Most modern marine ecology is ultimately based on unicellular phytopla
nkton, yet most large animals are unable to graze directly on even rel
atively large net phytoplankton; the repackaging effected by herbivoro
us mesozooplankton thus represents a key link in marine metazoan food
chains. Despite the deep taphonomic biases affecting plankton fossiliz
ation, there is a clear record of phytoplankton from at least 1800 m.y
ago. Proterozoic plankton are represented by small-to medium-sized sp
haeromorphic acritarchs and probably do not include many/most of the u
nusually large acritarchs that characterize the Neoproterozoic. The fi
rst significant shift in phytoplankton diversity was therefore the rap
id radiation of small acanthomorphic acritarchs in the Early Cambrian.
The coincidence of phytoplankton diversification with the Cambrian ra
diation of large animals points compellingly to an ecological linkage
between the two, particularly in light of recently discovered filter-f
eeding mesozooplankton in the Early Cambrian. The introduction of plan
ktic filter feeders would have established the second tier of the Elto
nian pyramid, potentially setting off the ''self-propagating mutual fe
edback system of diversification'' now recognized as the Cambrian expl
osion (Stanley 1973, 1976). By consuming significant percentages of ne
t phytoplankton and suspending it as animal biomass and non-aggregatin
g fecal pellets, mesozooplankton cause a net reduction in export produ
ction; a general introduction of zooplankton would therefore have redu
ced carbon burial and moderated the bloom and bust cycle that must hav
e characterized Proterozoic populations of net phytoplankton. The effe
ct of added trophic levels in Early Cambrian ecosystems can be viewed
as a serial application of the trophic cascade process observed in mod
ern lakes, whereby the introduction of higher trophic levels determine
s the accumulation of plant biomass at the base of the system. As such
, the major biogeochemical perturbations that mark the onset of the Ph
anerozoic might be considered a consequence, rather than a cause, of t
he Cambrian explosion; reduced C export due to zooplankton expansion e
xplains the otherwise anomalous drop in delta(13)C at the base of the
Tommotian. Cambrian acanthomorphic acritarchs likely derived from plan
ktic leiosphaerids exposed to mesozooplanktic grazing pressure, the or
namentation effectively increasing vesicle size without compromising b
uoyancy or surface-area:volume ratios. Alternatively, they may represe
nt an escape into the plankton through a miniaturization of the much l
arger Neoproterozoic acanthomorphs. An invasion of small benthic herbi
vores into the water column to exploit the phytoplankton accounts for
the origin of the mesozooplankton and may have been the key innovation
in the Cambrian explosion.