Callovian

The Callovian is a stage of the Middle Jurassic, part of the International Chronostratigraphic Chart. It spans roughly from 166.1 to 163.5 million years ago, making it the middle subdivision of the Jurassic. The Callovian follows the Bathonian and precedes the Oxfordian. The lower and upper boundaries are defined by Global Boundary Stratotype Section and Point (GSSP) markers, tied to distinctive biostratigraphic signals such as ammonite faunas, and mark the transition to and from adjacent stages.

During the Callovian, global climate was generally warm with relatively high sea levels, producing widespread epicontinental seas across Europe and other regions. This environment led to sedimentation of carbonate rocks, marls, and shales, especially in marine settings. The stage is well represented in European basins, where continuous shallow seas fostered the accumulation of marine sediments that preserve rich invertebrate assemblages.

Biostratigraphy for the Callovian relies heavily on ammonites and other marine microfossils to define regional zones and correlate strata globally. The fossil record also includes marine reptiles such as ichthyosaurs and plesiosaurs, and diverse invertebrate life that supports broad correlations with contemporaneous deposits elsewhere.

Subdivisions within the Callovian are based on fauna and lithology observed in different regions, with biostratigraphic zonation used to refine correlations. The Callovian ends with the onset of regression that leads into the Oxfordian, marking a transition within the broader Jurassic timeline.