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Key words: Echinodermata, Ophiuroidea, Protasteridae,
Lower Palaeozoic, Ordovician, Barrandian, palaeoecology, taphonomy.
Dense shallow-water ophiuroid populations or so-called "brittlestar beds" (the latter term has been applied to both living ophiuroid aggregations as well as to "hard" geological beds) occur in the fossil record from the Ordovician through the Jurassic. The brittlestar beds show a significant decline in the early Cretaceous and onward (Aronson 1989a). The last large fossil brittlestar beds were described from Upper Jurassic (Meyer 1988 and references therein). The well-known theory of "Mesozoic marine revolution" (see especially the original papers by Meyer and Macurda 1977, and Vermeij 1977) postulates a causal connection between the Cretaceous radiation of durophagous predators and the decline of shalow-water suspension- feeding echinoderms. Aronson (1987) stressed that there is a low proportion of individuals with regenerating arms in Palaeozoic and early Mesozoic ophiuroids and applied (Aronson 1989a) the "Mesozoic marine revolution" theory also to the occurrences of brittlestar beds. Aronson (1987, p. 190) points out that all recent predators of ophiuroids, including teleosts, brachyuran and paguroid crabs, some polychaetes, asteroids and other ophiuroids, cause sublethal damage to a more or less large proportion of their ophiuroid prey. Therefore, the few known examples of recent occurrences of large brittlestar aggregations in shallow waters are supposed to be anachronistic and are explained as a consequence of very unusual low- predation-level conditions (Aronson 1989b, 1991). Probably the best example (best described) of a dense Palaeozoic brittlestar bed comes from the Lower Mississippian strata of Ohio (Kesling and LeVasseur 1971) in which case the brittlestar community was formed exclusively of the protasterid species Strataster ohionensis Kesling et LeVasseur (Order Oegophiurida, Family Protasteridae). From the Ordovician (i.e. from the "pre-revolutionary" time, too) of the Barrandian area two protasterid brittlestar species have been known (Bohemura jahni Jaekel in the Zahorany Formation and Taeniaster bohemicus Petr in the Bohdalec Formation) to form relatively large populations. However, no "brittlestar beds" in the proper way were reported. At present, very important quantitative aspects of the brittlestar bed described by Kesling and LeVasseur (1971) from Ohio are comparable to a completely new and unique find at Praha 4 - Krc (Fig. 1), in the so-called Polyteichus facies of the Bohdalec Formation (Upper Ordovician). The new discovery represents the first true fossil brittlestar bed ever found on the territory of the Czech Republic.
Geological setting of the brittlestar locality
The locality "V podzámčí", yielding the discussed fossil brittlestar bed, was built of steep slopes of a road-cut exposure (Fig. 2), cutted in an unconsolidated rock material for the purpose of building a new street (now it is "V podzámci Street" at Praha 4 - Krc). The locality is now inaccessible (the outcrop does not exist because additional supporting soil material has been placed on the slope which is covered with vegetation now). The accessibility of the locality was possible only during years 1985-1990 when the site was discovered by the first of the authors. He has ascertained that the unconsolidated material was of Ordovician age (Bohdalec Formation) and formed of tectonically affected, micaceous clayey shales, containing locally admixture of carbonate and alternating rhythmically with either fine-grained sandstones or carbonate sandy siltstones. Most sandstone and siltstone beds were several centimetres thick, exhibiting subhorizontal or oblique bedding. Fossils were represented mainly by abundant brachiopod Heterorthina notata urbana (Havlícek) and the rare trilobite species Kloucekia phillipsi (Barrande). The most curious find was a bed of calcareous sandy siltstone (Fig. 3) which has yielded large numbers of the protasterid brittlestar Taeniaster bohemicus Petr in the form of dense accumulation. Other strata of calcareous sandy siltstone yielded clusters (up to 5 cm thick!) of highly fragmented brachiopods, bryozoans, and trilobites. Trace fossils were also abundant (e.g., Asteriacites, Cruziana, Rusophycus, Monomorphichnus, Bifungites, Planolites, Didymaulichnus - see Mikulás1988). On a bedding plane of calcareous sandstone, probably near that with the "brittlestar bed", the protasterid T. bohemicus was found in association with the trace fossil Asteriacites lumbricalis Schlotheim (Mikuláš1990). The brittlestar bed has been extensively sampled by two of the authors (RM and RJP) and the material is deposited in collections of the National Museum, Praha. The bed containing densely packed protasterids T. bohemicus reached thickness of about 60 mm. The proper accumulation of brittlestars was concentrated in a lamina in the middle part of the bed, while several further specimens of T. bohemicus were present also on the lower surface of it. The above mentioned bed was 3 metres long in the exposed outcrop, being limited at both sides by dislocations. The thickness and the palaeontological content of the bed remained stable at the section observed.
Palaeoecology of the brittlestar bed
Differences in preservation of the brittlestars were influenced by various stages of weathering of the rock. We have observed no evidence of breakage that would be expected with even short-distance transport or with any reworking of the original accumulation. This suggests that the remarkable dense aggregation of brittlestars represents really a fossil "brittlestar bed" preserved in situ. Many specimens are preserved oral side up, while others aboral side up (the same situation was observed by Kesling and LeVasseur 1971 in the above mentioned brittlestar aggregation from Ohio). Furthermore, from the many well-preserved individuals of T. bohemicus (Fig. 4, Pls. I-III) no one shows a sublethal damage (i.e. regenerating arm). We believe that this is a consequence of low predation rate in the Ordovician sea (rather than of the hypothetical possibility that the attacks were all lethal). On the other hand, we do not know whether it is only due to absence of predators or a consequence of production of a repellent by the ophiuroids. The existence of repellents in protasterid brittlestars cannot be excluded, especially because of observations on the Bohemian protasterid Bohemura jahni (the territory of which was invaded by no trilobites although they almost prevail in all the Zahorany Formation, see Petr 1989a) and on the American protasterid Strataster ohionensis (see the discussion of Kesling and LeVasseur 1971, p. 336). It must be stressed that the best specimens (Fig. 4) preserved almost all features of the skeletal morphology of T. bohemicus as described originally by Petr (1989b). Completely unknown were the tori and teeth (denticles). The denticles are so far unknown in all other Bohemian protasterids. On the material from Praha 4 - Krč we observed that each torus of T. bohemicus bears three long, slender denticles, the middle being distinctly longer than the other two (the number of teeth is the same as in Strataster ohionensis but they are more slender and more longer in the Bohemian species). The character of the disk in the individuals preserved supports the original theory of Hotchkiss (1970, p. 67) that marginal plates in Taeniaster (and in other protasterids, too - see Petr 1989b) did not exist and that the marginal thickening of the disk is simply from the folding of the disk at this locus. The species Taeniaster bohemicus was previously reported as being commonly found in the so-called "azimuthal orientation" or "Einsteuerung" (in original terminology of von Koeningswald, 1930). Therefore, previous finds were interpreted as a mode of preservation in which the brittlestars have been moved by more or less weak currents before becoming incorporated into the sediment (Petr 1989b). The new discovery supports the hypothesis of epifaunal mode of life in T. bohemicus and the existence of the brittlestar bed points to a discriminating suspension-feeding in this species. As expressed by Fell (1966), in recent seas the suspension-feeding ophiuroids commonly form aggregations because they are not dependent upon substratum as a limitation, since the suspended food may be brought to them by bottom currents. That is also the reason for apparent absence of brittlestar beds in the related species Bohemura jahni Jaekel (Zahořany and Letná Formation, see Petr 1988, 1989a). B. jahni led burrowing life habits and also its diet was very different. Although it was formerly supposed to be a discriminating suspension-feeder (Spencer 1934), this species represents the only really incontrovertible evidence of macrophagy in fossil echinoderms (Petr 1989a). This evidence has been observed on three unique specimens (first one holding a homoiostelean Dendrocystites sedgwicki by the arm and clearly feeding on the carpoid stele, second one grasping with its arm a conulariid theca while the soft parts of conulariid were oriented to the ophiuroid's mouth, and third exemplar coiling the arm around a theca of Sphenothallus sp.). Therefore, protasterids B. jahni were carnivores or carrion feeders (it is interesting because recent carnivorous and carrion-feeding ophiuroids fall generally into the category of foragers, see Lawrence 1987). A general negative phototaxis under conditions of strong illumination in recent ophiuroids was already indicated. In her classical "echinodermal treatise" Hyman (1955) wrote that "ophiuroids will seek the dark side of an aquarium or container or retreat into corners or hide under stones and plants if available. In a darkened aquarium they may be found moving about anywhere, but on illumination they retreat into the darkest available places. They react positively to dark walls even if these throw no shadow and will seek the dark half of containers painted half black and half white". Really, ophiuroids always exhibit negative response to light. Reef and rock-pool species shelter under algae, shells or stones, or within crevices, empty heads of coral skeletons or the cavities of sponges (Fell 1966). As reported by Grober (1989), within twenty minutes after sunset, many portions of the reef (off the Smithsonian field station in Panama´s San Blas Islands) are carpeted with brittlestars with their arms stretching into the bottom current. This seems to be very important. Possibly, the burrowing life habits of B. jahni in shallow waters may reflect its negative response to light, while the presence of epifaunal carpet of T. bohemicus at Praha 4 - Krc may reflect relatively low intensity of illumination and deep-water conditions of the Ordovician sea at that site. On the other hand, such a speculation may be completely wrong (in fact, there is no clear sedimentological reason for such an assumption). Moreover, T. bohemicus was previously reported from more fine-grained sediments than are those found at Praha 4 - Krč. It is not surprising because filter-feeding organisms are relatively independent of the substratum and because recent ophiuroids exhibit wide tolerance of depth change. But does it mean that at Praha 4 - Krč there was shallower water and possibly even more light? We argue that yes and, curiously, that it is not a paradox. The answer will be given by older experiments of Allee and Fowler (1932) which show that when brittlestars are placed in a tank with strong illumination, they congregate in clusters, of course, on the side away from the light. Moreover, Fell (1966) points to the fact that such a clustering is seen in shallow waters if there is little available cover. Therefore, seemingly paradoxically, the aggregation of ophiuroids may be commonly implied by strong illumination. Furthermore, there are many unanswered questions in the ecology of recent ophiuroids. For example, Fell (1941, cited in Fell 1966) reported that strong light had an narcotic effect (!) on young stages of the brittlestar Ophiomyxa. Therefore, the possible causal connection between predation, protection, negative phototaxis and illumination may be more complex. The idea that there were extremely shallow-water conditions during the sedimentation of the Polyteichus facies is not new. Several important sedimentological estimations of the palaeobathymetrical conditions in this facies were published by previous authors. Röhlich (1957) pointed to the fact that there is no graded bedding in the sandy material and after his description and interpretations of other authors (esp. Havlíček in Chlupáč et al. 1992), the sandy interbeds in the Polyteichus facies represent tempestites deposited above the storm wave base. Kukal´s (1963) observations led him to conclude that the sediments of the Polyteichus facies were laid down in the shallow platform environment and Havlíček (1982) suggested that there was a distinct bathymetrical differentiation of the Prague basin during the sedimentation of the Bohdalec Formation and that the sedimentation of the Polyteichus facies was largely controlled by the proximity of submarine elevations forming extremely shallow environment in some places. Moreover, our conclusions are in accordance also with previous palaeoichnological interpretations of the Polyteichus facies. Mikulás (1988, 1993) has described from the Polyteichus facies a typical Cruziana ichnofacies which is characteristic for shallow sublittoral above the storm wave base. The presence of some trilobite ichnofossils (Rusophycus) points to the presence of dalmanitid trilobites with large eyes which also indicate sufficient illumination. Preservation of articulated multielement skeletons of brittlestars generally requires rapid entombment in the sediment, and preservation in feeding position with prey in ophiuroid´s mouth as reported in B. jahni (the so-called "frozen" behaviour in terminology of Boucot 1990) indicates extremely rapid emplacement of sediment on the population of brittlestars, i.e. there is an "obrution deposit" in the Zahorany Formation (in terminology of Brett 1990). We infer that this is also the case with the brittlestar bed in the Bohdalec Formation - the unique aggregation of ophiuroids of the protasterid species Taeniaster bohemicus - discussed in the report above. The community met with a catastrophic death and the brittlestars were smothered by an extremely rapid sedimentation - unfortunately for the ophiuroids, luckily for the present authors.
Acknowledgements
Funding was provided by a grant of the Czech Grant Agency,
n. 205/94/0759 ("Czech Ordovician as a stratigraphical standard for the
Mediterranean region"). The authors are indebted to Prof. I. Chlupáč (Institute
of Geology and Palaeontology, Charles University) for his valuable comments
to our manuscript. Second of the authors (VP) is indebted to dr. Richard
B. Aronson (Smithsonian Institution, Washington, D.C.) and especially to
dr. Frederick H. C. Hotchkiss (Harvard, Massachusetts) for their generous
gifts of ophiuroid literature.
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