Geoscience Reference
In-Depth Information
their slow fall to the seafloor. In contrast, by 530 million years ago, zoo-
plankton and other pelagic animals emerged into the marine ecosystem
and generated rapidly settling fecal pellets; these transported the re-
mains of surface-living phytoplankton to the seafloor, to be discovered
by geochemists half a billion years later.
This is an attractive hypothesis, but it still needs testing. The history
of zooplankton evolution is poor to nonexistent. Such small organisms
leave a lousy fossil record. Nick Butterfield argues, however, that even if
their fossil record is bad, one still might be able to probe for their pres-
ence by focusing on what they might have eaten. In this case, there is
a rich record of potential foodstuffs, the so-called acritarchs, which are
believed to be the preserved casings of ancient algae or their resting-
stage equivalents. These undergo a dramatic increase in ornamentation
some 630 million years ago, which in Nick's view, signals a defense
against predation by newly evolved zooplankton. This timing wouldn't
quite fit Logan's results. If you recall, he and his colleagues argued that
the influence of animals on carbon cycling came later, sometime after
590 million years ago.
Clearly, our understanding of the relationship between oxygen and
animals is fuzzy. Many details need to be worked out. Among other
things, we need to explore in detail how a changing carbon cycle, driven
by animal evolution, would influence ocean oxygenation and its record,
which are revealed by many different proxy indicators. This will likely
require ocean modeling. However, it seems a real possibility that the
story of ocean oxygenation and its relationship to animal evolution has at
least as much to do with what was happening in the ocean as what was
happening in the atmosphere. And if this is true, it's equally possible
that motile animals evolved into an environment that was already “per-
missible” for some time before their appearance.
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It will be very excit-
ing to see how this story plays out.
