Geology Reference
In-Depth Information
through the water. This group includes modern fish such as trout, bass, and cat-
fish.
Figure 19-6:
Ar-
mored head bones
of a Dunkleo-
steus.
Lobe-finned fish:
This type of fish is much more rare today — and more fascinating
evolutionarily because they were the first step toward land-dwelling animals. One
type of modern lobe-finned fish, the lungfish, lives in freshwater habitats such as
streams or lakes and breathes through gills like other fish. However, when the wa-
ter dries up, this fish can burrow into the mud and breathe through a lung-type or-
gan until the water returns.
Among the extinct lobe-finned fish, the
crossopterygians
group is thought to have
given rise to amphibians, which were the first animals to live outside water. The
lobed fins of crossopterygians were muscular enough that scientists think they
could have propelled these fish across short distances on land, a precursor to
fully developed legs in amphibians. Other similarities between the skeletal and
tooth structures of these lobe-finned fish and early amphibians are still being stud-
ied as scientists fill in the details of early land-dwelling animals.
Part-time land dwellers: Amphibians venture out
of the water
In the middle Paleozoic, while the fish dominated the seas, amphibians evolved.
Amphi-
bians
are animals that breathe air and can move comfortably outside of water but still
