Markus Bertling

Late Jurassic coral/microbial reefs from the northern Paris Basin — facies, palaeoecology and palaeobiogeography

Abstract During the late middle Oxfordian, patch reefs grew on the northern margin of the Paris Basin. According to the facies analysis of the reef and inter-reef sediments, the environment was a warm, clear and agitated sea with highly episodic sedimentation. The bioherms were a short-lived phenomenon during the third phase of regional reefal development. Sequence stratigraphically, they are associated with a highstand system tract. Volumetrically and trophically dominant organisms were microbes now represented by massive clotted leiolite; ‘stalactitic’ hemispheroids with purely thrombolitic texture are restricted to open caves. Corals were of structural, reef-building importance due to their rapid upward growth. The patch reefs are characterised by thickets of ramose corals which developed a very open framework. In the vicinity of these patch reefs, though in hydrodynamically higher-energy environments, grew thickets of more stoutly branched corals; however, they are rarely preserved in situ and are generally represented as abundant coral rubble. The reef taxa are characterised by the notable absence of several groups (e.g. oysters, serpulids, bryozoans, pectinids) occurring at other localities where reefs of similar age developed in similar environmental conditions. The reefs also have strikingly modern aspects to them, in particular the presence of cryptic elements within caves and a sponge-dominated borer association. Dwellers belong to various life-form types although encrusting taxa are exceedingly rare. This may be explained by the presence of soft microbial films on most surfaces. The palaeoecological analysis suggests that the major controls on faunal composition and high diversity were elevated nutrient levels, highly episodic sedimentation and probably seasonal environmental disturbances. Structural and functional aspects of the reef community (grazers trigger framebuilders, borers trigger binders, binders hamper borers) allow ecological comparisons to be made with contemporaneous, as well as Recent, reefs. The unique combination of ecological factors resulted in a specialised, previously undescribed, community which differs from both Tethyan and northern localities in various aspects; these include cavities with cryptofauna, prominence of grazing gastropods and high faunal diversity in a microbially dominated build-up.

What's in a Name? Nomenclature, Systematics, Ichnotaxonomy

SUMMARY: Trace fossil diagnoses are subject to the principles of ichnotaxonomy, which seemingly lacks a standardized theoretical basis. The existing problems of this science, such as insufficient diagnoses of ichnotaxa or inadequate ichnotaxobases for newly introduced trace fossil names, are classified and solutions are suggested. A plea is made for a comprehensive two-level approach: similar trace fossils should have identical ichnotaxobases, and a universally acceptable framework of morphology-based ichnotaxobases should be sought. In both cases, a hierarchically organized system containing only geometrical criteria and principal types of substrate should be pursued. Biological affinity of trace makers, spatio-temporal distribution and other extrinsic factors, as well as inferences about the behaviour of the producers of trace fossils are to be avoided as criteria in ichnotaxonomy.

Arachnostega n. ichnog. -burrowing traces in internal moulds of boring bivalves (late jurassic, northern Germany)

KurzfassungGrabgänge an der Oberfläche von Mollusken-Steinkernen werden alsArachnostega gastrochaenae n. ichnog. n. ichnosp. beschrieben. Gegenwärtig ist das Ichnogenus nur aus dem norddeutschen Oberjura und der Nordsee bekannt. Die Synökologie der Erzeuger wird eingehend diskutiert: Es handelt sich um vermutlich opportunistische, detritusfressende Polychaeten, die der Coelobiten-Infauna zugerechnet werden müssen. Sie sind nicht in eine ökologische Sukzession eingebunden, da ein Sedimentationsereignis vorausgehen mußte. Auf den skeletalen Rahmen haben sie schwächend gewirkt, doch schnelle Zementation der weichen Internsedimente und Inkrustierer der Oberfläche behindern die Graborganismen. Abschließend werden biostratinomische und fossildiagenetische Aspekte erörtert.AbstractBurrowing traces in internal moulds of molluscs are described asArachnostega gastrochaenae n. ichnog. n. ichnosp. Currently, the ichnogenus is only known from the Late Jurassic and Recent of Northern Germany. The synecology of its polychaete producers is discussed in detail: they were probably opportunistic detritus-feeders and have to be regarded as infaunal coelobites. The tracemakers were not part of an ecological succession because their activity was facilitated by a sedimentation event. They weakened the skeletal frame but rapid cementation and encrusters inhibited them. Finally, aspects of biostratinomy and fossil diagenesis are sketched.

Ecology and distribution of the Late Jurassic Scleractinian Thamnasteria concinna (Goldfuss) in Europe

Abstract The quantitative investigation of coral associations in northern Germany revealed Thamnasteria concinna as their main element, necessitating an evaluation of all data for the spatio-temporal distribution of this scleractinian. Autecological criteria and sedimentological data indicate an optimum fitness of the coral in turbulent environments with low net-sedimentation rate. In these facies, no conflicting aims existed for T. concinna in exploiting light or plankton and resisting adverse hydrodynamic conditions. The most important synecological parameters of the associations studied are dominance patterns and diversity as well as the structure of guilds. T. concinna mostly constituted the builder guild, occurring as an ubiquist, though dominant only in shallow water. As recorded in the study area, strong dominance of a few taxa and low diversity indexes are typical of immature communities in unstable environments. T. concinna was well adapted to varying environmental factors showing characteristics of an r-strategist, thus being more successful than other corals in stressful facies. The distribution pattern of T. concinna was not controlled by single environmental factors, but rather the oscillation of many of them. This seasonality is a typical feature of the temperature realm. T. concinna actually was important only in temperate regions of the Late Jurassic neritic sea or its borders with the Tethyan region. Further south, the coral was just one of the many members of the Tethyan reefs where mature communities predominated. Because of sedimentological restrictions, this pattern is most clearly visible in the Oxfordian, whereas T. concinna was a negligible element of Tithonian reefs. Its evolutionary decline is due to the concentration of reefs on the (sub-)tropical Tethyan region.

ökologie und Taxonomie koralleninkrustierender Bryozoen des norddeutschen Malm

In the Late Oxfordian Korallenoolith of northern Germany, eight bryozoan taxa encrusting corals have been found, six of which had not been previously recorded in the area. In most cases, they can only be described using open nomenclature since the gonozooids are not known and the type material of existing species is too poorly preserved. “Stationary” bereniciform bryozoans were relatively good competitors adapted to a predictable environment. For this reason they thrived in the calm-water facies. However, the environment was disturbed frequently as proved by the joint occurrence with Stomatoporidae. These “mobile” taxa could grow fast on strongly three-dimensional surfaces, thus being able to survive even in highly agitated facies by escaping into cryptic habitats.The distribution of individual taxa is thus governed by the type and frequency of perturbations of their habitats. Bryozoans had a binding function in the fauna related to corals. Because of their low number and poor coverage, they mostly had little importance, however. This is due to the high water energy or high rate of sedimentation, which characterized the environments of the sampled localities. Thus, ideal habitats were not available. Nevertheless, the characteristic bryozoan associations of the European Jurassic became established together with typical associated faunal elements.KurzfassungDie Untersuchung der koralleninkrustierenden Bryozoen des norddeutschen Korallenoolith (Ober-Oxfordium) ergibt acht Taxa, von denen sechs bisher nicht von dort bekannt waren. Die meisten Formen können nur in offener Nomenklatur bezeichnet werden, da die Gonozooide nicht überliefert sind und vielfach das Typusmaterial bestehender Arten nicht entsprechend erhalten ist. Die „stationären“, relativ konkurrenzstarken bereniciformen Bryozoen waren an ein vorhersagbares Milieu angepaßt und gediehen daher in der Ruhigwasser-Fazies am besten; das gemeinsame Vorkommen mit Stomatoporiden belegt allerdings häufige Störungen. Diese „mobilen“ Vertreter wuchsen sehr schnell auf stark reliefierten Substraten und konnten damit auch im Hochenergie-Milieu in kryptischen Habitaten überleben.Die Verteilung der einzelnen Taxa wird somit durch die Art und Häufigkeit der sedimentologischen Beeinträchtigungen bestimmt. In der korallengebundenen Fauna fungierten die Bryozoen als Binder, hatten jedoch nach der geringen Zahl und Flächenausdehnung ihrer Kolonien meist nur geringe Bedeutung. Dies ist begründet in der zu hohen Wasserenergie oder Sedimentationsrate, die an den Probenpunkten meist geherrscht hat. Somit waren die zur Verfügung stehenden Habitate nicht ideal. Dennoch konnten sich die für den europäischen Jura charakteristischen Vergesellschaftungen der Bryozoen untereinander und mit der Begleitfauna herausbilden.