HISTORY Ordovarian Period (425-500 million

years ago)OSTRACODERMS

○ Slow, bottom-dwelling animals○ Covered with thick bony plates and scales○ Had very poorly developed fins and didn’t

have jaws○ Believed to be the first animal with backbone

Extinct about 250 million years ago

2 Groups of Fishes with movable jaws

PLACODERMS ACANTHODIANS

Appeared about 395 million years ago

Had thick, bony plates and paired fins

Their upper jaw was fused to the skull while the lower jaw was hinged and movable

Extinct about 345 million years ago

Class Chondrichthyes evolved

Appeared about 410 million years ago

Distinguished by the bony spines projecting in front their fins and by minute diamond-shaped scales

Class Osteichthyes evolved