Ichthyosauromorpha

{{short description|Clade of marine reptiles}}

{{Automatic taxobox

| taxon = Ichthyosauromorpha

| fossil_range =
Early Triassic - Late Cretaceous, {{fossilrange|earliest=269|251.3|90}} Inferred Permian origins

| image = HupehsuchusNanchangensis-PaleozoologicalMuseumOfChina-May23-08.jpg

| image_caption = Hupehsuchus nanchangensis

| image2 = Ichthyosaurus communis in London.jpg

| image2_caption = Ichthyosaurus somersetensis

| name = Ichthyosauromorpha

| authority = Motani et al., 2014

| subdivision_ranks = Subgroups

| subdivision = *{{extinct}}Baisesaurus

• {{extinct}}Hupehsuchia
• {{extinct}}Ichthyosauriformes

}}

The Ichthyosauromorpha are an extinct clade of Mesozoic marine reptiles consisting of the Ichthyosauriformes and the Hupehsuchia.

The node clade Ichthyosauromorpha was first defined by Ryosuke Motani et al. in 2014 as the group consisting of the last common ancestor of Ichthyosaurus communis and Hupehsuchus nanchangensis, and all its descendants. Their synapomorphies, unique derived traits, include: the presence of an anterior flange on the humerus and radius; the lower end of the ulna being as wide as or wider than the upper end, the forelimb being as long as or longer than the hindlimb, the hand having at least three quarters of the length of the upper arm and lower arm combined, the fibula extending behind the level of the thighbone, and the transverse process of the vertebral neural arch being reduced or absent.{{cite journal |title=A basal ichthyosauriform with a short snout from the Lower Triassic of China |author=Motani|year=2014 |journal=Nature |volume=517|issue=7535|pages=485–488|doi=10.1038/nature13866 |pmid=25383536|s2cid=4392798|url=http://www.nature.com/nature/journal/vaop/ncurrent/fig_tab/nature13866_F2.html|display-authors=etal|url-access=subscription}}

The Ichthyosauromorpha were previously thought to have likely originated in China during the upper Lower Triassic period, about 248 million years ago. However, a 2023 study recorded the fossils of a derived marine ichthyosauromorph (an ichthyopterygian) in earliest Triassic rocks of Spitsbergen, Norway, just 2 million years after the Permian–Triassic extinction event. The presence of such a derived ichthyosauromorph so early in the Triassic indicates that the ichthyosauromorphs (and possibly ichthyosauriformes, depending on divergence estimates) originated during the Permian and were thus survivors of the mass extinction as opposed to ecological successors that evolved following it.{{Cite journal |last=Kear |first=Benjamin P. |last2=Engelschiøn |first2=Victoria S. |last3=Hammer |first3=Øyvind |last4=Roberts |first4=Aubrey J. |last5=Hurum |first5=Jørn H. |date=2023-03-13 |title=Earliest Triassic ichthyosaur fossils push back oceanic reptile origins |url=https://www.cell.com/current-biology/abstract/S0960-9822(22)01990-X |journal=Current Biology |language=English |volume=33 |issue=5 |pages=R178–R179 |doi=10.1016/j.cub.2022.12.053 |issn=0960-9822 |pmid=36917937|doi-access=free }}

One branch consists of the Hupehsuchia, and the other of the Ichthyosauriformes, of which Cartorhynchus was a basal member. Other ichthyosauriforms were the Ichthyopterygia, containing the Ichthyosauria and allies. The last ichthyosaurs probably became extinct in the middle Cretaceous.

Their relationships with other reptiles are unresolved, due to their highly derived morphology and presumed ancient origins, even in their earliest known representatives,M. W. Maisch, Phylogeny, systematics, and origin of the Ichthyosauria—the state of the art. Palaeodiversity 3, 151–214 (2010). though they are usually considered to be diapsids.{{Cite journal |last=Moon |first=Benjamin C. |date=2019-01-17 |title=A new phylogeny of ichthyosaurs (Reptilia: Diapsida) |url=https://www.tandfonline.com/doi/full/10.1080/14772019.2017.1394922 |journal=Journal of Systematic Palaeontology |language=en |volume=17 |issue=2 |pages=129–155 |doi=10.1080/14772019.2017.1394922 |issn=1477-2019 |s2cid=90912678 |hdl-access=free |hdl=1983/463e9f78-10b7-4262-9643-0454b4aa7763}} A 2022 study on the early evolution of reptiles classified the Ichthyosauromorpha as basal archosauromorphs, forming a clade with the other marine reptile groups Thalattosauria and Sauropterygia as sister to the rest of the Archosauromorpha.{{Cite journal |last=Simões |first=Tiago R. |last2=Kammerer |first2=Christian F. |last3=Caldwell |first3=Michael W. |last4=Pierce |first4=Stephanie E. |date=2022-08-19 |title=Successive climate crises in the deep past drove the early evolution and radiation of reptiles |url=https://www.science.org/doi/10.1126/sciadv.abq1898 |journal=Science Advances |language=en |volume=8 |issue=33 |pages=eabq1898 |doi=10.1126/sciadv.abq1898 |issn=2375-2548 |pmc=9390993 |pmid=35984885}} A 2023 study describing the Triassic marine reptile Prosaurosphargis found a similar placement, albeit instead placing the three marine reptile groups within an expanded Archelosauria.{{Cite journal |last=Wolniewicz |first=Andrzej S |last2=Shen |first2=Yuefeng |last3=Li |first3=Qiang |last4=Sun |first4=Yuanyuan |last5=Qiao |first5=Yu |last6=Chen |first6=Yajie |last7=Hu |first7=Yi-Wei |last8=Liu |first8=Jun |date=2023-08-08 |editor-last=Ibrahim |editor-first=Nizar |title=An armoured marine reptile from the Early Triassic of South China and its phylogenetic and evolutionary implications |url=https://doi.org/10.7554/eLife.83163 |journal=eLife |volume=12 |pages=e83163 |doi=10.7554/eLife.83163 |issn=2050-084X|doi-access=free |pmc=10499374 }}