ABSTRACT

The evolutionary trends of the Early Silurian stricklandiid brachiopods from Anticosti Island and other regions of eastern North America are different from those of the classic Stricklandia-Costistricklandia lineage in several aspects: (1) Stricklandia from the uppermost Rhuddanian-lower Telychian carbonate strata of Anticosti Island had a much accelerated rate of crural plate (outer plate of old usage) reduction compared to the Stricklandia lens lineage of the British type area and the Baltic region; (2) reduction of umbones into a chisel-edge-shaped shell posterior to produce the Microcardinalia stock; and (3) development of various types of divaricate rugae and costae in the Microcardinalia (Chiastodoca) and Ehlersella lineages. The presence of the Stricklandia-Costistricklandia lineage in both Europe and North America and the lack of North American stricklandiids (e.g. Microcardinalia and Ehlersella) in Europe imply that pentamerid migration between Baltica and Laurentia may have been largely unidirectional (i.e. east to west) during early-middle Telychian (Early Silurian) time.