Ichthyosaurians diversity declined due to environmental volatility caused by climatic upheavals in the early Late Cretaceous, becoming extinct around the Cenomanian-Turonian boundary approximately 90 million years ago.

At the same time, he considered it a transitional form between fishes and crocodiles, not in an evolutionary sense, but as regarded its place in the scala naturae, the "Chain of Being" hierarchically connecting all living creatures.

[23] Geologist Charles Lyell, to the contrary, assumed that the Earth was eternal so that in the course of time the ichthyosaur might likely reappear, a possibility lampooned in a famous caricature by De la Beche.

Public awareness was increased by the works of the eccentric collector Thomas Hawkins, a pre-Adamite believing that ichthyosaurs were monstrous creations by the devil: Memoirs of Ichthyosauri and Plesiosauri of 1834[24] and The Book of the Great Sea-Dragons of 1840.

It turned out that Professor Buckland had been aware of this beforehand, and the museum was forced to reach a settlement with Hawkins, and gave the fake parts a lighter colour to differentiate them from the authentic skeletal elements.

In the surrounding park, life-sized, painted, concrete statues of extinct animals were placed, which were designed by Benjamin Waterhouse Hawkins under the direction of Richard Owen.

Although it was known that ichthyosaurs had been animals of the open seas, they were shown basking on the shore, a convention followed by many nineteenth century illustrations with the aim, as Conybeare once explained, of better exposing their build.

In 1905, the Saurian Expedition led by John Campbell Merriam and financed by Annie Montague Alexander, found twenty-five specimens in central Nevada, which were under a shallow ocean during the Triassic.

[34] The new method of cladistics provided a means to exactly calculate the relationships between groups of animals, and in 1999, Ryosuke Motani published the first extensive study on ichthyosaur phylogenetics.

[45] Since the 1980s, a close relationship was assumed between the Ichthyosauria and the Sauropterygia, another marine reptile group, within an overarching Euryapsida, with one such study in 1997 by John Merck showing them to be monophyletic archosauromorph euryapsids.

[50] The proposed relationships are shown by this cladogram: Hupehsuchia Cartorhynchus Ichthyopterygia The earliest ichthyosaurs are known from the Early and Early-Middle (Olenekian and Anisian) Triassic strata of Canada, China, Japan, and Spitsbergen in Norway, being up to 246 million years old.

These very early "proto-ichthyosaurs" had such a distinctive build compared to "ichthyosaurs proper" that Motani excluded them from the Ichthyosauria and placed them in a basal position in a larger clade, the Ichthyopterygia.

The overspecialisation of ichthyosaurs may be a contributing factor to their extinction, possibly being unable to 'keep up' with fast teleost fish, which had become dominant at this time, against which the sit-and-wait ambush strategies of the mosasauroids proved superior.

[64] A first extinction event in the beginning of the Cenomanian eliminated two of the three ichthyosaur feeding guilds still present: the 'soft-prey specialists' and the 'generalists', leaving only an unspecialized apex predator group.

The following cladogram is based on Motani (1999):[35] Utatsusaurus Parvinatator Chaohusaurus Grippia Cymbospondylus Mixosauria Shastasauria Toretocnemus Californosaurus Macgowania Hudsonelpidia Suevoleviathan Eurhinosauria Temnodontosaurus Stenopterygius Ichthyosaurus Ophthalmosauridae An alternative terminology was proposed by Maisch & Matzke in 2000, trying to preserve the traditional, more encompassing content of the concept Ichthyosauria.

[72] Basal Ichthyopterygia, like their land-dwelling ancestors, still had vertebrae that possessed a full set of processes that allowed them to interlock and articulate, forming a vertebral column supporting the weight of the body.

[72] As derived species no longer have transversal processes on their vertebrae—again a condition unique in the Amniota—the parapophyseal and diapophysael rib joints have been reduced to flat facets, at least one of which is located on the vertebral body.

[72] Basal forms have a forelimb that is still functionally differentiated, in some details resembling the arm of their land-dwelling forebears; the ulna and radius are elongated and somewhat separated; the carpals are rounded, allowing the wrist to rotate; the number of phalanges is within the range shown by land animals.

The pubic bone typically does not connect to the ischium behind it; the space in between is by some workers identified as the fenestra thyreoidea;[72] other researchers deny that the term is applicable given the general loose structure of the pelvis.

At least one specimen, R158 (in the collections of the Paleontologiska Museet, Uppsala University), shows the expected faded edges of a bacterial mat, so it has not been altered by preparators, yet still preserves a generally tuna-like body outline including a dorsal fin.

[83] In 2017, from the German Posidonia Shale the discovery was reported of 182.7-million-year-old vertebrae of Stenopterygius in a carbonate nodule, still containing collagen fibers, cholesterol, platelets, and red and white blood cells.

[85] Gastroliths, stomach stones that might have assisted digestion or regulated buoyancy, have only on a few occasions been found associated with ichthyosaur skeletons, once with a specimen of Nannopterygius and a second time in a Panjiangsaurus fossil.

[104] However, in 2013 a study concluded that the hyoid bone of ichthyosaurs, at the tongue base, was insufficiently ossified to support a suction feeding movement and suggested the alternative that such species were ram feeders, gathering food by constantly swimming forwards with a wide-open mouth.

In 1973, McGowan concluded that, because ichthyosaurs have a reversed tail fin asymmetry compared to sharks, they were apparently positively buoyant, lighter than water, which would be confirmed by their lack of gastroliths and of pachyostosis or dense bone.

[110] In 1987 however, Michael A. Taylor suggested an alternative hypothesis: as ichthyosaurs could vary their lung content, contrary to sharks (which lack a swimming bladder), they could also regulate their buoyancy.

In 1998, Darren Naish pointed out that the lungfish and the river dolphin actually do not use their fins in this way and that e.g. the modern humpback whale has very long front flippers, supported by a mosaic of bones, but that these nevertheless mainly serve as rudders.

[123] However, this last possibility is contradicted by the fact that, with modern animals, damage is not caused by a limited number of rapid ascension incidents, but by a gradual accumulation of non-invalidating degeneration during normal diving behaviour.

In 1990, Vivian de Buffrénil published a histological study, indicating that ichthyosaurs possessed a fibrolamellar bone structure, as with warm-blooded animals in general, typified by fast growth and a strong vascularisation.

[135] Other researchers have tended to assume that for at least derived ichthyosaurs Carrier's constraint did not apply, because of their stiff bodies, which seems to be confirmed by their good diving capacity, implying an effective respiration and oxygen storage system.

Air-breathing marine creatures must either come ashore to lay eggs, like turtles and some sea snakes, or else give birth to live young in surface waters, like whales and dolphins.