Westphalian (stage)

The Westphalian is a regional

The Langsettian, previously known as Westphalian A, is named after the village of Langsett in South Yorkshire, England. It marks the base of the Westphalian regional stage, as defined by the ammonoid Gastrioceras subcrenatum.^[6] The base of the Langsettian (and the Westphalian as a whole) has been dated to around 319.9^[9] or 319.2^[2] Ma.

The Langsettian corresponds to the Calymmotheca ("Lyginopteris") hoeninghausii assemblage zone.^[8] Many widespread plant species first appear near the base of the Langsettian, indicating a spike of diversification in tropical coal swamp habitats.^[8][10] Plant diversity steadily increases through the entire Langsettian, though this may be a result of ecological factors such as the receding coastline.^[8]

Duckmantian

The Duckmantian, previously known as Westphalian B, is named after the village of Duckmanton in Derbyshire, England. The base of the Duckmantian is defined by the ammonoid Anthracoceratites vaderbeckei.^[6] The boundary between the global Bashkirian and Moscovian stages (~315.2 Ma)^[4] corresponds to the mid-late part of the Duckmantian.^[9][4]

The lower-middle part of the Duckmantian corresponds to the Lonchopteris rugosa Assemblage-zone, the most diverse plant biozone in the Carboniferous coalfields of Europe. The rising diversity trend of the Langsettian continues into this biozone, with few notable changes in species composition. In the majority of European coalfields, plant diversity reached a plateau around halfway through the Duckmantian. Coal swamps became increasingly unstable in the Paripteris linguaefolia assemblage zone, which begins in the upper part of the Duckmantian.^[8][10]

Bolsovian

The Bolsovian, previously known as Westphalian C, is named after the town of Bolsover in Derbyshire. The base of the Bolsovian is defined by the ammonoid Donetzoceras aegiranum,^[6] and has an estimated age of around 313.8^[9] or 313.7^[5] Ma.

The Paripteris linguaefolia assemblage zone continues into the Bolsovian, and a decline in plant diversity is apparent across the entirety of Europe.

Westphalian D is often referred to as the Asturian, named after the Asturias region of northwest Spain.^[11] In most of Europe, Westphalian D is distinguished by plant fossils. Asturias is one of the few European regions with enough late Westphalian marine fossils to allow for precise correlations with other marine strata. The proposal to fully implement the name "Asturian" has yet to be ratified, as some stratigraphic difficulties in Spain are not fully resolved.^[4][3]

The lower part of the Asturian belongs to the Linopteris obliqua assemblage zone^[8] (sometimes termed the Linopteris bunburii zone).^[5] This biozone is notably lower in diversity than previous Westphalian biozones. Plant fossils are still common over much of Europe, with Neuropteris ovata as a particularly abundant species.^[8] An important ecological turnover occurs about halfway through the Asturian (~309 Ma),^[5] with the arrival of the Crenulopteris acadica assemblage zone^{[12][5][8][10]} (previously known as the Lobatopteris vestita zone).^[7] Lycopsid fossils become very rare, while marattialean ferns become abundant in coal swamp deposits.^[8] Many European coalfields were positioned in a foreland basin north of the Variscan orogeny. As mountain-building continued, uplift accelerated in the basin, endangering the survival of coal swamp environments.^[8][10]

Plant fossils (and coal deposits as a whole) are uncommon in the following "Cantabrian" substage of the Stephanian Stage.^[3] The end of the Asturian is a topic of strong debate; most estimates place the Westphalian-Stephanian boundary before the start of the Kasimovian global stage (~307 Ma),^[6][4][3] whereas a few place the boundary within the Kasimovian.^[5] U-Pb radiometric dating of tonstein beds in Spain estimate that the Asturian lasted from 310.7 to 307.5 Ma, ending just prior to the Kasimovian.^[3]

Life

The Westphalian interval is widely recognized for its coal deposits-- rocks that were deposited broadly across regions that were in low paleolatitudes. These deposits, from so-called "coal swamps" have yielded rich assemblages of fossils including spore-bearing and seed-bearing plants, fishes, and tetrapods.^[13][14][15] Amphibians were diverse and dominated some communities. The collapse of the rainforest ecology between the Moscovian and Kasimovian removed many amphibian species that did not survive as well in the cooler, drier conditions. Reptiles, however prospered due to specific key adaptations and underwent a major evolutionary radiation, in response to the drier climate that led to the rainforest collapse.^[16][17]

References