The Silurian System is split into the following divisions ➚ or Series. These are then split into further sub-divisions or Stages.
Inevitably the definition of series and stages will be heavily influenced by the area where the rocks were first studied and the Series first defined. In the case of the Silurian this was Shropshire, UK. The stages were initially defined by marked changes in rock formation, usually resulting itself from changes to depositional facies.
Ideally the divisions should be easy to map over the whole world so that rocks and fossils can be correlated. Very rarely does an event have a worldwide record in the rocks (e.g. massive volcanic eruption) , so it is the correlation of fossil finds that is normally used. In the case of the Silurian it is the first appearance of certain graptolite fossils that define most of the boundaries to the divisions. The occurrence of graptolites is reinforced by the presence of conodonts and brachiopods. The main confirmatory correlation is by the geo-chronological dating of volcanic ash (often occurring as bentonite) layers in the rocks. The ash has a fairly uniform composition over time and so, using the relative proportions of radio-active isotopes a time range can be measured. However the accuracy of this dating mechanism is fairly poor (it can be affected by later metamorphosis) and the time ranges have been adjusted over the last fifty years.
|Pridoli||Named after area in Czech Republic. Previously named Downton in the UK but after international agreement it was determined that Pridoli had a much better fossil record.||417-419Mya||The top of the series is marked by first appearance of the graptolite Monograptus uniformis thus defining the Silurian-Devonian boundary. See for a full definition ➚|
|Ludlow||Named after the historic town of Ludlow ➚ in southern Shropshire||419-423Mya||Marked by first appearance of the graptolite Neodiversograptus nilssoni.|
|Wenlock||Named after Wenlock Edge and town of Much Wenlock ➚ in Shropshire.||423-428Mya||Marked by first appearance of the graptolite Cyrtograptus centrifugus.|
|Llandovery||Named after the town of Llandovery ➚ in mid Wales, although the town of Llandovery is itself seated on Ordovician rocks.||428-440Mya||Marked by first appearance of the graptolite Parakidograptus acuminatus.|
This is how the stages are defined within each of these series.
|Pridoli||None||There are no agreed divisions for the Pridoli series.|
|Ludlow||Ludfordian||The Bohemograptus proliferation and also Saetograptus Leinardinesis graptolites mark the base of this stage (example location : Sunnyhill Quarry, Shropshire, UK). Named after village of Ludford.|
|Gorstian||Graptolites Tumescens and Incipiens mark the base of the stage (example location : Quarry at Pitch Coppice, Shropshire, UK). Named after Gortsy village SW of Ludlow.|
|Wenlock||Homerian||The base is defined by the first appearance of the graptolite Cyrtograptus lundgreni (example location : Whitwell Coppice within Coalbrookdale Formation, Shropshire, UK) Named after Homer Village - North of Much Wenlock.|
|Sheinwoodian||The Wenlock-Llandovery boundary is defined by the first occurrence of Cyrtograptus centrifugus Other graptolites in this stage include : Ellesae, Linnarssoni, Rigidus, Riccartonensis and Murchisoni. (example location : Hughley Brook start of Buildwas Formation, Shropshire, UK). Named after Sheinwood Village - North of Much Wenlock.|
|Llandovery||Telychian||The base of the Telychian is defined by the disappearance of the brachiopod Eocoelia intermedia and the apperance of Eocoelia intermedia Graptolites in this stage include : Crenulata, Griestoniensis, Crispus, Turriculatus. (example location : Pen-lan-Telych Farm near Llandovery, Powys, UK). Named after Telych Farm.|
|Aeronian||The base of the Rhuddanian is defined by the first occurrence of Monograptus triangulatus Graptolites in this stage include : Sedgwickii, Convolutus, Argenteus, Magnus, Triangulatus. (example location : Cwm-coed-aeron near Llandovery, Powys, UK) Named after the example location.|
|Rhuddanian||The base of the Rhuddanian and the whole Silurian System (the Ordovician-Silurian boundary) is defined by the first occurrence of Parakidograptus acuminatus and Akidograptus ascensus This boundary is currently defined at Dob's Linn, near Moffat, Scotland, UK. Graptolites in this stage include : Cyphus, Acinaces, Atavus, Acuminatus. (example location : Cefn-rhuddian farm Llandovery, Powys, UK) Named after the example location.|
It has been proposed that the Pridoli Series be subsumed within the Wenlock series, this is because it has such a short duration (at most 2Mya) and the other Palaeozoic Systems have three series rather than four for the Silurian.
The reason for the chequered history of the Pridoli is that the Silurian-Devonian boundary was an unconformity over most of the UK where the original stratigraphic work was done. The series was initially called the Downtonian series (named after Downton near Ludlow, Shropshire). Marine facies changed to sub-aerial and so there was a large change in the types of fossil deposited. This makes correlation of rocks in the UK and across the world difficult and so a different area was chosen that had uninterrupted deposition over the transitional time period, the area chosen happened to be in Pridoli, in the Czech Republic.
Note that the places mentioned for the Wenlock, Ludlow and Llandovery series are type areas but they are not the internationally agreed standard areas.