Editing Devonian (section)

=== Middle Devonian ===
The [[Middle Devonian]] comprised two subdivisions: first the [[Eifelian]], which then gave way to the [[Givetian]] at {{period start|Givetian}} Ma. During this time, the jawless [[agnathan]] fishes began to decline in diversity in freshwater and marine environments partly due to drastic environmental changes and partly due to the increasing competition, predation, and diversity of [[gnathostomata|jawed fishes]]. The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop such essential characteristics as well developed lungs and the ability to crawl out of the water and onto the land for short periods of time.<ref>{{Cite journal|last=Clack|first=Jennifer|date=13 August 2007|title=Devonian climate change, breathing, and the origin of the tetrapod stem group|url=https://academic.oup.com/icb/article/47/4/510/632798|journal=Integrative and Comparative Biology|volume=47|issue=4|pages=510–523|doi=10.1093/icb/icm055|pmid=21672860|quote=Estimates of oxygen levels during this period suggest that they were unprecedentedly low during the Givetian and Frasnian periods. At the same time, plant diversification was at its most rapid, changing the character of the landscape and contributing, via soils, soluble nutrients, and decaying plant matter, to anoxia in all water systems. The co-occurrence of these global events may explain the evolution of air-breathing adaptations in at least two lobe-finned groups, contributing directly to the rise of the tetrapod stem group.|doi-access=free}}</ref>