Origins, Evolution, and Diversification of Zooplankton

Fossil plankton are difficult to identify but have formed a major component of most marine ecosystems throughout geological time. The earliest fossil heterotrophs include planktic forms, and subsequent adaptive innovations quickly appeared in the plankton; these include metazoans and animals with hard parts. Movement into the plankton occurred sporadically throughout geological history and seems to have been independent of any biological or environmental forcing mechanism. Subsequent radiations and extinctions in the cohort of plankton closely reflect events in the benthos. The diversity of zooplankton rose quickly during the early Paleozoic era, but low plankton diversity characterized the late Paleozoic. Significant radiations during early Mesozoic times led to an overall increase in diversity through the Phanerozoic eon. As the composition of the zooplankton has changed, so has their effect on biogeochemical cycles.