Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by
bryozoa, diverse and abundant
brachiopods, the enigmatic
hederelloids,
microconchids and
corals. Lily-like
crinoids (animals, their resemblance to flowers notwithstanding) were abundant, and
trilobites were still fairly common. Among vertebrates, jaw-less armored fish (
ostracoderms) declined in diversity, while the jawed fish (gnathostomes) simultaneously increased in both the sea and
fresh water. Armored
placoderms were numerous during the lower stages of the Devonian Period and became extinct in the Late Devonian, perhaps because of competition for food against the other fish species. Early cartilaginous (
Chondrichthyes) and bony fishes (
Osteichthyes) also become diverse and played a large role within the Devonian seas. The first abundant genus of shark,
Cladoselache, appeared in the oceans during the Devonian Period. The great diversity of fish around at the time, have led to the Devonian being given the name "The Age of Fish" in popular culture.
The first
ammonites also appeared during or slightly before the early Devonian Period around 400
Mya.
Devonian - Wikipedia, the free encyclopedia
(Notice how everything "just appeared" there.)