Anarosaurus heterodontus - the least aquatic pachypleurosaur

Abstract | Pachypleurosauria are small-bodied (50 cm-120 cm) basal Eosauropterygia inhabiting epicontinental seas and intraplatform basins of the Tethys Ocean during the latest Early to late Middle Triassic (late Olenekian-Ladinian). Ladinian taxa show a paedomorphic and pachyostotic skeleton, both resulting from secondary aquatic adaptations. Some taxa occur in high individual numbers, which allowed for documenting viviparity and sexual dimorphism, the latter expressed in morphological differences. Pachypleurosaurs are a very diverse clade with currently 7 genera containing about 10 species. Some of these are well known, whereas others are so far only documented by one or two individuals. However, the origin of pachypleurosaurs, their position within Sauropterygia and the phylogenetic relationships between the European clade and the eastern Tethyan taxa are not well understood. Anarosaurus heterodontus, the pachypleurosaur occurring in Winterswijk, is one of the oldest pachypleurosaurs from a stratigraphical perspective. Regarding its morphology, it is one of the least aquatic adapted pachypleurosaurs, which might be the result of its lifestyle: living in near coastal, shallow marine habitats. A. heterodontus differs from other pachypleurosaurs in its special bone histology indicating - when compared to other pachypleurosaurs and modern reptiles - fast growth rates.