The fossil record and palaeoenvironmental significance of marine arthropod zooplankton

Arthropods are a major component of the marine zooplankton, functioning as intermediates between primary producers and tertiary consumers in pelagic food webs. They have likely occupied the water column ecospace since the early Cambrian, co-evolving with several major plankton groups through the Phanerozoic. Analysis of the fossil record of arthropods indicates that a zooplanktonic lifestyle has arisen independently across several arthropod groups, and that the arthropod body plan has shown adaptability to fundamental environmental change. Key anatomical adaptations and reproductive strategies that are compatible with and/or facilitate a zooplanktonic lifestyle include well-developed swimming appendages, buoyancy, an active metabolism supported by efficient respiration, visual or photoreception organs for use in detecting mates, food and predators, and the nurturing of young within the exoskeleton. Many of these characters are apparent in the earliest record of arthropods. The development and diversification of arthropod zooplankton into the food chain helped enable large secondary and tertiary consumers to enter the water column and also contributed to the export of organic matter to the seabed via the faecal stream. Although the fossil record of arthropod zooplankton is extremely poor for most of the Phanerozoic, their position in marine food webs suggests that they have been fundamental to rebuilding marine trophic structure following major extinction events, and to maintaining marine plankton diversity through Phanerozoic time.