A Comparison of Adaptive Radiations of Antarctic Fish with those of NonAntarctic Fish
9
to the tectonic and climatic history of the area. We will discuss this before
moving to a comparison of the radiations of Antarctic and lacustrine
species flocks.
Tectonic History
Although Antarctica is presently covered by a single ice sheet, the
underlying rocks comprise two crustal blocks of differing tectonic history:
East Antarctica and West Antarctica. The latter includes the Antarctic
Peninsula, whose position relative to East Antarctica has changed
significantly through time. There is also a cluster of very small crustal
plates (microplates) whose precise relationships and tectonic history are the
subject of much debate.
The main block of what is now Antarctica includes Precambrian
basement rocks, and in the Palaeozoic formed part of a single
supercontinent, Pangea. As far back as the Cambrian the southern edge of
East Antarctica and Australia supported a shallow-water marine fauna,
with a defined trilobite faunal realm [39].
Following the inception of the present cycle of seafloor spreading in the
Mesozoic, Pangea split into two major blocks, Laurasia and Gondwana,
separated by the Tethys Sea. At this point there were two quite distinct
areas of shallow water adjacent to the East Antarctic shield. One was the
long coastline formed by South America and the southern/western coast of
East Antarctica, facing the Pacific Ocean. The other was the
northerly/eastern coast of the East Antarctic shelf and Australia, where
they formed the southern boundary to Tethys. It is from the latter that the
faunal elements defining the Weddellian Province of the later Mesozoic
came. Since Gondwana remained as a single intact unit throughout the
Palaeozoic and into at least the Jurassic, those proto-Pacific coastal habitats
are considerably older than those around present-day East Antarctic (which
faced the Tethys Sea).
It seems likely that the initial opening between Antarctica and South
America/Africa was at about 150 Ma BP [40]. Initially this would have
been a shallow epicontinental sea, evidence for which comes from the
disjunct distribution of certain Jurassic endemics in both the southern
Andes and East AfricalMadagascar [41-44]. Faunal exchange between the
Pacific and TethyslWeddellian Provinces could thus have started in the
Jurassic, although it was not until much later that the Drake Passage
opened sufficiently for deep-water circulation to start.
By the early Eocene, fragmentation of Gondwana was well underway,
with Africa and India having moved substantially northwards. Australia
was still attached to East Antarctica and there was no deep-water
9
to the tectonic and climatic history of the area. We will discuss this before
moving to a comparison of the radiations of Antarctic and lacustrine
species flocks.
Tectonic History
Although Antarctica is presently covered by a single ice sheet, the
underlying rocks comprise two crustal blocks of differing tectonic history:
East Antarctica and West Antarctica. The latter includes the Antarctic
Peninsula, whose position relative to East Antarctica has changed
significantly through time. There is also a cluster of very small crustal
plates (microplates) whose precise relationships and tectonic history are the
subject of much debate.
The main block of what is now Antarctica includes Precambrian
basement rocks, and in the Palaeozoic formed part of a single
supercontinent, Pangea. As far back as the Cambrian the southern edge of
East Antarctica and Australia supported a shallow-water marine fauna,
with a defined trilobite faunal realm [39].
Following the inception of the present cycle of seafloor spreading in the
Mesozoic, Pangea split into two major blocks, Laurasia and Gondwana,
separated by the Tethys Sea. At this point there were two quite distinct
areas of shallow water adjacent to the East Antarctic shield. One was the
long coastline formed by South America and the southern/western coast of
East Antarctica, facing the Pacific Ocean. The other was the
northerly/eastern coast of the East Antarctic shelf and Australia, where
they formed the southern boundary to Tethys. It is from the latter that the
faunal elements defining the Weddellian Province of the later Mesozoic
came. Since Gondwana remained as a single intact unit throughout the
Palaeozoic and into at least the Jurassic, those proto-Pacific coastal habitats
are considerably older than those around present-day East Antarctic (which
faced the Tethys Sea).
It seems likely that the initial opening between Antarctica and South
America/Africa was at about 150 Ma BP [40]. Initially this would have
been a shallow epicontinental sea, evidence for which comes from the
disjunct distribution of certain Jurassic endemics in both the southern
Andes and East AfricalMadagascar [41-44]. Faunal exchange between the
Pacific and TethyslWeddellian Provinces could thus have started in the
Jurassic, although it was not until much later that the Drake Passage
opened sufficiently for deep-water circulation to start.
By the early Eocene, fragmentation of Gondwana was well underway,
with Africa and India having moved substantially northwards. Australia
was still attached to East Antarctica and there was no deep-water