Article: A biometric re-evaluation of the Silurian brachiopod lineage Stricklandia lens/S. laevis
Quantitative data from Norway, Estonia, and the Llandovery area in Wales are used to re-evaluate the existence of the Stricklandia lens/S. laevis part of the Stricklandia/Costistricklandia evolving lineage. Two of the directed trends originally used to discriminate subspecies in material of the type Llandovery area fall within the same range of variation at roughly contemporaneous horizons in the Lower Silurian of Norway, Wales, and Estonia. These are reduction of the outer plates relative to the inner plates, and reorientation of the inner plates. A third trend in the Llandovery material, increasing size of cardinalia, does not occur in the Norwegian material and is regarded as environmentally determined. The stricklandiids show large phenotypic variation within sample populations and correlation must be based on a minimum of ten specimens. The four lowermost subspecies of Williams (1951) occur abundantly in a nearly continuous section in Norway, providing material for refined biocorrelation through an index combining the two evolutionarily valid trends.