Stratigraphy

Sauropterygians, the group to which plesiosaurs belong, have the longest stratigraphic range of all Mesozoic marine reptiles. The sauropterygian fossil record spans the whole of the Triassic, Jurassic, and Cretaceous periods.

The stratigraphic ranges of the main groups of Mesozoic marine reptiles (modified from Benson et al. 2009). Ichthyosaurs became extinct during the Cretaceous Period, while squamates (represented by mosasaurs) originated late in the Mesozoic during the Cretaceous Period.

Basal non-plesiosaurian sauropterygians lived throughout the Triassic Period. The earliest possible true plesiosaur fossils are from Middle Triassic deposits (Benton 1993a). However, the oldest diagnostic plesiosaur fossils are from the Upper Triassic Rhaetian Stage (Taylor and Cruickshank 1993b, Storrs 1994, Storrs 1997, Wintrich et al. 2017). This includes one diagnostic plesiosaur taxon, Rhaeticosaurus (Wintrich et al. 2017). Another possible Triassic plesiosaur was named from the slightly older Norian stage (Alexeyisaurus Sennikov and Arkhangelsky 2010), but it was later regarded as an “undiagnostic sauropterygian” Wintrich et al. (2017).

The plesiosaur lineage as a whole 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. Some plesiosaur vertebrae once thought to be from the Palaeocene were wrongly dated (Lucas and Reynolds 1993).

Plesiosaurs through 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 of different plesiosaur groups. Click for full size images. N.B. these are draft charts.

Plesiosaur families through time

This chart shows the stratigraphic distribution of plesiosaur families based on the known ranges of valid genera referred to those families. Indeterminate material referred to these families may extend the ranges slightly but isn’t plotted here. The pliosaurid range consists of two adjoined sections, basal pliosaurids in the Early to Middle Jurassic, and derived (thalassophonean) pliosaurids in the Middle Jurassic to the Late Cretaceous (Turonian).

Plesiosaur genera through time

This chart shows the stratigraphic positions and ranges of all known valid (and well-known invalid) plesiosaur genera. Updated in July 2023. Some recently named genera may not yet be plotted. Click for the legible version. For genera where the exact horizon(s) are unknown I have made the range lines dotted.