Taphonomic variation within a Middle Triassic fossil lagerstätte (Cassina beds, Meride Limestone) at Monte San Giorgio

Abstract

Middle Triassic sediments at Monte San Giorgio, a UNESCO heritage site on the Switzerland-Italy border, have famously sourced well-preserved vertebrate fossils. Taphonomic studies of reptiles in the Besano Formation and in the Cava inferiore, Cava superiore and Cassina beds of the Meride Limestone, and fish in the Besano Formation have already elucidated changes in palaeoenvironment through time in sediments that predominantly comprise black shale and carbonate lithologies. Saurichthys skeletons from the Ladinian Cassina beds were scored for articulation and completeness, with the resulting data sorted to test for variation among layers and lithologies in the 2.7-m thick section. The greatest abundance of skeletons was found in finely laminated sediments, a reduced number in event bed sediments and an absence in volcanic ash deposits. No particular bed showed better or worse preservation. The final state of a skeleton entering the fossil record was the result of the mode of deposition and rate of sedimentation with the additional influence of features at the sediment surface, notably microbial mat growth and its ability to stick carcasses to substrate.