Fische und Saurier aus dem Hochgebirge : Fossilien aus der mittleren Trias bei Davos

Abstract

Since 1989, a small team of University of Zurich palaeontologists, together with passionate amateur workers, have collected well preserved vertebrate, invertebrate and plant fossils from the south eastern Swiss Alps (Austroalpine Silvretta nappe). The fossils are from the Middle Triassic Prosanto Formation, an important 100—200 m thick sequence of dark laminated limestone, marl and dolomite. The sediments were deposited in a small, 20—30 km wide restricted basin on a large shallow-water, carbonate platform (represented by the dolomites of the Vallatscha Formation) in the western Tethys. Radiometric dating (U-Pb) of a volcaniclastic layer indicates an age of 241 Ma (early Ladinian) for the most fossiliferous beds of the upper Prosanto Formation; the middle and lower parts are perhaps late Anisian. After initial collection from scree the fossils have since been gathered in systematic bed-by-bed excavations taking place over two to three weeks every summer. There are three main excavation sites, all located in the Ducan and Landwasser regions west and southwest of Davos (Canton Graubunden) at an altitude of 2600—2800 m above sea level. To date, several hundred small but well preserved skeletons of 25 different actinopterygian fishes, two actinistian fishes and two aquatic sauropterygian reptiles have been discovered, carefully prepared and studied, alongside as much if not more fragmentary or disarticulated material. Other finds include a shark tooth, a few echinoderms, cephalopods, brachiopods, several crustaceans, bivalves, gastropods, siliceous sponges, benthic foraminifera and dasycladacean algae, all pointing to a marine environment. The benthic organisms did not live on the seabed of the basin, being washed in with sediments disturbed by storm activity across the platform or slumping sediments around the basin edge. Rare insects and terrestrial reptiles, together with common plant remains indicate nearby islands or larger land areas. The Prosanto Formation has provided one of the best preserved fossil marine faunas of the Middle Triassic, well comparable to the classic marine vertebrate fauna from Monte San Giorgio, a UNESCO world heritage site on the border of southern Switzerland and northern Italy. Around the world, similar Middle Triassic marine vertebrate faunas are known from the German Muschelkalk, Spitsbergen, Greenland, North America and southwestern China.