Article: Evolutionary brachiopod lineages from the Llandovery Series of eastern Iowa
Markes E. Johnson
Stricklandia-Costistricklandia and Pentamerus-Pentameroides-Callipentamerus lineages provide stratigraphical control confirming the repetition of distinct faunas in the lower Silurian Hopkinton Dolomite of eastern Iowa. Traced laterally over the geographical extent of the outcrop area, the lineages also demonstrate that deposition of the Hopkinton Dolomite was phase-like and nearly synchronous. The prominent features that evolved in these brachiopods are the internal, skeletal structures. Changes in the cardinalia of the Iowa stricklandiids correspond to part of the known succession from the Llandovery type district in Wales and from Baltic regions. Variation in the external shell morphology of stricklandiids is interpreted as environmentally induced, and the separation of North American forms into the genera Microcardinalia and Plicostricklandia on this basis is considered unnecessary. The Iowa Llandovery section documents a previous prediction concerning migration and fusion of the outer plates in the Pentamerus-Pentameroides lineage. Pentamerids and stricklandiids formed member populations of 'perched' communities which were intermittently present in platform sea environments. While the illustration of 'insensibly graded' lineages is not possible in a stratigraphical sequence reflecting a history of shifting environments, the progressive changes noted in the Iowa stricklandiids and pentamerids are suggestive of phyletic evolution.