Thenode clade Ichthyosauromorpha was first defined byRyosuke Motaniet al. in 2014 as the group consisting of thelast common ancestor ofIchthyosaurus communis andHupehsuchus nanchangensis, and all its descendants. Theirsynapomorphies, unique derived traits, include: the presence of an anterior flange on thehumerus andradius; the lower end of theulna 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, thefibula extending behind the level of the thighbone, and the transverse process of the vertebralneural arch being reduced or absent.[2]
The Ichthyosauromorpha were previously thought to have likely originated in China during the upperLower Triassic period, about 248 million years ago. However, a 2023 study recorded the fossils of a derived marine ichthyosauromorph (anichthyopterygian) in earliestTriassic rocks ofSpitsbergen,Norway, just 2 million years after thePermian–Triassic extinction event. The presence of such a derived ichthyosauromorph so early in the Triassic indicates that the ichthyosauromorphs (and possiblyichthyosauriformes, depending on divergence estimates) originated during thePermian and were thus survivors of the mass extinction as opposed to ecological successors that evolved following it.[1]
One branch consists of the Hupehsuchia, and the other of the Ichthyosauriformes, of whichCartorhynchus was a basal member. Other ichthyosauriforms were theIchthyopterygia, containing theIchthyosauria and allies. The last ichthyosaurs probably became extinct in the middleCretaceous.
Their relationships with other reptiles are unresolved, due to their highlyderived morphology and presumed ancient origins, even in their earliest known representatives,[3] though they are usually considered to bediapsids.[4] A 2022 study on the early evolution of reptiles classified the Ichthyosauromorpha as basalarchosauromorphs, forming a clade with the other marine reptile groupsThalattosauria andSauropterygia as sister to the rest of the Archosauromorpha.[5] A 2023 study describing the Triassic marine reptileProsaurosphargis found a similar placement, albeit instead placing the three marine reptile groups within an expandedArchelosauria.[6]
