LATITUDINAL AND DEPTH ZONATION OF EARLY ORDOVICIAN GRAPTOLITES

Early Ordovician graptolites are inferred to have lived in two primary depth zones of the ocean waters. A shallow, epipelagic zone biotope contained species found preserved in all depth facies, whereas a deep, mesopelagic, zone biotope contained species now found only in deep-water facies (isograptid biofacies). An inshore biotope contained a few species that were apparently restricted to shelf-platform waters and which are now found in shelf sediments, along with members of the epipelagic biotope (didymograptid biofacies). The effects of surface water temperature on graptolite distribution, as measured by differences between high palaeolatitude (Europe/S. Britain - 'Atlantic province') and low palaeolatitude (Australasia/N. America - 'Pacific province') regions, was less marked than that of depth. A survey of over 80 species and species groups shows that a surprisingly large number of graptolites were pandemic and eurythermic. In the early Ordovician, individual species were more widespread than those of any other invertebrate group, hence their great value for long-distance correlation.