EVOLUTION AND BIOSTRATIGRAPHY OF MIDDLE ORDOVICIAN RADIOLARIANS

Well-preserved radiolarians are common in a number of samples from the Arenig of western Newfoundland and Spitsbergen (Svalbard). The faunas are dated by associated graptolites. The radiolarian faunas in Spitsbergen are from the latest early Ordovician Didymograptellus bifidus Biozone (Chewtonian). Comparable radiolarians are also present in the Cow Head Group of western Newfoundland, but the available samples range from the D. bifidus Biozone to the Isograptus victoriae maximodivergens Biozone (Castlemainian 4).

The radiolarian faunas are dominated by spumellarians of various development of their spheres. Spheres with unique sheet-like surfaces bearing pores with raised rims, similar to some modern collospaeroid species, are common, but species with one or more spheres constructed of round bars are abundant too. The typical labyrinthine shells of early Haplotaeniatumidae are moderately common in the Castlemainian, but seem to be lacking in the Chewtonian.

The diverse faunas represent the oldest known spumellarian dominated radiolarian faunas. The more primitive Archaeospicularia, dominating the Cambrian to Tremadoc faunas, are rare in the Arenig. A considerable gap in the faunal record exists from the early Tremadoc through most of the Arenig and the radiolarians from the D. bifidus Biozone are the oldest Arenig radiolarians found so far. The well-known Middle Ordovician radiolarian faunas with Proventocitum procerulum have been regarded as of Arenig age, but in the light of these newly discovered faunas and biostratigraphical revisions, Proventocitum appears to originate only in the early Darriwilian (latest Arenig).

A distinct and dramatic change in radiolarian skeleton structures and faunal composition took place in the late Tremadoc to early Arenig. The relatively unordered three-dimensional mesh of the Archaeospicularia was replaced by well ordered shell structures. It is currently unknown, whether this change represents a gradual replacement by new elements or a Tremadoc extinction event and re-diversification.