Geology Reference
In-Depth Information
When vertebrate fossils do show up in the fossil record, they are already full-fledged fish
with backbones. And due to the presence of an internal skeleton
(endoskeleton)
and oth-
er hard parts (such as teeth and bony scales), the evolution of the fishes is a well-de-
tailed story that I tell here.
Fish evolve body armor, teeth, and . . . legs?
The first fish were not much like the fish you see today. They had spinal cords but no
jaws and were called
ostracoderms
(which means “shell skin”) due to the bony plates
covering them. Ostracoderms are members of the group called
agnathans
and are dis-
tant relatives of the lamprey and hagfish: two modern fish that do not have jaws or bony
skin. Ostracoderms were bottom-feeders, skimming the surface of the seafloor sucking
up food while keeping their eyes (located on tops of their heads) peeled for predators.
They flourished through the early Paleozoic and lived alongside other fish groups
evolving at the same time. Figure 19-5 is a sketch of an ostracoderm.
The earliest fish with jaws belonged to a group called
acanthodians
that no longer exist.
The evolution of jaws appears to be related to the gill structure of early fish. Scientists
think the frontal gill supports made of cartilage or bone may have originally become
hinged to allow the gills to open wider, taking in more oxygen and also allowing the in-
take of more food. This feature proved advantageous to their survival and continued to
develop through natural selection, resulting eventually in bony hinged jaws. Scientists
are still sorting out the details but think that the acanthodians very likely led to later
groups of jawed fish, such as the placoderms, cartilaginous fish, and bony fish. (Keep
reading for details on these fish groups.)
While the fishes began to evolve in the early Precambrian, they reached their maximum
diversity in the Devonian period (about 400 million years ago). For this reason, the
Devonian is often called the “Age of the Fishes.” During the 50 million year span of the
Devonian period, all the major types of fish are present in the fossil record: ostraco-
derms, placoderms, cartilaginous fish, and bony fish.
Figure 19-5:
An os-
tracoderm, the
earliest fish.
