Editing Archosaur (section)

== Archosaurian domination in the Triassic ==
[[Synapsid]]s are a clade that includes [[mammals]] and [[pelycosaur|their extinct ancestors]]. The latter group are often referred to as ''mammal-like reptiles,'' but should be termed ''protomammals, stem mammals, or basal synapsids,'' because they are not true reptiles by modern [[cladistic]] classification. They were the dominant land vertebrates throughout the [[Permian]], but most perished in the [[Permian&ndash;Triassic extinction event]]. Very few large synapsids survived the event, but one form, ''[[Lystrosaurus]]'' (a [[herbivorous]] [[dicynodont]]), attained a widespread distribution soon after the extinction.<ref>{{cite journal |last1=Kulik |first1=Zoe T. |last2=Lungmus |first2=Jacqueline K. |last3=Angielczyk |first3=Kenneth D. |last4=Sidor |first4=Christian A. |date=2021-11-05 |title=Living fast in the Triassic: New data on life history in Lystrosaurus (Therapsida: Dicynodontia) from northeastern Pangea |journal=PLOS ONE |language=en |volume=16 |issue=11 |pages=e0259369 |doi=10.1371/journal.pone.0259369 |issn=1932-6203 |pmc=8570511 |pmid=34739492|bibcode=2021PLoSO..1659369K |doi-access=free }}</ref> Following this, archosaurs and other archosauriforms quickly became the dominant land vertebrates in the early [[Triassic]]. Fossils from before the mass extinction have only been found around the Equator, but after the event fossils can be found all over the world.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2019/01/190131084252.htm|title=Iguana-sized dinosaur cousin discovered in Antarctica - ScienceDaily}}</ref> Suggested explanations for this include:
* Archosaurs made more rapid progress towards erect limbs than synapsids, and this gave them greater stamina by avoiding [[Carrier's constraint]]. An objection to this explanation is that archosaurs became dominant while they still had sprawling or semi-erect limbs, similar to those of ''[[Lystrosaurus]]'' and other synapsids.{{Citation needed|date=December 2017}}
* Archosaurs have more efficient{{clarify|date=January 2021}} respiratory systems featuring unidirectional air flow, as opposed to the tidal respiration of synapids.<ref name=":0">{{cite journal |last1=Brocklehurst |first1=Robert J. |last2=Schachner |first2=Emma R. |last3=Codd |first3=Jonathan R. |last4=Sellers |first4=William I. |date=2020-03-02 |title=Respiratory evolution in archosaurs |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=375 |issue=1793 |pages=20190140 |doi=10.1098/rstb.2019.0140 |pmc=7017431 |pmid=31928195}}</ref> The ability to breathe more efficiently in [[Hypoxia (environmental)|hypoxic]] conditions may have been advantageous to early archosaurs during the suspected drop in oxygen levels at the end of the Permian.<ref name=":0" />
* The [[Early Triassic]] was predominantly arid, because most of the [[Earth]]'s land was concentrated in the [[supercontinent]] [[Pangaea]]. Archosaurs were probably better at conserving water than early synapsids because:
** Modern [[diapsid]]s (lizards, snakes, crocodilians, birds) excrete [[uric acid]], which can be excreted as a paste, resulting in low water loss as opposed to a more dilute urine. It is reasonable to suppose that archosaurs (the ancestors of crocodilians, dinosaurs and pterosaurs) also excreted uric acid, and therefore were good at conserving water. The aglandular (glandless) skins of diapsids would also have helped to conserve water.{{Citation needed|date=December 2017}}
** Modern mammals excrete [[urea]], which requires a relatively high urinary rate to keep it from leaving the urine by diffusion in the kidney tubules. Their skins also contain many glands, which also lose water. Assuming that early synapsids had similar features, e.g., as argued by the authors of ''[[Palaeos]]'', they were at a disadvantage in a mainly arid world. The same well-respected site points out that "for much of Australia's [[Plio-Pleistocene]] history, where conditions were probably similar, the largest terrestrial predators were not mammals but gigantic [[varanid]] lizards (''[[Megalania]]'') and land crocs."<ref name="WhiKazAOver">{{cite web|author1=White, T.  |author2=Kazlev, M. A. |title=Archosauromorpha: Overview |url=http://palaeos.com/Vertebrates/Units/270Archosauromorpha/270.000.html |publisher=Palaeos.com |access-date=6 September 2012 |url-status=dead |archive-url=https://web.archive.org/web/20101220204743/http://palaeos.com/Vertebrates/Units/270Archosauromorpha/270.000.html |archive-date=December 20, 2010 }}</ref>

However, this theory has been questioned, since it implies synapsids were necessarily less advantaged in water retention, that synapsid decline coincides with climate changes or archosaur diversity (neither of which tested) and the fact that desert dwelling mammals are as well adapted in this department as archosaurs,<ref>[[Darren Naish]], [http://tetzoo.com/podcast/2015/1/22/episode-38-a-not-too-shabby-podcarts Episode 38: A Not Too Shabby Podcarts] {{Webarchive|url=https://web.archive.org/web/20160127144449/http://tetzoo.com/podcast/2015/1/22/episode-38-a-not-too-shabby-podcarts |date=2016-01-27 }}</ref> and some cynodonts like ''[[Trucidocynodon]]'' were large sized predators.<ref>{{cite journal | last1 = Oliveira | first1 = T.V. | last2 = Soares | first2 = M.B. | last3 = Schultz | first3 = C.L. | year = 2010 | title = Trucidocynodon riograndensis gen. nov. et sp. nov. (Eucynodontia), a new cynodont from the Brazilian Upper Triassic (Santa Maria Formation) | journal = Zootaxa | volume = 2382 | pages = 1–71 | doi = 10.11646/zootaxa.2382.1.1 }}</ref>
A study favors competition amidst mammaliaforms as the main explanation for Mesozoic mammals being small.<ref>{{cite journal|doi = 10.1016/j.cub.2021.04.044|title = Mammaliaform extinctions as a driver of the morphological radiation of Cenozoic mammals|year = 2021|last1 = Brocklehurst|first1 = Neil|last2 = Panciroli|first2 = Elsa|last3 = Benevento|first3 = Gemma Louise|last4 = Benson|first4 = Roger B.J.|journal = Current Biology|volume = 31|issue = 13|pages = 2955–2963.e4|pmid = 34004143|s2cid = 234782605|doi-access = free| bibcode=2021CBio...31E2955B }}</ref>