12
J.T. Eastman and A. Clarke
for continental shelf organisms which would either become extinct, survive
in refugia not covered by ice, or move into deeper water on the continental
slope.
The critical piece of information in relation to the evolutionary history of
the Southern Ocean marine fauna is whether during one or more glacial
maxima in the past, the continental ice-sheet ever extended so far as to
cover all of the Antarctic continental shelf at once. We cannot yet answer
this question from geophysical or glaciological data. Nevertheless two
pieces of ecological evidence suggest that previous glacial maxima may
have covered a large proportion of the Antarctic continental shelf. The first
is that the peak of species richness for notothenioid fishes in East
Antarctica is at about 500-m depth [20). The second is that the bathymetric
ranges of many Antarctic marine organisms extended deeper than
elsewhere on the globe [50).
Taken together this evidence suggests (but no more) that glacial maxima,
perhaps as recently as the Pliocene or Pleistocene, may have severely
reduced or even eradicated available habitat for fish living on the Antarctic
continental shelf. If so, the radiation of notothenioid fish may be a much
more recent event than traditionally has been thought to be the case.
How Extensive Were Changes in Ice-Sheet Volume and the
Extent of Shallow-Water Marine Habitat in the Pliocene?
There has recently been considerable controversy over the extent, or even
existence, of deglaciation in Antarctica during the Pliocene. Although the
original evidence which came from diatom fossils has been shown to be
unreliable [46,51], other evidence has suggested some limited deglaciation
in Pliocene times (for example the Pecten conglomerate on Cockburn
Island, dated at 3.5-5.3 Ma BP: [52]). Some highly speculative
reconstructions of the Antarctic ice-sheet during the Pliocene suggest the
presence of large shallow epicontinental seas, which would have increased
greatly the habitat area available for shallow-water fish. On present
evidence, however, we can only conclude that the evidence for Pliocene
warming is suggestive. The possibility exists that shallow water habitats
were more extensive in the Pliocene, but we cannot yet be sure.
What is the Role of Milankovitch Cyclicity in the Evolution of
the Southern Ocean Fauna?
Gravitational interactions with other bodies in the solar system induce
variations in the Earth's orbital parameters. The main cycles have periods
of 23 kyr (precession), 41 kyr (obliquity) and ~ 100 kyr (eccentricity). It is
J.T. Eastman and A. Clarke
for continental shelf organisms which would either become extinct, survive
in refugia not covered by ice, or move into deeper water on the continental
slope.
The critical piece of information in relation to the evolutionary history of
the Southern Ocean marine fauna is whether during one or more glacial
maxima in the past, the continental ice-sheet ever extended so far as to
cover all of the Antarctic continental shelf at once. We cannot yet answer
this question from geophysical or glaciological data. Nevertheless two
pieces of ecological evidence suggest that previous glacial maxima may
have covered a large proportion of the Antarctic continental shelf. The first
is that the peak of species richness for notothenioid fishes in East
Antarctica is at about 500-m depth [20). The second is that the bathymetric
ranges of many Antarctic marine organisms extended deeper than
elsewhere on the globe [50).
Taken together this evidence suggests (but no more) that glacial maxima,
perhaps as recently as the Pliocene or Pleistocene, may have severely
reduced or even eradicated available habitat for fish living on the Antarctic
continental shelf. If so, the radiation of notothenioid fish may be a much
more recent event than traditionally has been thought to be the case.
How Extensive Were Changes in Ice-Sheet Volume and the
Extent of Shallow-Water Marine Habitat in the Pliocene?
There has recently been considerable controversy over the extent, or even
existence, of deglaciation in Antarctica during the Pliocene. Although the
original evidence which came from diatom fossils has been shown to be
unreliable [46,51], other evidence has suggested some limited deglaciation
in Pliocene times (for example the Pecten conglomerate on Cockburn
Island, dated at 3.5-5.3 Ma BP: [52]). Some highly speculative
reconstructions of the Antarctic ice-sheet during the Pliocene suggest the
presence of large shallow epicontinental seas, which would have increased
greatly the habitat area available for shallow-water fish. On present
evidence, however, we can only conclude that the evidence for Pliocene
warming is suggestive. The possibility exists that shallow water habitats
were more extensive in the Pliocene, but we cannot yet be sure.
What is the Role of Milankovitch Cyclicity in the Evolution of
the Southern Ocean Fauna?
Gravitational interactions with other bodies in the solar system induce
variations in the Earth's orbital parameters. The main cycles have periods
of 23 kyr (precession), 41 kyr (obliquity) and ~ 100 kyr (eccentricity). It is