Article: Variability in the Ordovician acritarch Dicrodiacrodium
Species of the Ordovician acritarch genus Dicrodiacrodium, with a single apical process, are evaluated critically, based on a review of published literature and new studies on material from Belgium, Bohemia, China, Germany, and Morocco. Investigations of large populations of such species show a wide variability between specimens. Biometric studies, including measurements on type area material, show that the subdivision into varieties and even into species is not justified. There is a complete gradation between all of the prescribed taxa. which are impossible to distinguish. Therefore, all specimens with a single apical process are classified as a single taxon. Dicrodiacrodium ancoriforme emend, nov. D. ancoriforme is very easy to recognize and of great biostratigraphical and palaeogeographical importance. Its First Appearance Datum (FAD) is in the Arenig Undulograptus sinodentatus/Didymograptus nexus graptolite Zone in South China (approximately equivalent to the British Arenig Isograptus gibberulus Biozone). Palaeobiogeographically, it is typical of the cold to temperate Gondwanan and peri-Gondwanan ('Mediterranean') Palaeoprovince.