Postcranial material of Nothosaurus marchicus from the Lower Muschelkalk (Anisian) of Winterswijk, The Netherlands, with remarks on swimming styles and taphonomy

摘要

The postcranial morphology of Nothosaurus from Winterswijk is described on the basis of three partially preserved skeletons. Because of the association with a diagnostic cranium, two of these can be confidently assigned to N. marchicus. Preserved humeri are recognized as morphotype II, which confirms the relationship between this humeral morphotype and N. marchicus. The occurrence of an additional nothosaur taxon in the Lower Muschelkalk, strong sexual dimorphism, or a broader size range of N. marchicus is evidenced again. The postcranial morphology of Nothosaurus is compared to published and new data on the basal pistosauroid (cf. Cymatosaurus) from the same locality. Numerous shared morphological characters of N. marchicus, Nothosaurus sp., and the basal pistosauroid from Winterswijk hamper assignment of isolated bones, which is presently only possible if a combination of features on multiple bones from a single individual can be assessed. Ontogenetic stage of the described skeletons is discussed as well. Differences between N. marchicus, the basal pistosauroid, and the morphologically similar, but smaller pachypleurosaur A. heterodontus are mainly related to the morphologies of the humerus, ulna, and the clavicle-interclavicle complex, and thereby indicate different modes of locomotion i.e., swimming styles. The pachypleurosaur and the basal pistosauroid were most likely anguilliform swimmers, whereas Nothosaurus also employed its forelimbs during swimming (paraxial swimming). Taphonomical observations on material of the Winterswijk locality indicate the presence of scavengers, although most disarticulated skeletons were transported by water and decayed and disintegrated in shallow water.