sequence more than 500-m thick with interbedded sandy levels and tuffitic layers. The Gesturi Marls were dated using micro-fossils to the Middle Langhian (Cherchi ; Iaccarino et al. ). In the Ussana area, the Gesturi Marls are locally heteropic with and followed by the Fangario Clays consisting of highly fossiliferous bathyal clays and marls (Spano & Barca ). The Fangario Clays are covered by the Pirri Sandstones, which mostly consist of sub-littoral well-cemented sandstones. This unit was dated to Serravallian and ends the second sedimentary cycle (Barca et al. ).

Material and methods

Two thin (c. 15 cm thick) stratigraphical intervals of the Marmilla Formation and Gesturi Marls were investigated in detail with respect to faunal abundance, taphonomy and sedimentology. The observed taphonomic signatures include disarticulation, fragmentation, abrasion and encrustation. Both the fabric of the deposits in plan view and in cross-section and the orientation of fossil material relative to the bedding planes were documented. Sedimentological features were observed on cut slabs and in thin section. Orientation data of echinoid spines were analysed using the program Steronet 7 (Allmendinger et al. ). Mineral phases of the sediments were evaluated using X-ray diffraction analysis for which the samples were lightly ground in an agate mortar. XRD analysis was performed on c. 200 mg of powdered samples using conventional θ–2θ equipment (Panalytical) with CuKα wavelength radiation (λ = 1.54060 Å) operating at 40 kV and 40 mA using an X'Celerator detector. Samples are stored at the Department of Earth Sciences of Cagliari University, Italy, under registration number MDLCA23519, 23520, 23521, 23522, 23523, 23530, 23531, 23532, with the exception of sample NHMW2013/0026/0001 which has been deposited at the Natural History Museum in Vienna, Austria.

Sedimentology of echinoid mass occurrence sections

Villanovaforru/Sardara (Gennas)

Fieldwork in the Villanovaforru/Sardara region revealed a thick clastic sedimentary sequence that starts with massive bioturbated sandy facies with conglomeratic intercalations, within which very abundant disarticulated and randomly oriented ostreid remains are found. The middle part of the sequence is characterised by alternations of metre-thick coarse to medium sandstones and heterolithic fine-grained levels showing parallel and undulated lamination and hummocky cross-stratification. The upper part of the sequence in which the echinoid beds are found consists of both massive and finely laminated siltstones and claystones and very fine sandstones. Massive medium to coarse-grained sandstones, c. one metre thick, are also sporadically present. The fossil content mainly consists of pectinids (Amusium), regular echinoid spines and disarticulated fish remains.
Detailed investigation of a c. 15-cm-thick interval of the Marmilla Formation, which contains the studied regular echinoid and spatangoid mass accumulations, shows yellow silicified mudstones exhibiting a millimetre-scale lamination. Generally, parallel and gently undulating laminae range from 1 to 5 mm in thickness and are not affected by bioturbation. The contacts between the laminated layers are sharp with no evidence for erosion.

Ussana area

The regular echinoid mass occurrence was found within a fine-grained sequence of the Gesturi Marls consisting of regular alternations of parallel and undulating laminated fine sandstones and mudstones. Sandstone levels are irregular, sometimes discontinuous and from a few millimetres to a centimetre in thickness. Often the lower bed contacts are sharp and erosive. The regular echinoid remains occur within a 3-cm-thick fine, massive sandstone level, the clasts of which are composed mainly of quartz grains and mm-sized clay chips that float within the sandstone bed. The grain sizes of the quartz grains range from 0.1 to 0.2 mm, and the degree of rounding varies from angular to sub-angular. Discontinuous silty levels can be observed within this sandstone bed. The lower bed contact is erosive, while the upper bed contact is sharp.

Results

Phymosomatid assemblage from the Marmilla Formation

The assemblage is mostly monospecific, comprising regular phymosomatid echinoids with sporadically occurring small bivalves. The echinoids are relatively small and have a large pentagonal apical opening and polygeminate ambulacral plates bearing a single large crenulated primary tubercle. The primary spines are slender and long (up to 60 mm), exceeding twice the test diameter in length and tapering only slightly, distally ending in a bluntly rounded point. The lower shaft is smooth and unornamented, whereas the distal two-thirds of the shaft are ornamented by faint and widely spaced longitudinal ridges. Secondary spines are shorter, 5–10 mm long.
The echinoid remains are exquisitely preserved as external and internal moulds and include such fine details as the crenulation of the tubercles, stereom surface details and the longitudinal striations of the spines. There is no evidence of encrustation, bioerosion or predation.

Preservational fabric

Complete echinoid tests are concentrated in a single bedding plane and form a 1-cm-thick bed (Fig. A). All tests lie parallel to bedding either on their oral or on aboral surface with 52% orientated in life position with the peristome directed downward (Fig. B). The few specimens that retain parts of their skeleton are not preserved in their original high magnesium calcite. Diffractometric analysis performed on test and spines' remains indicates that the skeletons, where present, have been partially replaced by SiO2.
Fig. 3. Phymosomatid echinoid assemblage from the Marmilla Formation, Gennas, Villanovaforru. A1, complete tests without spine attached (denuded tests) (NHMW2013/0026/0001). A2, partially articulated interambulacral plates. A3, single interambulacral plate. A4, intact spines. B, complete test with their spine canopy (MDLCA23521). C1, complete test retaining articulated spines. C2, single ambulacral plate (MDLCA23522). Scale bars equal 1 cm.
Fig. 4. A, data summarising the states of articulation of the phymosomatid remains. B, dorsoventral orientation of complete and denuded phymosomatid echinoid tests. C, data summarizing the states of articulation of the spatangoid remains. D, dorsoventral orientation of complete spatangoid tests.

Disarticulation and fragmentation

The echinoid remains show different states of articulation (Figs , A) ranging from tests lacking their apical disc, but retaining articulated spines (Fig. B, C), denuded coronas lacking spines, rare isolated ambulacral and interambulacral plates and disarticulated spines. Aristotle's lanterns can be observed through the peristome in specimens which are preserved with the oral side facing upwards. Apical plates were not observed. Isolated ambulacral and interambulacral plates are separated along plate boundaries. Disarticulated apical plates, isolated jaw elements as well as smaller elements such as peristomal plates or pedicellariae were not found. Fragmentation has not been observed.

Size frequency distribution, density and orientation

The test size frequency distribution (n = 45) is dominated by a single mode of between 16- and 28-mm test diameter (Fig. ). The test remains are found in high densities with an abundance of 140 individuals per m2. Spines show a wide size range from 5 mm to more than 60 mm in length. The disarticulated spines are randomly orientated in plan view (Figs A, 8A). In cross-section, the spines lie concordant to slightly oblique to bedding.
Fig. 5. Size frequency distribution of the phymosomatid echinoids in the accumulation at Gennas; N indicates the number of specimens.

Spatangoid assemblage from the Marmilla Formation

The echinoid bed is dominated by the spatangoid Brissopsis whose tests are flattened as a consequence of sediment loading. Brissopsis tests that have an oval outline are antero-posteriorly elongated with an anterior sulcus. The petals are shallowly sunken, distally closed and divergent. The test and spine remains are preserved as external or internal moulds and do not show evidence of abrasion. Fine details such as the crenulation of the perforate tubercles can be readily observed in the external moulds of both complete test and fragments. Details of the spines such as fine longitudinal striations are also preserved.

Preservational fabric

The monospecific Brissopsis assemblage lies on a single bedding plane (Fig. A). All the spatangoid tests are crushed. Despite this fact, taphonomic relevant features such as test orientation, disarticulation and surface preservation can be readily observed. The orientation of the specimens is with either the aboral side or oral side upwards with specimens in life position being slightly more common (60%) (Fig. D). Specimens lying on their sides are not present. The echinoid deposit ranges from matrix to shell-supported and can be described as densely packed to loosely packed deposit, sensu Kidwell & Holland.
Fig. 6. Spatangoid mass accumulation from the Marmilla Formation, Gennas, Villanovaforru. A, fragmented and complete specimens with the minute spines scattered on the bed (MDLCA23523); scale bar equals 1 cm. B, fragments of Brissopsis test showing fine details such as the crenulation of perforate tubercles (MDLCA23530). C1, portion of the petaloid, and C2, spines of Brissopsis (MDLCA23531).

Disarticulation and fragmentation

Different states of disarticulation prior to crushing can be inferred (Figs C, ). The spatangoid remains consist of the following: (1) complete tests preserved together with their spine canopies; (2) complete denuded tests lacking spines; (3) variously sized test segments composed of several articulated plates; these fragments include recognisable parts of the petalodium from the aboral side of the test and the enlarged plates of the plastron from the oral side of the test; and 4, disarticulated spines which are usually completely preserved from the base to the tip with only a few fragmented portions of the shaft present. Larger fragments and isolated plates do not show intraplate fragmentation.
Post-depositional crushing results in the flattening of tests. Crushing follows plate boundaries and results in fragment aggregates containing several associated plates. Furthermore, crushing results in the oral and aboral side of the test being flattened against one another.

Size frequency distribution, density and orientation

The fact that the tests are crushed precludes exact measurements of morphological parameters. Test lengths can be approximated and range from c. 1 to 7 cm, with size distribution dominated by a single mode. The complete test and fragments are found in high densities (Fig. A). There is no observed preferred orientation of complete test in plan view. Complete spines range from 0.1 to 1.2 cm in length. The spines are randomly oriented in plan view (Fig. B).

Regular echinoid accumulation from the Gesturi Marls

The accumulation is mostly monospecific, with spines and isolated coronal plates of an undetermined regular echinoid (Fig. ) very similar to the phymosomatoid preserved in the mass accumulation described above.
Fig. 7. Regular echinoid mass occurrence from the Gesturi Marls, Ussana. A1, incomplete test with spine attached; A2, intact and broken spines (MDLCA23519). B1, complete test with their spines attached. B2, denuded test (MDLCA23520). C, polished slab showing the regular echinoid spine bed (MDLCA23532). Scale bars equal 1 cm.

Preservational fabric

Abundant echinoid remains are present in the sandstone levels and tend to be rare in the mudstone-dominated levels. Spines reach highest densities in a c. 3-cm-thick sandstone level in the middle part of the sequence (Fig. C). All remains are concordant to sub-concordant to the bedding plane. The echinoid remains are usually, although not exclusively, in contact to one another. The echinoid-bearing deposit thus ranges from matrix to shell-supported and can be described as a densely packed to loosely packed deposit sensu Kidwell & Holland.

Disarticulation, fragmentation and abrasion

Echinoid remains are characterised mainly by complete and fragmented spines and isolated coronal plates. Fully articulated tests complete with their spine canopy and denuded coronas also occur, but are rare and poorly preserved (Fig. B). Various degrees of abrasion occur ranging from relatively pristine plates showing tubercle crenulations and well-preserved spines with longitudinal striation to considerably abraded plates and spines lacking fine surface details.

Size frequency distribution, density and orientation

The spines range from 5 to 65 mm in length and are largely intact. In plan view, the disarticulated spines are consistently bi-directionally orientated (Fig. C). The spines are concordant to slightly oblique to the bedding plane.
Fig. 8. Rose diagram indicating the orientation in plan view of complete spines. A, phymosomatid echinoid assemblage from the Marmilla Formation. B, spatangoid mass accumulation from the Marmilla Formation. C, regular echinoid assemblages from the Gesturi Marls, Ussana. N indicates the number of spines counted.

Discussion

All the fossil echinoid mass occurrences differ markedly with respect to the included taxa and skeletal architectures. Taphonomic, palaeoecological and sedimentological evidence, however, indicate that all three assemblages belonged initially to a highly dense, echinoid dominated biocoenosis within soft-bottom, siliciclastic shelf environments.

Phymosomatid echinoid assemblage

Based on the taphonomic features, the phymosomatids mass occurrence is interpreted to represent an obrution deposit, a rapidly buried autochthonous accumulation. The preservation of complete tests, some retaining their spine canopy, the preferred orientation of the specimens parallel to bedding and the absence of broken spines suggest minimal reworking by currents and short residence time of dead echinoids on the seafloor. This interpretation is supported by the wide size range of complete spines and their random orientation in plan view, which exclude long transport distances that would lead to size differentiation and orientation. Incipient post-mortem decay can be inferred by the fact that all echinoid tests lack the apical disc and that most of tests are denuded (81.8%). These features suggest that the echinoid population was subject to mass mortality before being permanently buried.
The lack of the apical disc in those specimens that retain the spines is, apparently, in contrast to the decay sequence as indicated by Greenstein. The preservation of apical disc in the phymosomatids is, however, particularly rare, as noted and Jagt & Deckers. This is due to the fact that the apical plates of phymosomatids are only loosely connected to the corona and thus more rapidly lost than in many other echinoid clades. Out of thousands of fossil Cretaceous phymosomatids present in European museum collections, only a handful of specimens are known to retain parts of their apical discs.
Obrution represents rapid burial of intact organisms (dead or alive) within a few hours or a few days and, as shown by Seilacher et al., is one of the main mechanisms preserving completely articulated multi-element skeletons in fossil lagerstätten. Obrution events permanently bury the organisms through a sudden influx of sediment, which can smother benthic communities preventing the mobile benthos from extricating themselves (Brett et al. ; Dornbos & Bottjer ). Echinoderms can be particularly sensitive to smothering because their water-vascular system (connected to the ambient seawater by the madreporite) is susceptible to clogging by fine sediments (Rosenkranz ; Rousseau & Nakrem ), as noted by Schäfer, a relatively thick (centimetres to decimetres) bed of sediments can entomb motile echinoderms such as echinoids, asteroids and ophiuroids. Examples of well-preserved mass accumulations of echinoderms associated with rapid sedimentation have been reported for different groups and time periods. These examples include Palaeozoic pelmatozoan echinoderms and edrioasteroids (Brett & Baird ; Taylor & Brett ; Shroat-Lewis et al. ; Zamora et al. ) as well as Mesozoic and Cenozoic echinoids, asteroids and ophiuroids (Aslin ; Shroat-Lewis ; Zatoń et al. ; Martínez et al. ; Jagt & Deckers ; Rousseau & Nakrem ).
Events that may cause obrution include turbidity currents, ash falls and storms (Brett ; Dornbos & Bottjer, ). During storms, large amounts of sediments are brought into suspension (Reineck & Singh ), and the fine-grained fraction is transported to the deeper shelf where it is deposited in low energy environments. The reconstruction of obrution events is consistent with sedimentological observations in the study area. The sedimentary succession at Gennas mainly consists of fine sandstones to mudstones characterised by both massive bedding as well as undulating and planar parallel lamination, hummocky cross-stratification and gutter casts, which record distal storm deposition in the transition zone to offshore settings (Mancosu ). The phymosomatid mass accumulation forms a single bed within a silty succession showing parallel, undulated lamination that is interpreted to be related to storm deposition (Mancosu ).

Spatangoid mass accumulation

The taphonomic signatures of the Brissopsis mass accumulation, such as the presence of intact tests with their spine canopy, denuded tests, both in life position and overturned, and fragments, suggest that the spatangoids were exhumed prior to final burial. It is, however, difficult to prove whether exhumation happened before or after the echinoids died. As described by Smith, when infaunal irregular echinoids undergo environmental stress, they often come up to the sediment surface, where they commonly die.
The fact that a taphonomic gradient is present from well-preserved complete tests with spines to highly fragmented material suggests the presence of an admixture of exhumed dead specimens and living specimens that were mobilised by currents. This interpretations is substantiated by both sedimentological features, such as parallel undulated lamination presumably related to storm deposition, (as previously discussed for the phymosomatid assemblages) and the fact that specimens are preserved in various orientations. The high percentage of complete, fragile spatangoid tests, some of them retaining spines, and the occurrence of minute spines scattered on the bed, however, precludes long transport. This assemblage is thus considered to be a parautochthonous accumulation within a deeper water, offshore setting.

Regular echinoid spine bed

The regular echinoid bed within the Gesturi Marls consists of disarticulated pristine and broken spines, with very rare complete tests. The taphonomic features allow high-energy depositional conditions to be inferred. In addition, the bi-directionality of spine orientation indicates that shell transport and reworking occurred. This interpretation is consistent with sedimentological observations with the sedimentary facies that indicate a shelf setting with alternating fine sandstones and laminated mudstones. The echinoid mass accumulation itself occupies the lower part of a fine-grained sandstone bed which show a gentle, erosive lower bed contact marking an episode of erosion. The origin of these features is interpreted to reflect storm activity which, as discussed by Kreisa and Aigner, can generate shell concentrations through transport, scouring and winnowing in shelf settings. The high percentage of pristine spines and the poorly sorted shell material, however, suggest that echinoid remains have not been transported very far which would lead to spine damage and sorting of the bioclasts. Therefore, this densely packed spine bed is interpreted to represent a parautochthonous deposit, consisting of both fresh and considerable reworked material, the origin of which could be the result of storm winnowing. As noted by Dattilo et al., storm winnowing alone is not effective at generating shell beds, particularly in deeper water environments where winnowing is diminished as the storm wave base is approached. It is thus argued that storm winnowing represents the latest stage of reworking of an already concentrated in situ echinoid shell bed. The taphonomic and sedimentological inferences of the highly dense regular echinoid accumulation of the Gesturi Marls is thus interpreted to represent a single or composite tempestite deposited in the distal part of the transition zone between shoreface and offshore environments.

Preservation potential of echinoid mass accumulations

The origin of exceptionally well-preserved echinoderm deposits is related to a complex interplay of factors including the test architecture, population ecologies such as gregarious behaviour and mass mortalities), environmental conditions affecting taphonomic processes, sedimentary environments as well as and other factors including time averaging (e.g. Brett & Baird ; Nebelsick ; Brett et al. ).
There are a wide variety of test architectures found among echinoids which affects preservation potentials including the degree of interlocking of coronal plates (Smith ). For example, clypeasteroid echinoids possess thick skeletons, a high degree of interlocking reinforced by calcareous needles penetrating into adjacent coronal plates and internal supports which connect the oral and aboral sides of the test (Seilacher ; Nebelsick & Kroh ; Belaústegui et al. ; Mancosu & Nebelsick ). These echinoids have a higher preservation potential than other irregular echinoids such as spatangoids that often disintegrates rapidly when subject to post-mortem transportation and reworking. The other end of the preservational spectrum is represented by regular echinoids with imbricate plates and slightly interlocking plates, such as echinothurioids, cidaroids and diadematoids which dissociate rapidly after soft tissue decay and have little chance of being preserved as complete tests (Smith ; Greenstein ).
Environment is also an important factor influencing the preservation potential of echinoids (e.g. Greenstein, ; Nebelsick ). As discussed by Kier, regular echinoids have a poor fossil record of tests if compared to that of irregular sea urchins. This is related to the fact that regular echinoids diversified as grazers on firm or rocky substrates in shallow water environments, which represent areas of active erosion, while irregular echinoids evolved and diversified as deposit feeders often buried within unconsolidated substrates in areas of active sedimentation (Smith ). Although the diversity of echinoids in hard substrates restricted to regular echinoids may be higher than those in soft substrates dominated (although not restricted to) by irregular echinoids, the differential preservation potential lead to a predominance of fragmented test material in hard substrates and the possibility for the preservation of complete test in soft substrates (e.g. Nebelsick ).
There are a number of examples of very good preservation of echinoderms within mass accumulations other than echinoids, such as ophiuroids, asteroids and crinoids. Again these are related to rapid burial related to storm events in shelf environments ranging from the transition zone to offshore (e.g. Ausich & Sevastopulo ; Twitchett et al. ; Zatoń et al. ; Martínez et al. ; Rousseau & Nakrem ). Mass accumulation of echinoids are known from shallow water Clypeasteroid deposits in the Cenozoic, while those of regular echinoids and other irregular echinoids are rare.
The role of skeletal architecture and environment in preservation potential of echinoids and in the origin of mass accumulation was emphasised for clypeasteroid echinoids by Seilacher, Moffat & Bottjer, Nebelsick & Kroh, Belaústegui et al., Belaústegui et al. and Mancosu & Nebelsick. These echinoids can frequently occur in well-preserved mass accumulations and represents the main contributors to echinoid shell beds in the shallow water sediments of the Miocene period. The origin of clypeasteroid mass accumulations is related to a number of factors including their gregarious behaviour, their robust test morphologies and their habitat in the shoreface sandy environments characterised by sediment movements, rapid burial and the physical concentration of skeletal material through transport, winnowing, reworking or amalgamation.
Examples of mass accumulations of thin-shelled spatangoids and regular echinoids are rare in the fossil record. Exquisite preservation of their complete articulated skeletons require exceptional environmental conditions of deposition, such as anoxic environmental conditions, low water temperature and more usually rapid influx of sediments in otherwise relatively calm background depositional environments (Donovan ; Kidwell & Baumiller ). When they do occur, these exceptional deposits of regular echinoids and spatangoids are commonly associated with rapid sedimentation (see Table ).
Table 1. Summary of published regular echinoids and spatangoids mass accumulations in siliciclastic environments; x denotes that the feature is present.
SpeciesPsammechinus philanthropus (Regular echinoid)Psammechinus dubius (Regular echinoid)Diademopsis helvetica (Regular echinoid)Echinocardium orthonotum (Spatangoid echinoid)Echinocardium leopolitanum (Spatangoid echinoid)
Preservation style
Intact test with spine attachedXXXXX
Denuded testXXXXX
Aristotle's lanternXXX  
Apical systemX    
PedicellariaeX    
Fragments   XX
Orientation of tests
Concordant  X  
ChaoticXX XX
Genesis of accumulationRapid burial by stormRapid burial by stormRapid burial by stormRapid burial by stormRapid burial by storm
Depositional environmentShorefaceDeeper offshoreTransition zoneShorefaceTransition zone
AgePlioceneMioceneJurassicPlioceneMiocene
LocalityVirginiaPolandGermanyVirginiaUkraine
ReferencesKier Radwański & Wysocka Bloos Kier Radwański & Wysocka
The preservation of a mass accumulation of the Lower Jurassic regular echinoid Diademopsis helvetica from the sub-littoral fine-grained sediments of the South German Basin is documented by Bloos. The origin of this well-preserved regular echinoid assemblage is due to a complex biostratinomic history that included several phases of sedimentation related to storm activity. Very well-preserved mass accumulations of the thin-shelled spatangoids Echinocardium leopolitanum and the regular echinoid Psammechinus dubius occur in shelf sediments of the Miocene of Ukraine and Poland (Radwański & Wysocka, ). These tests often preserve their spine canopy and, in the case of regular echinoids, the Aristotle's lantern. Kier reported a remarkably well-preserved echinoid accumulations of the camaradont echinoid Psammechinus philanthropus which are preserved complete with lanterns, apical system, spines and pedicellariae, and the spatangoid Echinocardium orthonotum from near shore, sandy sediments of the Lower Pliocene Yorktown Formation from Virginia.
Although complete regular echinoids and spatangoids remains can occur in high densities in different depositional settings, as shown in the above-mentioned studies, the preservation of such assemblages is potentially higher in deeper water environments. In shoreface environments above fair-weather wave base, high-energy depositional events occur repeatedly, thus continually obliterated once-buried echinoderm deposits. Regular echinoids and spatangoids in shoreface environments are thus more likely to be preserved as disarticulated remains. In contrast, deeper shelf setting, below the fair-weather wave base, is dominated primarily by sediment deposition related to storm events. In these environments, echinoderms can not only be entombed by obrution events, but also they are less likely to be subsequently exhumed by both high-energy events (storms) (Kidwell & Baumiller ).
The present study demonstrates the high preservation potential of both regular echinoids and thin-shelled spatangoids in the storm-dominated shelf setting, at moderate depth, from the distal part of the transition zone to the offshore environments, just below the storm wave base (Fig. ). The differences in sedimentary and taphonomic features between the assemblages presented herein (Table ) can be correlated to their position along an onshore–offshore gradient and to the related increase in palaeobathymetry. The regular echinoid spine accumulation of the Gesturi Marls is interpreted to have originated in the distal part of the transition zone, between the fair-weather and mean storm wave base, where major storms can erode, rework, transport and generate shell deposits. The well-preserved phymosomatid and spatangoid assemblages of the Marmilla Formation were generated in deeper, more distal, offshore environments, characterised by rapid influx of very fine-grained sediments without subsequent winnowing and reworking.
Table 2. Details of taxonomic, taphonomic and sedimentological features of the studied regular echinoid and spatangoid mass occurrences.
Locality & Stratigraphy
LocalityGennas (Marmilla Formation)Gennas (Marmilla Formation)Ussana (Gesturi Marls)
AgeLower MioceneLower MioceneMiddle Miocene
Sedimentary environmentSiliciclasticSiliciclasticSiliciclastic
Thickness of the accumulationSingle bedSingle bed3 cm
Diversity and size variation
Taxonomic compositionMonospecific (Regular echinoids)Monospecific (Spatangoids)Monospecific (Regular echinoids)
Sedimentary fabric
DensityHighHighHigh
Orientation of complete specimensConcordantConcordantConcordant
Spine orientation in plan viewRandomly orientedRandomly orientedBi-directional oriented
Spine orientation in cross section
ConcordantDominantDominantDominant
ObliquePresentPresentPresent
PerpendicularAbsentAbsentAbsent
Detailed taphonomy
Spine disarticulationLow to highLow to highHigh
Plate disarticulationLowLow to highHigh
FragmentationLowLow to highHigh
AbrasionAbsentAbsentPresent
EncrustationAbsentAbsentAbsent
BioerosionAbsentAbsentAbsent
PalaeoenvironmentOffshoreOffshoreTransition zone
Genesis of accumulationObrutionTempestiteComposite tempestite
Fig. 9. Physical factors influencing echinoid preservation and variation of taphonomic features and preservation potential for regular echinoid and thin-shelled spatangoid mass accumulations in siliciclastic shelf environments. FWB = Fair-weather wave base, SWB = Storm wave base.

Conclusions

A number of conclusions can be drawn.
1.
Three different echinoid mass occurrences from the shelf sediments of the Miocene of Sardinia were studied and compared. These consist of the following: a, a monospecific assemblage of excellently preserved phymosomatid regular echinoids; b, a monospecific assemblage containing both well-preserved and transported Brissopis, an irregular spatangoid echinoid; and c, a regular echinoid spine bed with rare and poorly preserved complete tests.
2.
The mass accumulations are interpreted to be the result of both the common occurrence if not gregarious behaviour of these echinoids as well as sedimentary events leading to excellent preservation in two of the cases.
3.
All the three studied echinoid deposits can be assigned to a storm-dominated shelf environment. The phymosomatid assemblage represents rapid burial through obrution of a highly dense, freshly dead community. The Brissopsis assemblage represents an accumulation of mixed origin, combining the test from a (par)autochthonous highly dense living community as well as transport tests. The third regular echinoid mass accumulation is considered to represent a composite tempestite.
4.
Although well-preserved mass accumulations of regular echinoids and thin-shelled spatangoids can occur in different depositional settings, they have higher preservation potentials in deeper water environments.
5.
Differences in the taphonomic signatures can be related to position along the onshore–offshore gradient. The echinoid beds dominated by phymosomatid echinoids and Brissopsis originated in a deeper depositional environment than those of the echinoid spine accumulation. The best preservation of regular echinoid and thin-shelled spatangoid mass accumulations occurs in the offshore, just below the storm wave base, and is due to low energy environments with rapid influx of fine-grained sediments and the absence of reworking, winnowing and biological disturbance.

Acknowledgements

We are grateful to Carlo Cabiddu and Luigi Sanciu for their guidance and help during the field investigation. The critical comments of two anonymous reviewers helped to improve the paper.

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Information & Authors

Information

Published In

Volume 48Number 11 January 2015
Pages: 8399

History

Received: 2 December 2013
Accepted: 7 May 2014
Published online: 4 July 2014
Issue date: 1 January 2015

Authors

Affiliations

Andrea Mancosu [email protected]
Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Via Trentino 51, 09127 Cagliari, Italy;
James H. Nebelsick [email protected]
Department of Geosciences, University of Tübingen, Sigwartstra,βe10 D-72076 Tübingen, Germany;
Andreas Kroh [email protected]
Naturhistorisches Museum Wien, Burgring 7, 1010 Vienna, Austria;
Gian Luigi Pillola [email protected]
Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Via Trentino 51, 09127 Cagliari, Italy;

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