Sauropterygians, the group to which plesiosaurs belong, have the longest stratigraphic range of all Mesozoic marine reptiles. Basal non-plesiosaurian sauropterygians occur throughout the Triassic Period. The earliest material identified as true plesiosaurian is known from Middle Triassic deposits (Benton, 1993). However, the oldest diagnostic plesiosaurs are from the uppermost Triassic (Taylor and Cruickshank, 1993b; Storrs, 1994, 1997).
The above image shows the stratigraphic ranges of the main groups of Mesozoic marine reptiles (modified from Benson et. al. 2009). The sauropterygian fossil record spans the whole of the Triassic, Jurassic, and Cretaceous periods. Ichthyosaurs became extinct during the Cretaceous Period, while squamates (represented by mosasaurs) originated late in the Mesozoic during the Cretaceous Period.
Plesiosaurs in time
The Mesozoic Era is divided into three time periods subdivided into epochs and ages. These units of time correspond to units of rock (systems, series, and ages, respectively). The charts below provide a summary of Jurassic and Cretaceous stratigraphy and shows the position and ranges plesiosaur taxa. Click for full size images. NB, these are draft charts and may contain errors.
Plesiosaur families in time
This chart shows plesiosaur family stratigraphic ranges based on the known ranges of valid genera referred to those families. Indeterminate material referred to these families may extent the ranges slightly. The Pliosauridae plot consists of two adjoined sections, basal pliosaurids in the Early to Middle Jurassic, and derived (thalassophonean) pliosaurids in the Middle Jurassic to Late Cretaceous.
Plesiosaur genera in time
This chart shows the position and ranges of valid plesiosaur genera. Some recently named genera are not yet plotted. Click for the legible version.
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, where their fossil record comes to an end at the K-T boundary. Plesiosaur vertebrae once regarded as Palaeocene in age were wrongly dated (Lucas and Reynolds, 1993).