1.1. General Introduction Plesiosaurs are a group of secondarily aquatic diapsid reptiles belonging to the clade Sauropterygia (Carroll, 1988). Diapsids (subclass Diapsida) are a monophyletic group supported by both morphological and molecular data (Lee, 2001) including lizards, snakes, crocodiles and birds, and represent a major sub-group of the amniote vertebrates, i.e. vertebrate organisms w...