Geology Reference
In-Depth Information
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Figure 21.22
Rhipidistian Crossopterygian The crossopterygians are the group from which the
amphibians are thought to have evolved.
Eusthenopteron
, a member of the rhipidistian crossopterygians,
had an elongate body and paired fi ns that could be used for moving about on land.
their swim bladder until the stream begins flowing or the
lake that they were living in fi lls with water. When they are
back in the water, lungfi sh then rely on gill respiration.
The
crossopterygians
(order Crossopterygii) are an
important group of lobe-fi nned fi sh, because it is probably
from them that amphibians evolved. However, the transi-
tion between crossopterygians and true amphibians is not as
simple as it was once portrayed. The group of crossopteryg-
ians that appears to be ancestral to amphibians are
rhipidis-
tians
(Table 21.2). These fi sh, reaching lengths of more than
2 m, were the dominant freshwater predators during the Late
Paleozoic.
Eusthenopteron
, a good example of a rhipidistian
crossopterygian and the classic example of the transitional
form between fish and amphibians, had an elongate body
that helped it move swiftly in the water and paired muscular
fi ns that many scientists thought could be used for moving
on land (
fi sh, as well as eurypterids and ammonoids, had a profound
effect on the marine ecosystem. Previously defenseless or-
ganisms either evolved defensive mechanisms or suffered
great losses, possibly even extinction.
For example, ostracoderms, although armored, would
have been easy prey for the swifter jawed fi sh. Ostracoderms
became extinct by the end of the Devonian, a time that co-
incides with the rapid evolution of jawed fi sh. Placoderms,
like acanthodians, greatly decreased in abundance after the
Devonian and became extinct by the end of the Paleozoic
Era. In contrast, cartilaginous fi sh and ray-fi nned bony fi sh
expanded during the Late Paleozoic, as did the ammonoid
cephalopods (Figure 21.10), the other major predators of the
Late Paleozoic seas.
INVADE THE LAND
Although amphibians were the fi rst vertebrates to live on land,
they were not the first land-living organisms. Land plants,
which probably evolved from green algae, fi rst evolved during
the Ordovician. Furthermore, insects, millipedes, spiders, and
even snails invaded the land before amphibians. Fossil evi-
dence indicates that such land-dwelling arthropods as scorpi-
ons and fl ightless insects had evolved by at least the Devonian.
The transition from water to land required animals to
surmount several barriers. The most critical were desicca-
tion, reproduction, the effects of gravity, and the extraction
Figure 21.22). The structural similarity between
crossopterygian fi sh and the earliest amphibians is striking
and one of the most widely cited examples of a transition
from one major group to another (
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Figure 21.23). However,
recent discoveries of older lobe-finned fish and tetrapods
(animals with four limbs), and newly published fi ndings of
tetrapod-like fish are filling in the gaps in the time of the
evolution between fi sh and tetrapods.
Before discussing this transition and the evolution of
amphibians, it is useful to place the evolutionary history of
Paleozoic fi sh in the larger context of Paleozoic evolutionary
events. Certainly, the evolution and diversifi cation of jawed
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