Cymbospondylus (meaning "cupped vertebrae") is an extinct genus of large ichthyosaurs, of which it is among the oldest representatives, that lived from the Lower to Middle Triassic in what are now North America and Europe.

It is in the beginning of the 20th century that more complete fossils were discovered through several expeditions launched by the University of California, and described in more detail by John Campbell Merriam in 1908, thus visualizing the overall anatomy of the animal.

Unlike cetaceans, Triassic ichthyosaurs like Cymbospondylus show that they reached large sizes very quickly after their appearance, probably because of the adaptive radiation of their prey, conodonts and ammonites, after the Permian–Triassic extinction.

All established species of Cymbospondylus are known from the fossil records of Nevada and Switzerland, with referred specimens without specific affiliations having nevertheless been discovered in Idaho, the rest of the Alps and Spitsbergen, an island in Norway.

In 1868, American paleontologist Joseph Leidy described two new genera of ichthyosaurs dating from the Middle Triassic on the basis of fossil vertebrae discovered in several localities in Nevada, United States, all of which were transmitted through the geologist Josiah Dwight Whitney.

[1] Between 1901 and 1907, the University of California sent ten expeditions across different corners of the United States to recover as many ichthyosaur fossils as possible dating from the Triassic, in order to be analyzed in more detail.

[4] In their work published in 2003, Christopher McGowan and Ryosuke Motani assert that the fossil vertebrae attributed to C. piscosus do not present distinctive characteristics to prove its validity.

[10][11] In 1927, a partial skeleton of a large ichthyosaur was discovered in the Grenzbitumenzone Member at the Monte San Giorgio fossil site in Switzerland and was mentioned by Bernhard Peyer in 1944.

[17] Twenty years later, in 1964, Emil Kuhn-Schnyder [de] published a photo of this same specimen and suggested that it shared affinities with Cymbospondylus, then only known from North America at that time.

[18] The specimen in question, cataloged as PIMUZ T 4351, was formally described for the first time in 1989 under the name C. buchseri by Paul Martin Sander [de], thus confirming the presence of the genus in Europe.

The specific epithet buchseri is named in honor of Fritz Buchser, a member of the Museum of Paleontology at the University of Zurich, the latter having prepared the holotype skeleton in 1931 as his first major professional achievement.

The species in question is named in honor to Elizabeth Nicholls, an American-Canadian paleontologist specializing in Triassic marine reptiles, who made a major contribution to the ichthyosaurs that lived during this period.

[10] In his thesis published in 2017, Wolniewicz redescribed C. nichollsi anatomically and considered it a "subjective junior synonym of C. piscosus",[21] but its observation is not shared and the species is maintained valid in subsequent publications.

This discovery was a partial skeleton that was briefly mentioned in a 2013 in a secondary article describing the large contemporary ichthyosaur Thalattoarchon,[22] as well as in a 2018 histological study, where it is among the specimens analyzed.

The species name duelferi was chosen in honor of Olaf Dülfer, fossil preparer who made many practical contributions to research on Mesozoic marine reptiles.

[20] After preparation of the fossils, the specimen, cataloged as LACM DI 157871, consists of a large complete skull, some cervical vertebrae, the right humerus as well as fragments of the shoulder girdle.

[5] In his 1868 paper describing Cymbospondylus, Leidy also named another ichthyosaur as Chonespondylus grandis, based on a fragment of a caudal vertebra found at Star Canyon in the Humboldt Range.

[34][5] In 1873, John Whitaker Hulke described a species of Ichthyosaurus, I. polaris, named after two sets of vertebrae associated with rib fragments that were discovered on Isfjorden, Spitsbergen, an island in Norway.

[5] In a review of German ichthyosaurs published in 1916, Friedrich von Huene described two other species of Cymbospondylus whose fossils were discovered in the Muschelkalk of the town of Laufenburg in the state of Baden-Württemberg.

[48][49] In 2002, paleontologists Chun Li and Hai-Lu You named a new species as C. asiaticus based on a complete skull discovered in the Xiaowa Formation, located in Guizhou Province, China, and it was considered as the most recent known representative of the genus.

They mention that the latter does not share any notable commonalities with the three then recognized species of the genus at the time, namely C. petrinus, C. buchseri and C. nichollsi, and suggest that it would in fact be a junior synonym of Guizhouichthyosaurus tangae.

Ichthyosaurs were superficially similar to dolphins and had flippers rather than legs, but the oldest representatives (with the exception of the parvipelvians, more advanced) do not have dorsal fins, or would have one but relatively poorly developed.

[11][27][66] The antiquity as well as such an imposing size for an animal dating from the beginning of the Middle Triassic makes C. youngorum qualify as "the first aquatic giant" according to Lene Liebe Delsett and Nicholas David Pyenson.

In their phylogenetic analysis the authors did not recover a definite placement for C. buchseri, leading them to state that further study was needed to determine whether the Swiss species belonged to the genus.

They argue that the long neck and torso would create drag in water while the laterally-flattened tail lacking the lunate fluke of later ichthyosaur taxa was more suited for an undulating swimming style.

In their research they suggest that the elongated flexible bodies of early ichthyosaurs were built to support an undulating swimming style while the powerful tail would provide bursts of speed, both of which they cite as being possible adaptations to ambush prey.

Direct evidence for its diet exists for the medium-sized Cymbospondylus buchseri from Switzerland, which was found with its stomach contents exclusively consisting of hooks belonging to soft-bodied coleoid cephalopods.

[20] During this period, the Fossil Hill Member represented the eastern part of the Panthalassan Ocean, and the proven presence of archosaurs like Benggwigwishingasuchus shows that the region would have been coastal.

During the time when the animal lived, when the Besano Formation was being deposited, the region where Monte San Giorgio is would have been a marine lagoon, located in a basin on the western side of the Tethys Ocean.

[84][87] Unlike the Fossil Hill Member in Nevada, ichthyosaurs do not represent the most diverse marine reptiles in the Besano Formation, the latter being limited only to Besanosaurus, C. buchseri, Phalarodon and Mixosaurus,[88] their abundance in the middle part of this zone correlating with the time when the lagoon was deepest.