Page 12 - Fish and Amphibians (Britannica Illustrated Science Library)

8 GENERAL CHARACTERISTICS FISH AND AMPHIBIANS 9

Earliest Forms Dunkleosteus Dorsal

The Arthrodira—with a jointed Its head was fin The tail was not
protected by
bout 470 million years ago, the first fish appeared. Unlike today's neck—were armored fish that strong armor. protected by
STREAMLINED SHAPE predominated in the late Devonian scales.
fish, they did not have a jawbone, fins, or scales. Hard plates The shape of Pteraspis Period. The Devonian predator
A covered the front part of the fish and formed a protective shield. shows that it was an Dunkleosteus was an arthrodiran
excellent swimmer. placoderm that lived over 300 million
They also had a solid, flexible dorsal spine that allowed them to propel years ago. Its head was encased in This area of the body
themselves. Later, in the Silurian Period, fish appeared that had a an impressive set of plates 1.2 inches had neither armor
(3 cm) thick, with razor-sharp bony nor scales.
jawbone. Known as the gnathostomata, they were large predators. plates that served as teeth.

FIERCE JAW It had a lobed tail, similar
Dunkleosteus was a to a shark's tail, which
fierce predator that indicates that it was a
CONICAL NOSE EYES devoured any type of powerful swimmer.
Pteraspis Its streamlined shape Very small, prey, including sharks.
helped the fish move. located on both DORSAL SPIKE
The fish without a jawbone, sides of the head. Located on the fish's
Pteraspis, was about 6.5 back, it worked like a
inches (16 cm) long and lived in dorsal fin. It also had strong
the seas of Europe, Asia, and North jaws with bony teeth.
America. These fish were most
abundant during the Devonian Period.
They had bodies with armor that DORSAL SPINES
covered their heads, and they had a These helped the fish
streamlined shape. The shell had a co- to stay balanced while
nical nose that helped the fish to move. swimming.
16 feet

6.5 inches (16 cm) (5 m)
WING SHIELD
LENGTH OF THE FISH
Scientific Pteraspis
name
MOUTH
Diet Small organisms Having no jawbone, TAIL
The shape of the tail
Habitat Sea, then rivers and lakes they fed on small helped balance the
organisms.
Range Europe, Asia, North America weight of the armor.
Period Early Devonian
EVOLUTION OF THE JAWBONE cranium
LATERAL LINE
The development of the jawbone was a The evolution of the jawbone
long evolutionary process that involved modified the configuration Sensory organs are
changes in the diet of fish to include not of the skull. present on both sides
only small organisms but also other fish. of the body and on top
of the armor.
PRIMITIVE
1
VERTEBRATE
Marine
The first fish
lamprey Chimaeriformes Cheirolepis Pycnodus Sole
had no jawbone.
Fossil
Fish with lungs appeared
in the Mesozoic Era Evolution Holocephali Cheirolepididae Pycnodontiformes Holostei Teleostei
ELASMOBRANCHIMORPH (200 million years ago). Wing shield
2 Dunkleosteus Sharks and rays Eusthenopteron Chondrostei NEOPTERYGII
The formation of the Similar to amphibians, these In the Devonian Period (Pteraspis)
jawbone permitted new Placoderms species breathe with lungs ocean fish began to
feeding habits, and the and are now considered diversify. Coelacanths appeared, Placoderms Elasmobranchii Sarcopterygii
fish evolved from living fossils. The line as well as the earliest bony fish Lamprey
herbivore to carnivore. through the center of the and the first cartilaginous fish, ACTINOPTERYGII
Acanthodii
photo of the fossil is the including sharks. In this period
fish's lateral line. the three main groups of Jawless CARTILAGINOUS FISH BONY FISH
fish
BONY FISH gnathostomad fish also Devonian
3
They already had a appeared: the placoderms,
FOSSILIZED GNATHOSTOMATA
specialized jawbone Modern fish LUNGFISH SCALES chondrichthyes, and THIS PERIOD SAW AN EXPLOSION IN
like fish of today. Dipterus valenciennesi osteichthyes. VERTEBRATA THE DIVERSITY OF FISH SPECIES.