Devonian

The Devonian period is most notable for the explosion of fish diversity. Jawless fish, armored placoderms, cartilaginous fish, and early bony fish dominated the oceans. By the middle of the Devonian, the first tetrapods—amphibians that could move between land and water—had evolved from fish. The fossil record shows early forms like Tiktaalik, which had both fish-like and amphibian features, such as lungs and rudimentary limbs.

The Devonian also saw the dramatic expansion of land plants, including the evolution of the first true forests. These forests were dominated by early tree-like plants, such as Archaeopteris, which had woody tissues and leaves but reproduced with spores. The development of forests had a profound impact on the carbon cycle, as the burial of plant material led to significant changes in atmospheric carbon dioxide levels, contributing to climate changes later in the period.

The Devonian ended with a series of mass extinctions, which primarily affected marine species, especially reef-building organisms such as stromatoporoids and tabulate corals. The exact causes of these extinctions are debated but may include anoxia (lack of oxygen) in the oceans, rapid climate change, and possibly the impacts of asteroid collisions.