8 GENERAL CHARACTERISTICS
FISH AND AMPHIBIANS 9
Earliest Forms
Dunkleosteus
bout 470 million years ago, the first fish appeared. Unlike today's fish, they did not have a jawbone, fins, or scales. Hard plates covered the front part of the fish and formed a protective shield. They also had a solid, flexible dorsal spine that allowed them to propel themselves. Later, in the Silurian Period, fish appeared that had a jawbone. Known as the gnathostomata, they were large predators.
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STREAMLINED SHAPE The shape of Pteraspis shows that it was an excellent swimmer.
Its head was protected by strong armor.
The Arthrodira—with a jointed neck—were armored fish that predominated in the late Devonian Period. The Devonian predator Dunkleosteus was an arthrodiran placoderm that lived over 300 million years ago. Its head was encased in an impressive set of plates 1.2 inches (3 cm) thick, with razor-sharp bony plates that served as teeth.
Dorsal fin
The tail was not protected by scales.
This area of the body had neither armor nor scales.
It had a lobed tail, similar to a shark's tail, which indicates that it was a powerful swimmer.
FIERCE JAW CONICAL NOSE Its streamlined shape helped the fish move.
Pteraspis The fish without a jawbone, Pteraspis, was about 6.5 inches (16 cm) long and lived in the seas of Europe, Asia, and North America. These fish were most abundant during the Devonian Period. They had bodies with armor that covered their heads, and they had a streamlined shape. The shell had a conical nose that helped the fish to move.
EYES Very small, located on both sides of the head.
DORSAL SPIKE Located on the fish's back, it worked like a dorsal fin.
Dunkleosteus was a fierce predator that devoured any type of prey, including sharks.
It also had strong jaws with bony teeth.
DORSAL SPINES These helped the fish to stay balanced while swimming.
16 feet (5 m)
6.5 inches (16 cm) WING SHIELD
LENGTH OF THE FISH
Scientific Pteraspis name Diet
Small organisms
Habitat
Sea, then rivers and lakes
Range
Europe, Asia, North America
Period
Early Devonian
EVOLUTION OF THE JAWBONE The development of the jawbone was a long evolutionary process that involved changes in the diet of fish to include not only small organisms but also other fish.
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MOUTH Having no jawbone, they fed on small organisms.
TAIL The shape of the tail helped balance the weight of the armor.
cranium
PRIMITIVE VERTEBRATE The first fish had no jawbone.
LATERAL LINE Sensory organs are present on both sides of the body and on top of the armor.
The evolution of the jawbone modified the configuration of the skull.
Marine lamprey
ELASMOBRANCHIMORPH The formation of the jawbone permitted new feeding habits, and the fish evolved from herbivore to carnivore.
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BONY FISH They already had a specialized jawbone like fish of today.
Placoderms
Modern fish
Fossil Fish with lungs appeared in the Mesozoic Era (200 million years ago). Similar to amphibians, these species breathe with lungs and are now considered living fossils. The line through the center of the photo of the fossil is the fish's lateral line. FOSSILIZED LUNGFISH SCALES Dipterus valenciennesi
Evolution In the Devonian Period ocean fish began to diversify. Coelacanths appeared, as well as the earliest bony fish and the first cartilaginous fish, including sharks. In this period the three main groups of gnathostomad fish also appeared: the placoderms, chondrichthyes, and osteichthyes.
Wing shield (Pteraspis)
Chimaeriformes
Cheirolepis
Holocephali
Cheirolepididae
Dunkleosteus
Placoderms
Sharks and rays
Eusthenopteron
Elasmobranchii
Sarcopterygii
Lamprey Acanthodii Jawless fish
CARTILAGINOUS FISH
GNATHOSTOMATA VERTEBRATA
Pycnodus
Sole
Pycnodontiformes
Chondrostei
Holostei Teleostei
NEOPTERYGII
ACTINOPTERYGII BONY FISH
Devonian THIS PERIOD SAW AN EXPLOSION IN THE DIVERSITY OF FISH SPECIES.