in the Devonian Seas
and plant life teemed in the vast ocean that covered much of the globe.
Ammonoid cephalopods first appeared
in the Early Devonian, along with siliceous sponges.
Brachiopods (which arose in the Cambrian) reached
their maximum diversity as did hundreds of rugose coral
species. Extensive reef-building produced some of
the largest reef complexes in Earth’s history. Trilobites
are also known from this time, but most groups had disappeared by
the end of the Devonian, leaving only a select few genera to continue
on through the Carboniferous and Permian.
The first radiation of vertebrates with jaws
occurred during the Devonian, which is why it is sometimes known as
the “Age of Fishes.” Although they first appeared in the
Silurian, spiny acanthodians and armored
reached their peak diversity during the Devonian and began to dwindle
in numbers later in the Paleozoic. Some, like the giant Dunkleosteus,
reached nearly three meters in length and were menacing marine predators.
Placoderms had no teeth, instead relying on self-sharpening
bony plates in their jaws that performed the function of teeth. Chondrichthyan
(cartilaginous) fishes, such as sharks, were Ordovician survivors
that thrived during the Devonian. A few shark scales have been dated
to the Late Ordovician, but the earliest shark teeth are Early Devonian,
and the first relatively modern-looking sharks had evolved by the
Middle Devonian. The first true bony fishes (osteichthyans)
appear as well, as both lobe-finned (sarcopterygians)
and ray-finned (actinopterygians)
forms. The lobe-finned fish are of particular interest because they
gave rise to the first land vertebrates, around 360 million years