selected for survival, which is why certain species survive one major extinc-
tion after another. This is particularly true for marine species such as sharks,
which originated in the Devonian around 400 million years ago and have sur-
vived every mass extinction since.
Fish comprise more than half the species of vertebrates, both living and
extinct.They include the jawless fish (lampreys and hagfish), the cartilaginous
fish (sharks, skates, rays, and ratfish), and the bony fish (salmon, swordfish, pick-
erel, and bass). The ray-finned fish are by far the largest group of living fish
species. Fish progressed from rough scales, asymmetrical tails, and cartilage in
their skeletons to flexible scales, powerful advanced fins and tails, and all-bone
skeletons, much like they are today.
The jawless fish, which first appeared in the Ordovician, are the earliest
known vertebrates, having been in existence for 470 million years. They had
a flexible rod similar to cartilage, which functioned as a spine along the back.
They were probably poor swimmers, however, and restricted to shallow water.
Bony plates surrounded the head for protection from invertebrate predators.
However, the additional weight forced the fish to live mostly on the seafloor,
where they sifted bottom sediments for food particles.
The development of jaws about 460 million years ago revolutionized
predation. Giant jawed vertebrates, some of which were monsters in their day,
climbed to the very topof the food chain. The extinct placoderms (Fig.92),
which reached 30 or more feet in length, were ferocious giants that preyed on
smaller fish.They lived in shallow, freshwater streams and lakes.They had cam-
ouflage of red scales that helped them blend in well with their reddish brown
habitat. They had well-developed articulated jaws and thick armor plating
around the head that extended over and behind the jaws. One of these groups
gave rise to land animals, emphasizing the great importance jaws played in ver-
tebrate evolution.
Figure 92 The extinct
placoderms were giants
measuring 30 feet in
length.DEVONIAN FISH