A new pachypleurosaur from the Early Ladinian Prosanto Formation in the Eastern Alps of Switzerland

Nicole Klein; Heinz Furrer; Iris Ehrbar; Marta Torres Ladeira; Henning Richter; Torsten M Scheyer

doi:10.1186/s13358-022-00254-2

A new pachypleurosaur from the Early Ladinian Prosanto Formation in the Eastern Alps of Switzerland

Swiss J Palaeontol. 2022;141(1):12. doi: 10.1186/s13358-022-00254-2. Epub 2022 Jul 13.

Authors

Nicole Klein¹, Heinz Furrer¹, Iris Ehrbar¹, Marta Torres Ladeira¹, Henning Richter², Torsten M Scheyer¹

Affiliations

¹ Palaeontological Institute and Museum, University of Zurich, Karl Schmid-Strasse 4, CH-8006 Zurich, Switzerland.
² Diagnostic Imaging Research Unit (DIRU), Clinic for Diagnostic Imaging, Department of Clinical Diagnostics and Services, Vetsuisse Faculty, University of Zurich, Winterthurerstrasse 258c, CH-8057 Zurich, Switzerland.

Abstract

The Alpine Prosanto Formation (Middle Triassic) cropping out in the Ducan region in eastern Switzerland has yielded a rich fish and reptile fauna. Here, we present new pachypleurosaur remains from the upper part of the formation (Early Ladinian), similar to the previously known pachypleurosaurs from the Middle Triassic UNESCO World Heritage Site of Monte San Giorgio in southern Switzerland/northern Italy. From these remains, a new pachypleurosaur species, Prosantosaurus scheffoldi nov. gen. et spec., is described on the basis of six fairly complete skeletons, one disarticulated specimen and an isolated skull. As is typical for pachypleurosaurs and most other Triassic marine reptiles, the new taxon is based to a large degree on a combination of characters (e.g., nasals articulating broadly with the anterior margins of the prefrontals and lacking posterior processes; postorbitals with rounded anterior processes that articulate with the postfrontals anterolaterally) rather than on many unambiguous autapomorphies, although a few of the latter were found including (1) a premaxilla which is excluded from entering both the external and internal nares and (2) a parietal, which is distinctly longer than wide and carrying distinct anterolaterally angled processes. Phylogenetic relationships of the new taxon are tested within European Pachypleurosauria, revealing that the new species is the sister taxon to a clade including Serpianosaurus, Proneusticosaurus, and the monophyletic Neusticosaurus spp. Mapping of palaeogeographic and stratigraphical distribution of valid European pachypleurosaurs shows that a formerly proposed scenario of migration of pachypleurosaurs from the eastern Palaeotethys during the Olenekian into the Germanic Basin and a subsequent diversification and invasion during the Anisian into the intraplatform basins of the South Alpine realm must be re-assessed. The exceptional preservation and preparation of the Ducan fossils further allow the description of tooth replacement patterns for the first time in a European pachypleurosaur species. The "alveolarization" of replacement teeth, the horizontal replacement pattern, and the subsequent remodelling of the functional alveoli during tooth replacement supports the monophyly of Sauropterygia as discussed before.

Supplementary information: The online version contains supplementary material available at 10.1186/s13358-022-00254-2.

Keywords: Alpine Triassic; Character combination; European pachypleurosaur diversity; Phylogeny; Stratigraphic and palaeogeographic distribution; Tooth replacement.