Palaeoecology of the Graptoloidea

Graptoloids (Class Graptolithina) were colonial pterobranch hemichordates and formed a major component of the early Paleozoic macrozooplankton. By analogy with modern pterobranchs, they filtered food particles from the water mass in which they lived by means of ciliated lophophores. They are inferred to have been able to migrate vertically through the water column for feeding efficiency and predator avoidance as by modern euphausiids but their means of locomotion is still highly conjectural. The rhabdosomes which housed the colonies grew by distal accretion and large examples are inferred to have taken up to several years to have reached full size. The few studies of survivorship suggest a relatively constant mortality of individuals within the community. Tubular and blister-like outgrowths of cortical tissue are seen rarely and are inferred to be a response to parasitism, and rare faecal packages with crumpled rhabdosomes suggest ingestion by predators. The function of the monospecific aggradations of conjoined colonies (synrhabdosomes), seen on rare bedding planes, is uncertain, but is unlikely to have been part of their normal life history and ecology. Monospecific assemblages of unconnected colonies on single bedding planes are not rare, however, and suggest localised populations of single species in the overlying water column in life. Other bedding planes have a range of species resulting from mass mortality from volcanic eruptions and suggest a diversity of species in the water column. 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.