A Comparison of Adaptive Radiations of Antarctic Fish with those of NonAntarctic Fish
11
seawater temperatures then cooled to the present fully polar temperatures.
There is some evidence for a general warming in the Pliocene, which may
have led to some retreat of the ice-cap but this is still debated (see below).
Key Problems in the Climatic History of the Antarctic in
Relation to the Evolution of the Marine Fauna
The broad features of the tectonic and climatic history of Antarctica during
the Tertiary are now reasonably well established. Nevertheless there
remain important unanswered questions in relation to the evolution of the
Southern Ocean marine fauna.
When Did Widespread Continental Glaciation Start?
Prior to the establishment of a significant ice sheet in Antarctica, the
continental shelves would have been shallower, shallow seas would
probably have been more extensive, and there would have been a greater
range of shallow-water habitats available including estuaries and mudflats.
All of these are likely to have allowed a more diverse fauna than exists
today, and fossil evidence indicates that early Tertiary fish fauna in
Antarctica was indeed diverse, including many chondrichthyans and other
groups not represented today [1,47]. Ancestral notothenioids probably
inhabited the shelf of Gondwana, and later Antarctica, during the late
Cretaceous or early Tertiary, although their representation in the late
Eocene (40 Ma) fossil fish fauna from Seymour Island is debatable [47,
48].
Although there is evidence from glacial deposits of glaciers discharging
at sea-level in the Eocene, these do not necessarily indicate the presence of
ice sheets [46]. Evidence from sea-level changes suggests the growth and
decay of substantial ice sheets in the Eocene, but isotopic data give no
support to significant variations in ice volume prior to about 36 Ma BP.
On present evidence, there are thus indications that widespread
continental glaciation started around the Eocene/Oligocene border. It is at
this time that there may have been significant changes in habitat area, and
many habitat types such as rivers and estuaries may have been eradicated,
with consequent impacts on the diversity of the fish fauna.
How Far Have Ice-Sheets Extended in the Past?
The present ice-sheet covers over 99% of Antarctica, but does not extend
over all of the continental shelf. Geophysical evidence has shown that in
the past, ice-sheets have extended fully to the edge of the Antarctic
continental shelf [49]. This would obviously eradicate all available habitat
11
seawater temperatures then cooled to the present fully polar temperatures.
There is some evidence for a general warming in the Pliocene, which may
have led to some retreat of the ice-cap but this is still debated (see below).
Key Problems in the Climatic History of the Antarctic in
Relation to the Evolution of the Marine Fauna
The broad features of the tectonic and climatic history of Antarctica during
the Tertiary are now reasonably well established. Nevertheless there
remain important unanswered questions in relation to the evolution of the
Southern Ocean marine fauna.
When Did Widespread Continental Glaciation Start?
Prior to the establishment of a significant ice sheet in Antarctica, the
continental shelves would have been shallower, shallow seas would
probably have been more extensive, and there would have been a greater
range of shallow-water habitats available including estuaries and mudflats.
All of these are likely to have allowed a more diverse fauna than exists
today, and fossil evidence indicates that early Tertiary fish fauna in
Antarctica was indeed diverse, including many chondrichthyans and other
groups not represented today [1,47]. Ancestral notothenioids probably
inhabited the shelf of Gondwana, and later Antarctica, during the late
Cretaceous or early Tertiary, although their representation in the late
Eocene (40 Ma) fossil fish fauna from Seymour Island is debatable [47,
48].
Although there is evidence from glacial deposits of glaciers discharging
at sea-level in the Eocene, these do not necessarily indicate the presence of
ice sheets [46]. Evidence from sea-level changes suggests the growth and
decay of substantial ice sheets in the Eocene, but isotopic data give no
support to significant variations in ice volume prior to about 36 Ma BP.
On present evidence, there are thus indications that widespread
continental glaciation started around the Eocene/Oligocene border. It is at
this time that there may have been significant changes in habitat area, and
many habitat types such as rivers and estuaries may have been eradicated,
with consequent impacts on the diversity of the fish fauna.
How Far Have Ice-Sheets Extended in the Past?
The present ice-sheet covers over 99% of Antarctica, but does not extend
over all of the continental shelf. Geophysical evidence has shown that in
the past, ice-sheets have extended fully to the edge of the Antarctic
continental shelf [49]. This would obviously eradicate all available habitat