Ichthyosauromorpha
Temporal range:
Early Triassic - Late Cretaceous, Inferred Permian origins[1]
Hupehsuchus nanchangensis
Ichthyosaurus somersetensis
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Neodiapsida
Clade: †Ichthyosauromorpha
Motani et al., 2014
Subgroups

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

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.[2]

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.[1]

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,[3] though they are usually considered to be diapsids.[4] 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.[5] 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.[6]

Taxonomy

Phylogeny

The internal phylogenetic structure of the Ichthyosauromorpha is shown by this cladogram:

Ichthyosauromorpha 

Hupehsuchia

 Ichthyosauriformes 

Nasorostra

Ichthyopterygia

References

  1. 1 2 Kear, Benjamin P.; Engelschiøn, Victoria S.; Hammer, Øyvind; Roberts, Aubrey J.; Hurum, Jørn H. (2023-03-13). "Earliest Triassic ichthyosaur fossils push back oceanic reptile origins". Current Biology. 33 (5): R178–R179. doi:10.1016/j.cub.2022.12.053. ISSN 0960-9822. PMID 36917937.
  2. ↑ Motani; et al. (2014). "A basal ichthyosauriform with a short snout from the Lower Triassic of China". Nature. 517 (7535): 485–488. doi:10.1038/nature13866. PMID 25383536. S2CID 4392798.
  3. ↑ M. W. Maisch, Phylogeny, systematics, and origin of the Ichthyosauria—the state of the art. Palaeodiversity 3, 151–214 (2010).
  4. ↑ Moon, Benjamin C. (2019-01-17). "A new phylogeny of ichthyosaurs (Reptilia: Diapsida)". Journal of Systematic Palaeontology. 17 (2): 129–155. doi:10.1080/14772019.2017.1394922. hdl:1983/463e9f78-10b7-4262-9643-0454b4aa7763. ISSN 1477-2019. S2CID 90912678.
  5. ↑ Simões, Tiago R.; Kammerer, Christian F.; Caldwell, Michael W.; Pierce, Stephanie E. (2022-08-19). "Successive climate crises in the deep past drove the early evolution and radiation of reptiles". Science Advances. 8 (33): eabq1898. doi:10.1126/sciadv.abq1898. ISSN 2375-2548. PMC 9390993. PMID 35984885.
  6. ↑ Wolniewicz, Andrzej S; Shen, Yuefeng; Li, Qiang; Sun, Yuanyuan; Qiao, Yu; Chen, Yajie; Hu, Yi-Wei; Liu, Jun (2023-08-08). Ibrahim, Nizar (ed.). "An armoured marine reptile from the Early Triassic of South China and its phylogenetic and evolutionary implications". eLife. 12: e83163. doi:10.7554/eLife.83163. ISSN 2050-084X. PMC 10499374.
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