BIGG

of

Britain and Ireland

Graptolites

Biozones and Index Fossils

Silurian

 

Ludlow 427.4-423.6Ma

Bohemograptus Zone

This is currently the last of the graptolite zones as dscribed by Holland and Palmer (1974) highlighted by only containing a single species and is the youngest Graptolite zone in the UK. It is a question how B.bohemicus tenuis is present here and in the Scanicus zone but not seen in other time zones?

Leintwardinensis Zone

Once was the final graptolite zone in the U.K. Mainly containing the type fossil only but others have been found to occur at the same time. Defined by Marr (1892) in the Lake district has been widely recognised in the UK. Mainly by S.l. leintwardinensis and the very low diversification of only 3 confirmed species.

Tumescens/ Incipiens Zone

Both key graptolites (incipiens and tumescens) appear earlier than this zone and often are not found together. There are also related shelf deposits in this horizon. Established by Wood (1900) in Wales but is difficult to define as it relates to an interregnum that may not always be obvious. Graptolite diversity is massively reduced compared to earlier zones. The only new species is Saetograptus Incipiens.

Scanicus Zone

Sometimes difficult to find the base and so referred to as the nilssoni-scanicus zone. Sudden abundance of certain key species. Established by Wood (1900) is widely employed as a biozone but is problematic as it characterised by the extinction of many nilssoni taxa and an increase in Cucullograptus scanicus and Saetograptus chimaera chimaera. (Rickards 1976)

Nilssoni Zone

Originally defining the fauna between the Wenlock and Aymestry limestones the zone was redefined by Wood (1900). Upper part of the zone contains L.progenitor and lower part by N.nilssoni. First zone of the Ludlovian with a big species input. Most of the recent Ludlow work has been carried out by the BGS and has focused on the Nilssoni as it marks the Ludlow but only approximately. Noted by its noted biodiversity compared to the Wenlock, with the appearance of Saetograptus being paramount.

ludlow