Stratigraphy
Sauropterygians have the longest stratigraphic range of any group of Mesozoic marine reptiles. Basal sauropterygians occur throughout the Triassic and material identified as possible plesiosaurs occurs in Middle Triassic deposits (Benton, 1993). However, the oldest diagnostic plesiosaurs do not occur until the uppermost Triassic (Taylor and Cruickshank, 1993b; Storrs, 1994, 1997).
The plesiosaur lineage reached a worldwide distribution during the early Jurassic Period and plesiosaurs achieved their maximum diversity during the Late Jurassic (Sullivan, 1987). They persisted successfully to the end of the Cretaceous Period - their fossil record comes to an end at the K-T boundary. Plesiosaur vertebrae of putative Palaeocene age were wrongly dated (Lucas and Reynolds, 1993).
Geological time scale
Chronostratigraphy is the division of the rock record into discrete units of time (as opposed to discrete units, or formations, of rocks - lithostratigraphy). The Mesozoic Era is divided into several systems, which are themselves subdivided into series, which are divided into stages. These units of rock correspond to geochronological units of time (Periods, Epochs and ages, respectively.) These terms are used throughout this site so the table below provides a summary of Mesozoic chronostratigraphy/geochronology for reference.
