The palaeogeographical spreading of the acritarch genusVeryhachium during the Ordovician

SERVAIS, T., ; LI, J.; MOLYNEUX, S.G.; RUBINSTEIN, CLAUDIA V.; VECOLI, M.; YAN, K.

Lund

Simposio; International Geoscience Programme (IGCP) Project 591, Annual Meeting, Lund, Suecia; 2013

Lund University & IGCP Project 591,

The acritarch genus Veryhachium Deunff 1954, originally described from the Ordovician of Britanny, western France, is one of the most frequently recorded organic-walled microphytoplankton genera. Over 250 specific and subspecific taxa are recorded from the Cambrian to the Oligocene, and the genus is generally abundant and widespread throughout the Phanerozoic. The detailed analyses of the first occurrences of Veryhachium in different localities from different palaeocontinents providedprecise first appearance data (Servais et al. 2007). In addition, it is also possible to describe the palaeobiogeographical radiation of the genus from the South Pole to all Ordovician palaeocontinents: first Veryhachium specimens appeared in the middle part of the Tremadocian (Early Ordovician), and possibly earlier in the early Tremadocian, at levels where graptolites of the R. flabelliformis group occur, in localities from North Africa, which was located at the South Pole during the earliest Ordovician.  Subsequently, Veryhachium is also present in the late Tremadocian in localities of the Gondwanan margin in North Africa and Avalonia (English Lake District and Rügen Island, Germany), which are all considered to have been located at high latitudes (> 60°) in the southern hemisphere. In China, located at intermediate latitudes (between 30° and 60° S), Veryhachium has not been recorded below the lowermost Floian (lowermost Arenig), and in Argentina, located at similar latitudes, not below the middle Floian (lower-middle Arenig). Veryhachium has not been recorded outside of peri-Gondwana during the Tremadocian. The first occurrence on another palaeocontinent, Baltica, is recorded in the lower Floian, but the genus did not become common untilthe Darriwilian (Middle Ordovician). This suggests that Veryhachium first appeared in the Tremadocian at high latitudes before it radiated to lower latitudes of the Gondwanan margin (China and Argentina) and Baltica during the Floian, to become cosmopolitan by the end of the Middle Ordovician.  After the Middle Ordovician, the genus became a major component of most open marine acritarch assemblages throughout the Palaeozoic. For instance, in the Permian, it is the most frequently recorded microphytoplankton taxon. Veryhachium dominates most open marine acritarch assemblages with numerous taxa belonging to the ?Veryhachium-Micrhystridium complex.? The relatively slow expansion of Veryhachium in the Ordovician, over some 20 million years from a regional distribution in high latitudes in the early-middle Tremadocian to a cosmopolitan distribution in the Upper Ordovician, raises the question of the usefulness of the genus Veryhachium for biostratigraphical correlations. Although of significant importance for regional stratigraphical correlations in the earliest Ordovician, intercontinental correlations using the genus should be analysed with greatest care.