Biocénoses à triopsidés(Crustacea, Branchiopoda) du Permien du bassin de Lodève (France)

Abstract

Triopsids (Crustacea, Branchiopoda), which are abundant in the “Saxonian” facies of the Salagou Formation of the Lodève Basin and dated to the Thuringian, Saxonian or Artinskian by different workers, are described here. From thousands of carapace and thoraco-abdominal fragments and more rarely from intact specimens, we identify and describe two new species: Triops cancriformis permiensis nov. ssp. and Lepidurus occitaniacus nov. sp., which are morphologically similar to modern species. On the basis of the ecological preferences of modern triopsids and the sedimentological characteristics of their environment in Permian times, we confirm the occurrence of shallow, temporary expanses of water during the deposition of the Salagou Formation. These pools were either neutral or slightly alkaline and were scattered across a playa type environment and endured with the same characteristics during deposition of the formation which is more than 1000 m thick in the south of the basin. Aridity that prevailed during this long period of sedimentation had been suggested earlier from the study of sedimentological data (dessication cracks, rubefaction) and of Isopodichnus ichnites attributed to notostracean triopsids. This is now confirmed by the discovery of large numbers of these crustaceans in crescent-shaped megaripples from the base to the top of the Salagou Formation. The organism fragments are located in minor sequence limestone horizons within the sandstone body. The fossiliferous strata formed during restrictive stages of the living environment, essentially from moulting. Triopsids and Isopodichnus,, viewed as their trace fossilw, are not found together in the same strata showing that the fossilisation conditions for the organisms and their tracks differed.