Preservation of Pachypleurosauridae (Reptilia; Sauropterygia) from the Middle Triassic of Monte San Giorgio, Switzerland

The pachypleurosaurids Neusticosaurus, known from three species found in three closely spaced horizons in the Meride Limestone, and Serpianosaurus, found in the Besano Formation, were used to determine variation in the environmental conditions at Monte San Giorgio through time. Although all of the taxa show similar progression in disarticulation and loss of completeness of the nine designated anatomical units, the extent of the progression varies among the species. As Neusticosaurus and Serpianosaurus are almost identical in body plan, taphonomic variation must reflect differences in the surrounding environment. However, the hydrology and general conditions of the depositional setting of each horizon is similar suggesting the setting did not change fundamentally. Preservational variation instead infers small scale, subtle differences: the final form of the fossil is a combination of variation in sedimentation rate, increasing during episodic event bed deposition relative to normal background deposition, residence on the sediment, and exposure to current activity in bottom waters, the effects of which increased as soft tissue decay became more extensive. Overall, Neusticosaurus from the three horizons in the lower Meride Limestone exhibit better preservation due to shorter residence and more rapid sedimentation, compared to Serpianosaurus from the upper part of the Besano Formation.