This paper offers a re-description of one of the oldest (Lower Hettangian, Lower Jurassic) plesiosaurians (Reptilia: Sauropterygia) from Germany and discusses its phylogenetic relevance. Lindwurmia thiuda, new genus and species, is a moderately sized plesiosaur (estimated size between 2 and 3 m long) exhibiting several plesiomorphic features. Although it presents a short and robust premaxillary rostrum, constricted at the premaxilla-maxilla suture as observed in Rhomaleosauridae, its skull and skeleton share a great number of characters with basal sauropterygians and basal plesiosaurians: premaxillae and maxillae in contact posterior to the external nares, absence of contact between the vomers and the maxillae posterior to the internal nares, cervical zygapophyses wider than the vertebral centrum width and curved anterior border of the humerus. Phylogenetic analyses variably place Lindwurmia as sister taxon to Anningasaura and in a basal position to all other plesiosaurians or recover it among rhomaleosaurids. Most of the oldest plesiosaurians are known from the Hettangian or Hettangian-possibly Sinemurian strata of the UK (Lyme Regis and Street); Lindwurmia thus represents one of the oldest European taxa found outside of the UK.
Keywords: Germany; Hettangian; Lindwurmia; Lower Jurassic; Plesiosauria.