Postcranial morphology and growth of the pachypleurosaur Anarosaurus heterodontus (Sauropterygia) from the Lower Muschelkalk of Winterswijk, The Netherlands

Sauropterygia from the Muschelkalk are only found in lag deposits known as bone beds, and most of the material consists of isolated bones. Alpha taxonomy of Sauropterygia from the Germanic Basin which include Pachypleurosauria is thus based mainly on skull morphology of a few specimens. Articulated or associated postcranial material of pachypleurosaurs, associated with diagnostic skull material, is very rare in the Germanic Basin and currently occurs in larger numbers only in the Lower Muschelkalk of Winterswijk (Gelderland Province, The Netherlands), which continuously produces new material. For the first time, the morphology of several partially articulated skeletons of the pachypleurosaur Anarosaurus heterodontus is described and compared. Some of those specimens have skull material attached; others were identified as pachypleurosaurs on the basis of their long bone histology. The current study revealed that postcranial bones of A. heterodontus feature a diverse morphology reflecting differences during ontogeny. Thus, A. heterodontus specimens could be assigned to size classes (I-III). However, on the basis of morphology, histology, and maximal known size of isolated skulls and humeri, none of these specimens represent fully grown individuals. Growth mark counts of midshaft-femur samples, morphologically assigned to size class III, document that this size class was reached within the first year of life. Size class III continued into the second year of life, and then afterwards skeletal maturity was reached. Thus, a juvenile A. heterodontus grew very fast, which is also indicated by its bone tissue type, composed of a high number of radial vascular canals and a fast-deposited bone matrix. The assignment of isolated bones from Lower to Middle Muschelkalk localities to A. heterodontus is now possible with an extensive amended diagnosis of this taxon. This largely contributes to the understanding of taxonomical diversity and distribution. Morphological comparison of the postcranial skeleton of A. heterodontus with that of the two other valid pachypleurosaurs from the Germanic Basin, Anarosaurus pumilio and Dactylosaurus, supports their close phylogenetic relationship. Furthermore, the skeleton of A. heterodontus has no morphological or histological aquatic adaptation such as pachyostosis or pachyosteosclerosis and thus represents the least degree of aquatic adaptation within Pachypleurosauria.