Sea levels were high, and there was a great variety of fish and other marine organisms. The Devonian fish included the Agnatha (jawless fish), the Acanthodii (spiny fish), the Placoderms (armoured fish), the Chondrichthyes (cartilaginous fish), and the early Osteichthyes (bony fish). The Devonian strata are divided into lower, middle and upper subdivisions.
During the Devonian the evolution of fish into tetrapods occurred. Various terrestrial arthropods also became established. The first seed-bearing plants spread across dry land, forming huge forests.
Tectonic and volcanic activity was high. A series of extinction events occurred towards the end of the Devonian, including a major event at the Frasnian-Famennian boundary in the Late Devonian, about 364 mya. The supercontinent Pangaea was just starting to form.