Since 1935, the Westphalian has been split into four substages, from oldest to youngest: Langsettian (Westphalian A), Duckmantian (Westphalian B), Bolsovian (Westphalian C), and "Asturian" (Westphalian D). These substages are defined by
guide fossils, or "index fossils," particularly
plant macrofossils,
miospores, and
ammonoids. The Westphalian exhibits distinctive changes in plant diversity, and many plant macrofossil
assemblage zones have been defined across an area encompassing Europe,
Turkey, and the
Maritime Provinces of Canada.
Langsettian 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. or 319.2 Plant diversity steadily increases through the entire Langsettian, though this may be a result of ecological factors such as the receding coastline. Ma. The
Paripteris linguaefolia assemblage zone continues into the Bolsovian, and a decline in plant diversity is apparent across the entirety of Europe. 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. The lower part of the Asturian belongs to the
Linopteris obliqua assemblage zone (previously known as the
Lobatopteris vestita zone). Lycopsid fossils become very rare, while
marattialean ferns become abundant in coal swamp deposits. 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. Plant fossils (and coal deposits as a whole) are uncommon in the following "Cantabrian" substage of the Stephanian Stage. 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), whereas a few place the boundary within the Kasimovian. 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. ==Life==