The nature and resolution of Jurassic Ammonite biozones

Abstract

Biostratigraphical studies of Jurassic faunas have a long history and can be traced back to the origin of stratigraphyitself. Progressive biostratigraphical refinement now allows very fine discrimination and by comparing radio-metric methods of dating of stage duration a good idea of the time involved in biostratigraphical subdivisions can now be obtained. The identification of ammonite biohorizons probably represents the ultimate in Jurassic biostratigraphy. Different types of biozones are used in the Jurassic; the main types are described and examples chosen from the Jurassic to illustrate the use of total and local range biozones, combined range biozones, partial range biozones, consecutive range biozones and assemblage biozones. Contiguous zonal index species are seldom directly phylogenetically related; the reasons for this are partly historical, but more often reflect the richness of the faunas. Phylogenetic contiguity is more frequent amongst sub-biozonal index species and it would be expected that at the level of the biohorizon phylogenetic contiguity would be the rule. Examples show that this is not always the case.