USING SPIDER TAPHONOMY TO DECIPHER THE PALEOSALINITY OF THE CRATO FORMATION

The Crato Formation (CF) of Brazil is a Fossil-Lagerstätte and a significant source of Cretaceous spiders. The CF is interpreted as a lacustrine environment deposited in an arid to semi-arid setting, and is represented by deposits of laminated limestones. Two other notable deposits in which spiders are present are the Green River Formation (GRF) and Florissant Formation (FF), which are interpreted as varying from fresh to marine lacustrine environments of the Eocene. The three formations are the result of similar depositional settings; however, the leg orientation (curled versus extended) of spiders preserved in the CF differs from spiders in the GRF and FF. This study investigates the cause of this difference in preservation, which preliminary research suggests may be related to water salinity. Over 200 fossil spiders from the GRF, FF, and CF were photographed, and the leg orientation for each specimen was determined. GRF and FF spiders commonly display extended legs, while those in the CF typically have legs curled under the body. Spider legs lack extensor muscles, and extension is controlled by a hydraulic mechanism, so subaerial death usually results in curled legs due to rigor mortis of the flexor muscles. The majority of spiders preserved in the CF and GRF are believed to be araneoids. Spiders preserved in the FF are of various families. To attempt to explain the leg orientation of spiders in the CF, living Mangora maculata (family Araneidae) spiders were submerged in three different salinities: 1) fresh water 2) marine 3) hypersaline. Should leg flexion be promoted by hypersaline water, for example, this would suggest hypersaline conditions during deposition of the CF. Spiders naturally float on the surface of water, so forced submersion was required to break the surface tension. Once submerged, many spiders had an initial reaction to contract the legs. After a period of four days, however, leg orientation changed and remained stable. Spiders submerged in fresh and marine waters typically exhibited extended legs. Spiders in hypersaline conditions exhibited mostly curled legs. These results suggest the difference in leg orientation appears to be driven by salinity. It is suggested that, in Fossil-Lagerstätte with abundant spider fossils, such as the CF, leg curling in spiders could be useful as an indicator of salinity.