Editing Devonian (section)

==Subdivisions==
The Devonian Period is formally broken into Early, Middle and Late subdivisions. The rocks corresponding to those [[series (stratigraphy)|epoch]]s are referred to as belonging to the Lower, Middle and Upper parts of the Devonian System.

=== Early Devonian ===
[[File:Mollweide Paleographic Map of Earth, 405 Ma (Emsian Age).png|thumb|A map of Earth in the [[Emsian]] stage of the Early Devonian (405 million years ago)]]
The [[Early Devonian]] lasted from {{period start|Early devonian}} to {{period end|Early devonian}} Ma. It began with the [[Lochkovian]] Stage from {{period start|Lochkovian}} to {{period end|Lochkovian}} Ma, which was followed by the [[Pragian]] from {{period start|Pragian}} to {{period end|Pragian}} Ma and then by the [[Emsian]], which lasted until the Middle Devonian began at {{period start|Middle devonian}} Ma.<ref name=ICSC>{{cite journal |last1=Cohen| first1= K.M.| last2= Finney|first2 = S.C.|last3 = Gibbard| first3 = P.L.| last4 =Fan|first4 = J.-X. |year=2013 |title=The ICS International Chronostratigraphic Chart| journal = Episodes|volume = 36| issue =3| pages= 199–204 | doi= 10.18814/epiiugs/2013/v36i3/002|url=https://stratigraphy.org/ICSchart/Cohen2013_Episodes.pdf |archive-url=https://web.archive.org/web/20150913145806/http://www.stratigraphy.org/ICSchart/Cohen2013_Episodes.pdf |archive-date=2015-09-13 |url-status=live |access-date=7 January 2021|doi-access= free}}</ref>
During this time, the first [[Ammonoidea|ammonoids]] appeared, descending from [[Bactritida|bactritoid]] [[Nautiloidea|nautiloids]]. Ammonoids during this time period were simple and differed little from their nautiloid counterparts. These ammonoids belong to the order [[Agoniatitida]], which in later epochs evolved to new ammonoid orders, for example [[Goniatite|Goniatitida]] and [[Clymeniida]]. This class of [[cephalopod]] molluscs would dominate the marine fauna until the beginning of the [[Mesozoic]] Era.

=== 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>

=== Late Devonian ===
[[File:Mollweide Paleographic Map of Earth, 370 Ma (Famennian Age).png|thumb|250px|right|A map of Earth in the [[Famennian]] stage of the Late Devonian (370 million years ago)]]
Finally, the Late Devonian started with the [[Frasnian]], from {{period start|Frasnian}} to {{period end|Frasnian}} Ma, during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing [[Famennian]] subdivision, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian at {{period end|Devonian}} Ma.<ref name=ICSC/>