DID SPONGES HAVE CRABS? THE FAUNA OF A UNIQUE SPONGE-ALGAL CYLINDRICAL BIOHERM COMPLEX FROM THE UPPER JURASSIC OF ROMANIA

Exposures of Oxfordian (lower Upper Jurassic) carbonate rocks in central Dobrogea, Romania, record a remarkable sponge-algal bioherm complex and its associated fauna. The area of exposure, situated near the village of Cheia, is dissected by the Cheia River. The region lies on the fault-bounded Moesian Platform which is a microplate originating southeast of modern Romania. The deposits are included within the Visterna Member of the Casimcea Formation. The biostratigraphy of the formation was outlined by Bărbulescu (1961-1989). Detailed biofacies analysis was defined by Hermann (1996), and biozonation of the formation was largely defined by Bărbulescu (in Dragastan et al., 1998). Individual cylindrical sponge-algal bioherms, with diameters of approximately 30 m, stand approximately 25 m above the valley floor. Each bioherm has a hollow core filled by a limonitic carbonate breccia, and each is separated from adjacent bioherms by flanking, well-bedded talus. The bioherms formed near the distal margin of a carbonate ramp in water depths below normal wave base. Sponge biostromes were formed seaward and coral patch reefs and lagoonal deposits were formed landward of the sponge-algal bioherms. The fauna associated with the sponge-algal bioherms includes prosopid and dynomenid crabs; ammonites; belemnites; pectinid pelecypods; terebratulid, rhynchonellid, craniid, and thecideiid brachiopods; bryozoans; serpulids; and cidarid echinoids. Neither the crabs nor the belemnites has been reported previously. The associated benthic faunal elements tend to be small, probably occupying niches within the bioherms, and represent a biocoenosis. Diversity is low. Abundance of specimens is uniform and moderate in all facies. The architectural framework of the sponge-algal bioherm complex is uniquely preserved so that the composition and distribution of benthic organisms within the complex can be clearly delineated. This site is the only known cylindrical sponge-algal bioherm complex. The work was supported by collaborative grants from the National University Research Council of Romania to Lazăr and the National Science Foundation to Feldmann and Schweitzer.