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Taphonomic comparison between Recent and fossil sand dollars
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Palaeogeography, Palaeoclimatology, Palaeoecology 149 (1999) 349–358
Taphonomic comparison between Recent and fossil sand dollars James Nebelsick * Institute and Museum of Geology and Palaeontology, Sigwartstrasse 10, D-72076 Tu¨bingen, Germany Received 28 January 1997; revised version received 9 July 1997; accepted 8 June 1998
Abstract The taphonomy of a Recent and a fossil sand dollar are compared. The recent Echinodiscus auritus originates from a shallow-water carbonate environment in the Red Sea. The fossil Parascutella ho¨barthi is found in micaceous sands of the Lower Miocene Austrian Molasse Zone. Both species show strong similarities in constructional morphology including the flattened overall shape, details of the surface morphology as well as the presence of an internal support system. Three taphonomic features are remarkable different: (1) lethal and non-lethal predation (especially high in the recent E. auritus; (2) encrustation of the test (especially by coralline algae in the fossil Parascutella); and (3) radial cracking on implosion of the body cavity caused by sediment loading on the fossil tests. Differences in taphonomic signatures can be correlated with depositional environment (predation and encrustation) as well as factors related to burial of the fossil specimens (sediment loading and diagenesis). 1999 Elsevier Science B.V. All rights reserved. Keywords: taphonomy; echinoids; clypeasteroids; Miocene; predation; encrustation
1. Introduction The preservation of the echinoids as in all fossils is dependent on the nature and intensity of taphonomic processes affecting the organisms (Fig. 1), whereby a ‘poorly preserved’ skeleton can, in fact, convey more information concerning the palaeoecology of the fossil and its environment. The preservation of echinoids can be related to three main factors (Fig. 2). These are skeletal morphology, environmental factors, and temporal effects (Nebelsick, 1995). In this study a direct taphonomic comparison is made between a fossil sand dollar (Parascutella ho¨barthi) from the Lower Miocene of the Austrian Molasse Zone and a Recent species from the Red Sea (Echinodiscus auritus), allowing the relative importance of these factors to be evaluated and compared. This direct comparison is Ł E-mail:
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appropriate due to similarities not only in the constructional morphology (flattened form with internal support systems) and test surface characters, but also in the environments in which they are found (shallow, subtidal sands). Numerous Recent observations concerning the biological and physical factors affecting the preservation of these sea-urchin tests can thus be incorporated in the comparison. Echinoderms obviously represent a special case among invertebrates as far as their skeletal construction is concerned. Although unified by the possession of a stereom and a multiplated skeleton, echinoderms show a wide range of skeletal morphologies and preservation potentials. This has been shown in an increasing number of modern studies concerning specific echinoderm taxa or taphonomic processes affecting them (Meyer and Meyer, 1986; Kidwell and Baumiller, 1990; Donovan, 1991; Greenstein, 1991, 1992; Gordon and Donovan, 1992; Nebel-
0031-0182/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 8 ) 0 0 2 1 1 - 9
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Fig. 1. Schematic diagram of the taphonomic processes leading to different preservations of an echinoid. The presence of a taphonomic feature is indicated by the thicker line (modified after Nebelsick, 1995).
sick, 1992, 1996; Nebelsick and Kampfer, 1994). The sand dollars represent extremely flattened forms of clypeasteroid sea-urchins (Durham, 1966; Mooi, 1989) and are among the most successful groups of Recent echinoderms (Smith, 1984; Mooi, 1990). These irregular echinoids show a wide geographical distribution occurring in both tropical and temperate settings (Ghiold and Hoffman, 1984, 1986). They are generally restricted to shallow-water, higher-energy environments. They include both deposit feeding, endobenthic forms (such as Echinarachnius, Echinodiscus, Encope and Melitta) (Bell and Frey, 1969; Stanley and James, 1971; Ebert and Dexter, 1975; Bentley and Cockcroft, 1995), as well as the partially exposed, suspension-feeding Dendraster ex-
centricus, an echinoid well known from the west coast of North America (Birkeland and Chia, 1971; Timko, 1976; Smith, 1981; Beadle, 1989). Sand dollars have an extensive fossil record (Wagner, 1974; Zinsmeister, 1980; Dodd et al., 1985; McKinney, 1985) including such extinct genera as Scutella and Parascutella which are characteristic for many Miocene rocks of the Mediterranean and Caribbean areas as recorded not only in numerous historical monographs, but also in the modern literature (Philippe, 1983; Boggild and Rose, 1984; Poddubiuk and Rose, 1984; Courville et al., 1988). These sand dollars can occur as isolated specimens, but are often found accumulated in great numbers in so called ‘Scutella-sands’ (as in the study area).
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m. It is most common in a relatively restricted area with poorly sorted, fine sands (Piller and Mansour, 1990) where it reaches maximum densities of 2–3 per m2 . Both juveniles and adults are found in the same environment. They live shallowly buried in the substrate and, if extracted, bury themselves within 10 min. This distribution is similar to that observed for another species of Echinodiscus (E. bisperforatus) from the South African south coast (Bentley and Cockcroft, 1995). 2.2. Parascutella ho¨barthi from the Lower Miocene Austrian Molasse Fig. 2. Taphonomic triangle showing three factors affecting taphonomic processes (modified after Nebelsick, 1995).
2. Material and methods 2.1. Echinodiscus auritus from the Red Sea Echinodiscus auritus Leske, 1778 is a large IndoPacific sand dollar reaching a maximum size of 15 cm. It shows a number of highly developed test characteristics. These include deeply incised posterior paired lunules, a very complex polyfurcating food groove system leading to the centrally positioned peristome (mouth) (Fig. 3) and a cover of highly differentiated minute spines. The periproct (anus) is located midway between the peristome (mouth) and the posterior ambitus (rim). It has an extensive, dense, internal support system consisting of skeletal connections bridging the gap between the oral (lower) and apical (upper) surface. The internal supports are distributed between the central cavity containing the main internal organs of the sea-urchin and the ambitus. Cage experiments showed that dead Echinodiscus loses its minute spines within a few days (Nebelsick and Kampfer, 1994). It is also susceptible to plate disarticulation, which, once initiated, quickly leads to the disintegration of the complete test into larger and smaller fragments (single plates). Live and dead Echinodiscus were studied in the Northern Bay of Safaga, Red Sea, Egypt, a shallow-water carbonate environment (Nebelsick, 1992, 1996; Nebelsick and Kampfer, 1994). This species is found in coastal sands in depths of less than 10
P. ho¨barthi (Ku¨hn, 1936) is a Miocene sand dollar restricted to a single locality in the Molasse Zone of Lower Austria (Ku¨hn, 1936; Schaffer, 1962; Steininger, 1971). The accumulation of a large number of specimens allows detailed palaeoecological and taphonomic analysis. Numerous specimens were examined from museum collections (Naturhistorisches Museum, Vienna, Austria; Institute of Palaeontology, Univ. Vienna, Ho¨barth Museum in Horn, Austria, and the Krahuletz Museum in Eggenburg, Austria). Additional specimens were recovered from field excavations which delivered important sedimentological, palaeoecological and taphonomic information. The sediments in which these echinoids are found are known as the ‘Scutellensand’ and belong to the Loibersdorf Formation. This Formation is part of a complex series of highly differentiated, shallow-water sediments near the southern margin of the Bohemian massif (Steininger and Roetzel, 1991). They are Eggenburgian in age, a regional stratigraphic stage of the Paratethys corresponding to the Burdigalian. Sedimentary data show very poorly sorted, micaceous sands representing a shallow-water nearshore environment (R. Roetzel, pers. commun., 1996). These sediments have never been lithified, except for concretionary build-ups of which the echinoids often serve as a nucleus. The biota is dominated by the name-giving Parascutella. Other fossils include coralline algae, mostly in association with the sea-urchin tests, bryozoans, calcitic-shelled bivalves including oysters and pectinids as well as the spines of regular cidaroid echinoids and fragments of irregular spatangoid echinoids. The oysters
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Fig. 3. Schematic morphological comparison of Echinodiscus and Parascutella. Note the differences in the ambitus (with and without lunules), the complexity of the food groove system and the position of the anus.
are commonly found in gregarious clumps. Moulds of gastropods and aragonitic-shelled bivalves can be found in association with the lithification occurring around the echinoid tests. P. ho¨barthi is a relatively small sand dollar reaching a maximum size of 8 cm. It possesses only slight indentation along the ambitus and its food groove system is relatively simple (Fig. 3). The tuberculation (and hence spine differentiation, see Smith, 1984) is not as complex as for Echinodiscus. The periproct is located along the posterior ambitus thus distinguishing this genus from Scutella which possess a periproct halfway between the peristome and posterior ambitus. The taphonomic comparison of the echinoids was accomplished by visual recognition of different fea-
tures found on the tests (Fig. 4). In all, 23 modern specimens and 73 fossil specimens were analyzed. Additional information was gained by scanning electron microscope analysis of surface characters (Fig. 5). The results of the comparison are shown in Fig. 6.
3. Results 3.1. Taphonomy of Echinodiscus auritus Dead tests of E. auritus show a very wide range of taphonomic features (Nebelsick and Kampfer, 1994). Their preservation ranges from complete tests with very well preserved surface features (tubercles
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Fig. 4. (A) Apical view of recent Echinodiscus auritus showing massive predation of the apical surface and breakage of the oral surface. Note the scratch marks leading to the wound as well as the non-lethal predation (arrow) in the anterior of the test (scale bar D 2 cm). (B) Fossil Parascutella ho¨barthi. Note the non-lethal predation (arrow) which is present as cuspate-shaped marks along the ambitus of the test. (C) P. ho¨barthi showing encrustation by coralline algae (scale bar D 2 cm). (D) P. ho¨barthi showing implosion of the central area of the test.
and ambulacral pores) to highly abraded, heavily encrusted specimens. A remarkable taphonomic feature is lethal predation resulting in a characteristic, hollowed-out, central area and paired grooves leading to the wound (Fig. 4A). This feature (30% of complete tests) is most likely caused by fish. Non-lethal predation of the ambitus (Fig. 4A) is also common (60%). Other taphonomic features observed were encrustation (43%), corrosion (26%), bioerosion (22%) and abrasion (13%). Fragments of these echinoids are found in a wide range of preservation and sizes ranging from ‘pie-slices’ of the tests to single plates (see Nebelsick and Kampfer, 1994). 3.2. Taphonomy of Parascutella ho¨barthi P. ho¨barthi occurs as complete tests, fragments of the test as well as spines and jaw elements.
Although specimens are very common, the sand dollars do not form a coquina, rarely touching one another in the sediment. The taphonomic features recognized include lethal and non-lethal predation, encrustation, abrasion of the ambitus, presence of radial interplate crack marks and concentric central collapse structures. Non-lethal predation (15% of the specimens, Fig. 6) is seen as cuspate irregularities of the ambitus (Fig. 4B), while lethal predation is present either as rounded holes which cut through the echinoid skeleton or, in one case, with the removal of a large area of the oral surface. Abrasion of the test is seen at the ambitus (30%) and in the removal of small surface features such as tubercles. Encrustation by coralline algae (Fig. 4C) is intense and was found in more than 80% of the specimens. The oral surface is more heavily encrusted than the apical surface. Encrustation is initiated as
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Fig. 5. Scanning electron microscope pictures of tubercle and stereom morphology from the oral surface of recent Echinodiscus auritus (A and C) and fossil Parascutella ho¨barthi (B and D). Both species show perforated, sunken tubercles. (scale bars: A and B D 200 µm; C D 50 µm, D D 40 µm).
small nodules on several points of the surface. These then coalesce to extend above the surface of the test, often flaring out beyond the ambitus. The echinoid-red algal associations can reach sizes of over 10 cm. Attachment to the echinoid test is often restricted to a few points. The algal nodules sometimes grow so much as to incorporate a number of different biogenic particles. Post-burial taphonomic features include radial cracking and skeletal implosion of the test (Fig. 4D), grain indentation as well as cement formation. Radial cracking of the test invariably follows plate boundaries and is often first formed between the rows of ambulacral plates beyond the petals. In some cases the complete test is splayed out with all the plate rows separated. Skeletal implosion is characterized by the crushing of that part of the test not supported by internal supports which roughly corresponds to the area covered by the petalodium. This feature is accompanied by concentric cracks which again
correspond to plate boundaries. Grain indentation can totally mask the surface of the test destroying such features as tuberculation or tube feet pores. The echinoids often serve as a nucleus for sediment lithification, which can encompass the neighbouring sediment and biogenic components.
4. Discussion In both localities, the Northern Bay of Safaga, Red Sea, Egypt and the Lower Miocene Loibersdorf Formation in situ accumulations of echinoderm skeletons are present. This is corroborated by the fact that they do not form a coquina, and by Recent observation of live and dead specimens occurring together. There are similarities as well as differences in the taphonomic patterns of the recent and fossil sand dollars. Strong differences in preservation are present for (1) predation, (2) encrustation, and
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is augmented by the powers of regeneration and wound-healing common to all echinoderms (compare Ebert, 1988). 4.2. Encrustation Although encrustation is evident in both fossil and Recent sand dollars, it shows a very different pattern on each. Encrustation of the sand dollars is obviously post-mortal in both cases. In living echinoids, it is restricted to the naked spines of cidaroids as well as rare cases of barnacle encrusting sand dollars (Seilacher, 1979; Philippe, 1983). Both studied echinoids show a high differentiation of the oral and apical surfaces. The recent encrustation of Echinodiscus by bryozoans, foraminifera and serpulids is more diverse, but not as extensive as that of the fossil Parascutella by coralline red algae. The prevalence of active coralline algal growth to lower surfaces is a common feature in shallow-water marine habitats (pers. observation). 4.3. Fracturing and grain impression
Fig. 6. Comparison of taphonomic features found in recent Echinodiscus auritus from the Red Sea fossil Parascutella ho¨barthi from the Lower Miocene of the Austrian Molasse.
(3) radial fracturing, test implosion as well as grain impressions. These three features correspond neatly to the classification of taphonomic processes into necrolysis, biostratinomy and diagenesis (see Dodd and Stanton, 1990). 4.1. Predation Predation of sea-urchins by a number of different organisms including crabs, gastropods, diverse fish, birds, otters and other echinoderms, is a well known phenomenon (Moore, 1956; Quinn, 1965; Birkeland and Chia, 1971; Zinsmeister, 1980; Hughes and Hughes, 1981; Tegner and Levin, 1983; Courville et al., 1988; Gibson and Watson, 1989; Frazer et al., 1991; McNamara, 1994; Nebelsick and Kowalewski, 1999). The fact that non-lethal predation is recorded in living and fossil sand dollars, shows that both are robust enough to withstand repeated attacks. This
Taphonomic patterns resulting from sediment loading could only be observed for the fossil specimens. The radial interplate cracks and skeletal implosions are post-burial events resulting from sediment loading of the test. Bioturbation within the sediment can lead to plate disarticulation. The observed radial and concentric fracturing is, however, symmetrical suggesting an applied uniform, vertical pressure to the test. Grain impressions in which sediment particles are pressed into the skeleton are the result of pressure solution and represent a diagenetic feature. This results in the differential preservation of test surface features (Fig. 5B,D). Syntaxial cement can play an important role in local cementation of the fossil sediments, perhaps leading to the fact that the echinoids often act as a nucleus for lithified sediment clumps in an otherwise sandy, unlithified sediment.
5. Conclusions The differences and similarities between the recent Echinodiscus from the Red Sea and the fos-
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sil Parascutella from the Miocene can be assessed by comparing constructional morphologies, environmental factors and temporal effects related to diagenesis and sediment loading. 5.1. Constructional morphology Both tests show similar constructional morphologies. The excellent preservation of both Recent and fossil sand dollars as complete tests is primarily due to the internal support system which not only connects the oral and apical surfaces, but also forms a meshwork connecting the separate plates to each other. The presence of this system differentially affects the studied specimens. It allows the Recent Echinodiscus to be preserved as a ‘complete test’ despite being hollowed out by predators; it becomes evident in the fossil Parascutella when sediment pressure depresses the central area leaving the rim complete. Both Recent and fossil sand dollars have better preservation potentials than other echinoderms with which they occur. 5.2. Environmental factors Differences in environmental factors are a more important control of taphonomic patterns than variations related to constructional morphology for the studied material. The coarser fossil sediment may represent a higher-energy environment which would explain the higher degree of abrasion recorded for the fossil specimens. The difference in predation patterns is another factor related to the environment. The fact that E. auritus originates from a tropical environment with a very wide facies variation including reefs may enhance the presence of different potential predators. Both echinoids offer a similar substrate for encrusters as far as general size and surface morphology is concerned. The differences in encrustation patterns thus also represent differences in the ambient environment. The importance of dead Parascutella tests as a substrate for coralline algae, which would otherwise be offered little other suitable surfaces, has to be emphasized. 5.3. Time-related effects Temporal effects affecting the sea-urchins range from short-term time-averaging (see Nebelsick,
1992) to long-term diagenetic effects. A major difficulty in comparing these two species is that taphonomic processes would still have acted upon the Recent Echinodiscus had they not been removed from the study area. This may explain the under- or overrepresentation of certain taphonomic features. Thus the spines found in situ in the recent echinoids will have fallen off the test within a few days if left in the sediment. Continued surface residence time will also increase the chance of abrasion and encrustation. This comparison of recent and fossil echinoids also shows how post-burial factors, including sediment loading and grain indentations, can reduce the possibility of recognizing important taphonomic features such as small-scale alterations of surface characters.
Acknowledgements I would like to thank T. Haunold, S. Kampfer, L. Nebelsick, P. Pervesler, W. Piller, R. Roetzel and F. Steininger. This study was supported by the Austrian Science Foundation Projects: P5877 and P7507-Geo and P8869-GEO.
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Palaeogeography, Palaeoclimatology, Palaeoecology 149 (1999) 349–358
Taphonomic comparison between Recent and fossil sand dollars James Nebelsick * Institute and Museum of Geology and Palaeontology, Sigwartstrasse 10, D-72076 Tu¨bingen, Germany Received 28 January 1997; revised version received 9 July 1997; accepted 8 June 1998
Abstract The taphonomy of a Recent and a fossil sand dollar are compared. The recent Echinodiscus auritus originates from a shallow-water carbonate environment in the Red Sea. The fossil Parascutella ho¨barthi is found in micaceous sands of the Lower Miocene Austrian Molasse Zone. Both species show strong similarities in constructional morphology including the flattened overall shape, details of the surface morphology as well as the presence of an internal support system. Three taphonomic features are remarkable different: (1) lethal and non-lethal predation (especially high in the recent E. auritus; (2) encrustation of the test (especially by coralline algae in the fossil Parascutella); and (3) radial cracking on implosion of the body cavity caused by sediment loading on the fossil tests. Differences in taphonomic signatures can be correlated with depositional environment (predation and encrustation) as well as factors related to burial of the fossil specimens (sediment loading and diagenesis). 1999 Elsevier Science B.V. All rights reserved. Keywords: taphonomy; echinoids; clypeasteroids; Miocene; predation; encrustation
1. Introduction The preservation of the echinoids as in all fossils is dependent on the nature and intensity of taphonomic processes affecting the organisms (Fig. 1), whereby a ‘poorly preserved’ skeleton can, in fact, convey more information concerning the palaeoecology of the fossil and its environment. The preservation of echinoids can be related to three main factors (Fig. 2). These are skeletal morphology, environmental factors, and temporal effects (Nebelsick, 1995). In this study a direct taphonomic comparison is made between a fossil sand dollar (Parascutella ho¨barthi) from the Lower Miocene of the Austrian Molasse Zone and a Recent species from the Red Sea (Echinodiscus auritus), allowing the relative importance of these factors to be evaluated and compared. This direct comparison is Ł E-mail:
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appropriate due to similarities not only in the constructional morphology (flattened form with internal support systems) and test surface characters, but also in the environments in which they are found (shallow, subtidal sands). Numerous Recent observations concerning the biological and physical factors affecting the preservation of these sea-urchin tests can thus be incorporated in the comparison. Echinoderms obviously represent a special case among invertebrates as far as their skeletal construction is concerned. Although unified by the possession of a stereom and a multiplated skeleton, echinoderms show a wide range of skeletal morphologies and preservation potentials. This has been shown in an increasing number of modern studies concerning specific echinoderm taxa or taphonomic processes affecting them (Meyer and Meyer, 1986; Kidwell and Baumiller, 1990; Donovan, 1991; Greenstein, 1991, 1992; Gordon and Donovan, 1992; Nebel-
0031-0182/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 8 ) 0 0 2 1 1 - 9
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Fig. 1. Schematic diagram of the taphonomic processes leading to different preservations of an echinoid. The presence of a taphonomic feature is indicated by the thicker line (modified after Nebelsick, 1995).
sick, 1992, 1996; Nebelsick and Kampfer, 1994). The sand dollars represent extremely flattened forms of clypeasteroid sea-urchins (Durham, 1966; Mooi, 1989) and are among the most successful groups of Recent echinoderms (Smith, 1984; Mooi, 1990). These irregular echinoids show a wide geographical distribution occurring in both tropical and temperate settings (Ghiold and Hoffman, 1984, 1986). They are generally restricted to shallow-water, higher-energy environments. They include both deposit feeding, endobenthic forms (such as Echinarachnius, Echinodiscus, Encope and Melitta) (Bell and Frey, 1969; Stanley and James, 1971; Ebert and Dexter, 1975; Bentley and Cockcroft, 1995), as well as the partially exposed, suspension-feeding Dendraster ex-
centricus, an echinoid well known from the west coast of North America (Birkeland and Chia, 1971; Timko, 1976; Smith, 1981; Beadle, 1989). Sand dollars have an extensive fossil record (Wagner, 1974; Zinsmeister, 1980; Dodd et al., 1985; McKinney, 1985) including such extinct genera as Scutella and Parascutella which are characteristic for many Miocene rocks of the Mediterranean and Caribbean areas as recorded not only in numerous historical monographs, but also in the modern literature (Philippe, 1983; Boggild and Rose, 1984; Poddubiuk and Rose, 1984; Courville et al., 1988). These sand dollars can occur as isolated specimens, but are often found accumulated in great numbers in so called ‘Scutella-sands’ (as in the study area).
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m. It is most common in a relatively restricted area with poorly sorted, fine sands (Piller and Mansour, 1990) where it reaches maximum densities of 2–3 per m2 . Both juveniles and adults are found in the same environment. They live shallowly buried in the substrate and, if extracted, bury themselves within 10 min. This distribution is similar to that observed for another species of Echinodiscus (E. bisperforatus) from the South African south coast (Bentley and Cockcroft, 1995). 2.2. Parascutella ho¨barthi from the Lower Miocene Austrian Molasse Fig. 2. Taphonomic triangle showing three factors affecting taphonomic processes (modified after Nebelsick, 1995).
2. Material and methods 2.1. Echinodiscus auritus from the Red Sea Echinodiscus auritus Leske, 1778 is a large IndoPacific sand dollar reaching a maximum size of 15 cm. It shows a number of highly developed test characteristics. These include deeply incised posterior paired lunules, a very complex polyfurcating food groove system leading to the centrally positioned peristome (mouth) (Fig. 3) and a cover of highly differentiated minute spines. The periproct (anus) is located midway between the peristome (mouth) and the posterior ambitus (rim). It has an extensive, dense, internal support system consisting of skeletal connections bridging the gap between the oral (lower) and apical (upper) surface. The internal supports are distributed between the central cavity containing the main internal organs of the sea-urchin and the ambitus. Cage experiments showed that dead Echinodiscus loses its minute spines within a few days (Nebelsick and Kampfer, 1994). It is also susceptible to plate disarticulation, which, once initiated, quickly leads to the disintegration of the complete test into larger and smaller fragments (single plates). Live and dead Echinodiscus were studied in the Northern Bay of Safaga, Red Sea, Egypt, a shallow-water carbonate environment (Nebelsick, 1992, 1996; Nebelsick and Kampfer, 1994). This species is found in coastal sands in depths of less than 10
P. ho¨barthi (Ku¨hn, 1936) is a Miocene sand dollar restricted to a single locality in the Molasse Zone of Lower Austria (Ku¨hn, 1936; Schaffer, 1962; Steininger, 1971). The accumulation of a large number of specimens allows detailed palaeoecological and taphonomic analysis. Numerous specimens were examined from museum collections (Naturhistorisches Museum, Vienna, Austria; Institute of Palaeontology, Univ. Vienna, Ho¨barth Museum in Horn, Austria, and the Krahuletz Museum in Eggenburg, Austria). Additional specimens were recovered from field excavations which delivered important sedimentological, palaeoecological and taphonomic information. The sediments in which these echinoids are found are known as the ‘Scutellensand’ and belong to the Loibersdorf Formation. This Formation is part of a complex series of highly differentiated, shallow-water sediments near the southern margin of the Bohemian massif (Steininger and Roetzel, 1991). They are Eggenburgian in age, a regional stratigraphic stage of the Paratethys corresponding to the Burdigalian. Sedimentary data show very poorly sorted, micaceous sands representing a shallow-water nearshore environment (R. Roetzel, pers. commun., 1996). These sediments have never been lithified, except for concretionary build-ups of which the echinoids often serve as a nucleus. The biota is dominated by the name-giving Parascutella. Other fossils include coralline algae, mostly in association with the sea-urchin tests, bryozoans, calcitic-shelled bivalves including oysters and pectinids as well as the spines of regular cidaroid echinoids and fragments of irregular spatangoid echinoids. The oysters
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Fig. 3. Schematic morphological comparison of Echinodiscus and Parascutella. Note the differences in the ambitus (with and without lunules), the complexity of the food groove system and the position of the anus.
are commonly found in gregarious clumps. Moulds of gastropods and aragonitic-shelled bivalves can be found in association with the lithification occurring around the echinoid tests. P. ho¨barthi is a relatively small sand dollar reaching a maximum size of 8 cm. It possesses only slight indentation along the ambitus and its food groove system is relatively simple (Fig. 3). The tuberculation (and hence spine differentiation, see Smith, 1984) is not as complex as for Echinodiscus. The periproct is located along the posterior ambitus thus distinguishing this genus from Scutella which possess a periproct halfway between the peristome and posterior ambitus. The taphonomic comparison of the echinoids was accomplished by visual recognition of different fea-
tures found on the tests (Fig. 4). In all, 23 modern specimens and 73 fossil specimens were analyzed. Additional information was gained by scanning electron microscope analysis of surface characters (Fig. 5). The results of the comparison are shown in Fig. 6.
3. Results 3.1. Taphonomy of Echinodiscus auritus Dead tests of E. auritus show a very wide range of taphonomic features (Nebelsick and Kampfer, 1994). Their preservation ranges from complete tests with very well preserved surface features (tubercles
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Fig. 4. (A) Apical view of recent Echinodiscus auritus showing massive predation of the apical surface and breakage of the oral surface. Note the scratch marks leading to the wound as well as the non-lethal predation (arrow) in the anterior of the test (scale bar D 2 cm). (B) Fossil Parascutella ho¨barthi. Note the non-lethal predation (arrow) which is present as cuspate-shaped marks along the ambitus of the test. (C) P. ho¨barthi showing encrustation by coralline algae (scale bar D 2 cm). (D) P. ho¨barthi showing implosion of the central area of the test.
and ambulacral pores) to highly abraded, heavily encrusted specimens. A remarkable taphonomic feature is lethal predation resulting in a characteristic, hollowed-out, central area and paired grooves leading to the wound (Fig. 4A). This feature (30% of complete tests) is most likely caused by fish. Non-lethal predation of the ambitus (Fig. 4A) is also common (60%). Other taphonomic features observed were encrustation (43%), corrosion (26%), bioerosion (22%) and abrasion (13%). Fragments of these echinoids are found in a wide range of preservation and sizes ranging from ‘pie-slices’ of the tests to single plates (see Nebelsick and Kampfer, 1994). 3.2. Taphonomy of Parascutella ho¨barthi P. ho¨barthi occurs as complete tests, fragments of the test as well as spines and jaw elements.
Although specimens are very common, the sand dollars do not form a coquina, rarely touching one another in the sediment. The taphonomic features recognized include lethal and non-lethal predation, encrustation, abrasion of the ambitus, presence of radial interplate crack marks and concentric central collapse structures. Non-lethal predation (15% of the specimens, Fig. 6) is seen as cuspate irregularities of the ambitus (Fig. 4B), while lethal predation is present either as rounded holes which cut through the echinoid skeleton or, in one case, with the removal of a large area of the oral surface. Abrasion of the test is seen at the ambitus (30%) and in the removal of small surface features such as tubercles. Encrustation by coralline algae (Fig. 4C) is intense and was found in more than 80% of the specimens. The oral surface is more heavily encrusted than the apical surface. Encrustation is initiated as
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Fig. 5. Scanning electron microscope pictures of tubercle and stereom morphology from the oral surface of recent Echinodiscus auritus (A and C) and fossil Parascutella ho¨barthi (B and D). Both species show perforated, sunken tubercles. (scale bars: A and B D 200 µm; C D 50 µm, D D 40 µm).
small nodules on several points of the surface. These then coalesce to extend above the surface of the test, often flaring out beyond the ambitus. The echinoid-red algal associations can reach sizes of over 10 cm. Attachment to the echinoid test is often restricted to a few points. The algal nodules sometimes grow so much as to incorporate a number of different biogenic particles. Post-burial taphonomic features include radial cracking and skeletal implosion of the test (Fig. 4D), grain indentation as well as cement formation. Radial cracking of the test invariably follows plate boundaries and is often first formed between the rows of ambulacral plates beyond the petals. In some cases the complete test is splayed out with all the plate rows separated. Skeletal implosion is characterized by the crushing of that part of the test not supported by internal supports which roughly corresponds to the area covered by the petalodium. This feature is accompanied by concentric cracks which again
correspond to plate boundaries. Grain indentation can totally mask the surface of the test destroying such features as tuberculation or tube feet pores. The echinoids often serve as a nucleus for sediment lithification, which can encompass the neighbouring sediment and biogenic components.
4. Discussion In both localities, the Northern Bay of Safaga, Red Sea, Egypt and the Lower Miocene Loibersdorf Formation in situ accumulations of echinoderm skeletons are present. This is corroborated by the fact that they do not form a coquina, and by Recent observation of live and dead specimens occurring together. There are similarities as well as differences in the taphonomic patterns of the recent and fossil sand dollars. Strong differences in preservation are present for (1) predation, (2) encrustation, and
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is augmented by the powers of regeneration and wound-healing common to all echinoderms (compare Ebert, 1988). 4.2. Encrustation Although encrustation is evident in both fossil and Recent sand dollars, it shows a very different pattern on each. Encrustation of the sand dollars is obviously post-mortal in both cases. In living echinoids, it is restricted to the naked spines of cidaroids as well as rare cases of barnacle encrusting sand dollars (Seilacher, 1979; Philippe, 1983). Both studied echinoids show a high differentiation of the oral and apical surfaces. The recent encrustation of Echinodiscus by bryozoans, foraminifera and serpulids is more diverse, but not as extensive as that of the fossil Parascutella by coralline red algae. The prevalence of active coralline algal growth to lower surfaces is a common feature in shallow-water marine habitats (pers. observation). 4.3. Fracturing and grain impression
Fig. 6. Comparison of taphonomic features found in recent Echinodiscus auritus from the Red Sea fossil Parascutella ho¨barthi from the Lower Miocene of the Austrian Molasse.
(3) radial fracturing, test implosion as well as grain impressions. These three features correspond neatly to the classification of taphonomic processes into necrolysis, biostratinomy and diagenesis (see Dodd and Stanton, 1990). 4.1. Predation Predation of sea-urchins by a number of different organisms including crabs, gastropods, diverse fish, birds, otters and other echinoderms, is a well known phenomenon (Moore, 1956; Quinn, 1965; Birkeland and Chia, 1971; Zinsmeister, 1980; Hughes and Hughes, 1981; Tegner and Levin, 1983; Courville et al., 1988; Gibson and Watson, 1989; Frazer et al., 1991; McNamara, 1994; Nebelsick and Kowalewski, 1999). The fact that non-lethal predation is recorded in living and fossil sand dollars, shows that both are robust enough to withstand repeated attacks. This
Taphonomic patterns resulting from sediment loading could only be observed for the fossil specimens. The radial interplate cracks and skeletal implosions are post-burial events resulting from sediment loading of the test. Bioturbation within the sediment can lead to plate disarticulation. The observed radial and concentric fracturing is, however, symmetrical suggesting an applied uniform, vertical pressure to the test. Grain impressions in which sediment particles are pressed into the skeleton are the result of pressure solution and represent a diagenetic feature. This results in the differential preservation of test surface features (Fig. 5B,D). Syntaxial cement can play an important role in local cementation of the fossil sediments, perhaps leading to the fact that the echinoids often act as a nucleus for lithified sediment clumps in an otherwise sandy, unlithified sediment.
5. Conclusions The differences and similarities between the recent Echinodiscus from the Red Sea and the fos-
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sil Parascutella from the Miocene can be assessed by comparing constructional morphologies, environmental factors and temporal effects related to diagenesis and sediment loading. 5.1. Constructional morphology Both tests show similar constructional morphologies. The excellent preservation of both Recent and fossil sand dollars as complete tests is primarily due to the internal support system which not only connects the oral and apical surfaces, but also forms a meshwork connecting the separate plates to each other. The presence of this system differentially affects the studied specimens. It allows the Recent Echinodiscus to be preserved as a ‘complete test’ despite being hollowed out by predators; it becomes evident in the fossil Parascutella when sediment pressure depresses the central area leaving the rim complete. Both Recent and fossil sand dollars have better preservation potentials than other echinoderms with which they occur. 5.2. Environmental factors Differences in environmental factors are a more important control of taphonomic patterns than variations related to constructional morphology for the studied material. The coarser fossil sediment may represent a higher-energy environment which would explain the higher degree of abrasion recorded for the fossil specimens. The difference in predation patterns is another factor related to the environment. The fact that E. auritus originates from a tropical environment with a very wide facies variation including reefs may enhance the presence of different potential predators. Both echinoids offer a similar substrate for encrusters as far as general size and surface morphology is concerned. The differences in encrustation patterns thus also represent differences in the ambient environment. The importance of dead Parascutella tests as a substrate for coralline algae, which would otherwise be offered little other suitable surfaces, has to be emphasized. 5.3. Time-related effects Temporal effects affecting the sea-urchins range from short-term time-averaging (see Nebelsick,
1992) to long-term diagenetic effects. A major difficulty in comparing these two species is that taphonomic processes would still have acted upon the Recent Echinodiscus had they not been removed from the study area. This may explain the under- or overrepresentation of certain taphonomic features. Thus the spines found in situ in the recent echinoids will have fallen off the test within a few days if left in the sediment. Continued surface residence time will also increase the chance of abrasion and encrustation. This comparison of recent and fossil echinoids also shows how post-burial factors, including sediment loading and grain indentations, can reduce the possibility of recognizing important taphonomic features such as small-scale alterations of surface characters.
Acknowledgements I would like to thank T. Haunold, S. Kampfer, L. Nebelsick, P. Pervesler, W. Piller, R. Roetzel and F. Steininger. This study was supported by the Austrian Science Foundation Projects: P5877 and P7507-Geo and P8869-GEO.
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