| System (period) | Series | Stage (age) | Lower boundary, Ma | |
|---|---|---|---|---|
| Devonian | Lower | Lochkovian | 419.2±3.2 | |
| Silurian | Pridoli |  423.0±2.3
| ||
| Ludlow | Ludfordian | 425.6±0.9
| ||
| Gorstian | 427.4±0.5
| |||
| Wenlock | Homerian | 430.5±0.7
| ||
| Sheinwoodian | 433.4±0.8
| |||
| Llandovery | Telychian | 438.5±1.1
| ||
| Aeronian | 440.8±1.2
| |||
| Rhuddanian | 443.8±1.5
| |||
| Ordovician | Upper | Hirnantian | older | |
| Subdivisions and "golden spikes" according to IUGS as of September 2023[1] | ||||
The Wenlock is a second series of the Silurian system, corresponding to the Wenlock epoch of the Silurian period. It began after the Telychian age of the Llandovery at 433.4 ± 0.8 Ma (million years ago) and lasted until the Gorstian age of the Ludlow at 427.4 ± 0.5 Ma (IUGS 2023). Fossils of very early terrestrial organisms are known from this epoch.
The Wenlock is named after Wenlock Edge in Shropshire, UK.
Definition[]
The Global Stratotype Section and Point (GSSP) of the Wenlock series and the Sheinwoodian stage was ratified in 1980. The beginning of the epoch is approximately related to the the base of the Cyrtograptus murchisoni Graptolite Biozone. The type section is located in Hughley Brook, UK.[2]
Subdivisions[]
The Wenlock divided into two stages: Sheinwoodian and Homerian. In geochronology, both correspond to the same ages.
Paleogeography[]
Gondwana remained the largest continent, stretching from the South Pole to the equator. Smaller continents, Laurentia, Baltica and Avalonoa, were moving to each other near the equator. Glaciers that formed in the Ordovician gradually melted, causing sea levels to rise. Vast and extended, but shallow seas formed near the continents. The North Pole most likely remained free of land.
Wenlock life[]
In the wide, shallow seas, massive coral reefs formed where life flourished. Numerous stenolaemates (Chasmatopora, Eridotrypa, Fenestrellina), bivalves (Nuculites, Mytilacra, Modiolopsis) and gastropods (Platyceras, Murchisonia, Hormotoma) are known from the marine Wenlock deposits. Cephalopods like Orthoceras and Dawsonoceras hunted in the water column. Trilobites (Encrinurus, Trimerus, Dalmanites, Calymene) lived near the coasts and in the open ocean. Ostracods (Drepanellina, Paraechmina, Velibeyrichia) and eurypterids (Eusarcana, Pterygotus, Erettopterus) are also known from deposits of this epoch.
The first land plants appeared in the Ordovician (according to some assumptions even earlier), but well-developed vascular plants (tracheophytes) can only be traced in fossil record from the Wenlock. Cooksonia, the oldest known tracheophyte, is known from the Wenlock and younger sediments. The development of terrestrial flora led to the evolution of terrestrial fauna. Scorpion Palaeophonus and myriapod Pneumodesmus are among the earliest land animals. It is possible that terrestrial gastropods already existed at that time.
Vertebrates lived in water. The number and variety of fish increased, among which jawed fish began to predominate.
References[]
- ↑ "Latest version of international chronostratigraphic chart". International Commission on Stratigraphy. https://stratigraphy.org/chart#latest-version.
- ↑ "GSSP Table - Paleozoic Era". https://timescalefoundation.org/gssp/index.php?parentid=77.