Stratigraphy


Sauropterygians have the longest stratigraphic range of any group of Mesozoic marine reptiles. Basal non-plesiosaurian sauropterygians occur throughout the Triassic Period. The earliest material identified as belonging to true plesiosaurs 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 throughout the whole of the Triassic, Jurassic, and Cretaceous periods. Ichthyosaurs became extinct during the Cretaceous Period. Squamates (represented by mosasaurs) originated late in the Mesozoic during the Cretaceous Period.

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 actually wrongly dated (Lucas and Reynolds, 1993).

Geological time scale

The Mesozoic Era is divided into three time periods subdivided into epochs and ages. These units of time also correspond to units of rock (systems, series, and ages, respectively) that go by the same names. The table below provides a summary of the order of Mesozoic stratigraphy. From left to right the columns show: Period/System, Epoch/Series, Age/Stage.

Cretaceous

Upper/Late

Maastrichtian

Campanian

Santonian

Coniacian

Turonian

Cenomanian

Lower/Early

Albian

Aptian

Barremian

Hauterivian

Valanginian

Berriasian

Jurassic

Upper/Late

Tithonian

Kimmeridgian

Oxfordian

Middle/Mid

Callovian

Bathonian

Bajocian

Aalenian

Lower/Early

Toarcian

Pliensbachian

Sinemurian

Hettangian

Triassic

Upper/Late

Rhaetian

Norian

Carnian

Middle/Mid

Ladinian

Anisian

Lower/Early

Scythian