EPIBIONTS ON DROMIOPSIS RUGOSA (DECAPODA: BRACHYURA) FROM THE LATE MIDDLE DANIAN LIMESTONES AT FAKSE QUARRY, DENMARK: NOVEL PREPARATION TECHNIQUES YIELD AMAZING RESULTS

ABSTRACT-

Application of new preparation techniques for cleaning and study of fossil crabs to Dromiopsis rugosa (Schlotheim, 1820), from the late middle Danian limestone in the Fakse quarry, Denmark, has revealed remarkable detail of the carapace surface and epibionts infesting inner and outer surfaces of the carapace. Epibionts, identified as clionid sponges, scleractinian corals, cheilostome and ctenostome bryozoans, serpulid worms, and brachiopods, are interpreted as having attached to molted carapaces after the molted carapace had been released.

INTRODUCTION

FAKSE LIMESTONE quarry is situated east of the village of Fakse in Zealand, Denmark. It constitutes, together with the nearby Stevns Klint, the type locality of the Danian Stage, the lowermost stage of the Paleogene. The Fakse quarry exposes a section through a complex of bryozoan and coral mounds of late middle Danian age (Willumsen, 1995; Surlyk and Håkansson, 1999). The azooxanthellate coral mounds are developed in the narrow epicontinental seaway that lay within the Tornquist-Sorgenfrei lineament (Berthelsen, 1992; Willumsen, 1995). Extensive bryozoan mounds that developed in this seaway during the early and middle Danian were associated with azooxanthellate coral mounds at Fakse and at Limhamn, Sweden (Bernecker and Weidlich, 1990; Willumsen, 1995).

The coral-rich carbonates are highly fossiliferous. The fauna and lithofacies have been described by numerous authors, among them Rosenkrantz (1938), Rosenkrantz and Rasmussen (1960), Asgaard (1968), Floris (1979, 1980), Bernecker and Weidlich (1990), Willumsen (1995), and Surlyk and Håkansson (1999). This coral-rich limestone is also one of the most abundant sources for anomuran and brachyuran decapods from this interval. The decapods have been described or mentioned many times (Schlotheim, 1820; Reuss, 1859; von Fischer-Benzon, 1866; Segerberg, 1900; Woodward, 1901; Förster, 1975; Rasmussen, 1972; Jagt et al., 1993; Collins and Jakobsen, 1994). The total known crab fauna of Fakse consists of 20 species (Jakobsen and Collins, 1997).

In the course of studying the crabs, it was noted that many of the specimens were infested with epibionts. Some of these epibionts were situated on the outer surface of the carapaces whereas others were on the inner carapace surface, clearly a result of postmortem encrustation. In order to study properly the interaction of the crabs and the epibionts, special preparation techniques were developed to better expose the crab surfaces and the encrusting organisms. The purposes of this work are to describe these novel preparation techniques in order to enhance exposed surfaces of decapods and other organisms; to catalogue the epibionts found on a single species of crab, Dromiopsis rugosa (Schlotheim, 1820); and to discuss the nature of living versus postmortem encrustations. The study is based upon collections in the Geological Museum, Copenhagen, Denmark. A total of 1,096 specimens were studied, consisting of molds of the interior of the carapace and molds of the exterior of the carapace with partially or entirely preserved surface granulation.

PRESERVATION

The Fakse Limestone consists of two main facies, bryozoan and coral limestone, with a wide range of subfacies due to depositional differences and local variation in diagenesis (see Bernecker and Weidlich, 1990; Willumsen, 1995). The crab material consists almost entirely of disintegrated molted parts. The original cuticle has been converted to a structureless, chalky mass, leaving the molds of the exterior and interior with imprints of the cuticle. Only when found in matrix-poor cavities in the limestone are the crab remains exposed with a uniformly thin, fibrous calcite cement covering the original surface. It is remarkable that no evidence of corpses, or specimens with at least some associated legs, has been recognized within the crab assemblage at Fakse, apart from two specimens of the brachyuran Raniliformis baltica (Scgerberg, 1900) and two undescribed macrurans. The raninids and the macrurans, both of which tend to be burrowing organisms, lack epibionts. The burrowing behavior may account for their having been preserved with appendages as opposed to being disarticulated. The majority of specimens exhibit pre-burial fragmentation as a result of the high-energy paleoenvironment. High-energy conditions are also indicated by breakage and collapse of the scleractinian corals, none of which has ever been observed in growth position (Floris, 1980). Breakage and collapse of the corals are caused by two factors. Bioerosional activity primarily by boring sponges (?Entobia Bronn, 1837) within the coral stems weakens the skeletons. In combination with bioerosion, the turbulence on the seafloor breaks up the coral branches, subsequently leading to total collapse. Indication of very early lithificalion, as suggested by Willumscn (1995), is demonstrated in the crabs by partial sediment infilling of the carapace and absence of compaction.