Palaeoecology of the Graptoloidea

The onshore-offshore and depth-facies preference of some 200 Ordovician species has led to an ecological model in which the graptoloid habitat is partitioned vertically and horizontally in waters of the neritic and pelagic realms. One group of species (group 1) is largely confined to a deep water biotope (mesopelagic zone, characterised by dysaerobic water) where they are thought to have exploited high bioproductivity denitrification zones. A second group (group 2) occupies an epipelagic biotope and is confined to waters of the epipelagic zone (aerobic water). A third, and less well-defined, group (group 3) occupies the inner shelf waters of the epipelagic zone only. The median duration of species in the deep water biotope is significantly shorter than that of species in the epipelagic biotope suggesting greater environmental stress and instability in the dysaerobic zone over geological time, consistent with models of disruption of the density structure and circulation patterns of the oceans from climatic instability. The diversity and abundance of species were enhanced in both the epipelagic and deep water biotopes in regions of marginal upwelling where nutrients were brought from the deep to the shallow water zones. It was in these regions that the classical graptolitic shale developed.