Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides.
Ichthyosaurians thrived during much of the Mesozoic era; based on fossil evidence, they first appeared around 250 million years ago (Ma) and at least one species survived until about 94 million years ago, into the Late Cretaceous. During the Early Triassic epoch, ichthyosaurs and other ichthyosauromorphs evolved from a group of unidentified land reptiles that returned to the sea, in a development similar to how the mammalian land-dwelling ancestors of modern-day dolphins and whales returned to the sea millions of years later, which they gradually came to resemble in a case of convergent evolution. Ichthyosaurians were particularly abundant in the Late Triassic and Early Jurassic periods, until they were replaced as the top aquatic predators by another marine reptilian group, the Plesiosauria, in the later Jurassic and Early Cretaceous, though previous views of ichthyosaur decline during this period are probably overstated. Ichthyosaurians diversity declined due to environmental volatility caused by climatic upheavals in the early Late Cretaceous, becoming extinct around the Cenomanian-Turonian boundary approximately 94 million years ago.
Scientists became aware of the existence of ichthyosaurians during the early 19th century, when the first complete skeletons were found in England. In 1834, the order Ichthyosauria was named. Later that century, many finely preserved ichthyosaurian fossils were discovered in Germany, including soft-tissue remains. Since the late 20th century, there has been a revived interest in the group, leading to an increased number of named ichthyosaurs from all continents, with over fifty genera known.
Ichthyosaurian species varied from in length. Ichthyosaurians resembled both modern fish and dolphins. Their limbs had been fully transformed into flippers, which sometimes contained a very large number of digits and phalanges. At least some species possessed a dorsal fin. Their heads were pointed, and the jaws often were equipped with conical teeth to catch smaller prey. Some species had larger, bladed teeth to attack large animals. The eyes were very large, for deep diving. The neck was short, and later species had a rather stiff trunk. These also had a more vertical tail fin, used for a powerful propulsive stroke. The vertebral column, made of simplified disc-like vertebrae, continued into the lower lobe of the tail fin. Ichthyosaurians were air-breathing, warm-blooded, and bore live young. Many, if not all, species had a layer of blubber for insulation.
Like other ancient marine reptiles, such as those in the clades Mosasauria and Plesiosauria, the genera in Ichthyosauria are not part of the clade Dinosauria.
History of discoveries
Early finds The first known illustrations of ichthyosaur bones, vertebrae, and limb elements were published by Edward Lhuyd in his of 1699. Lhuyd thought that they represented fish remains.Lhuyd, E., 1699, Lithophylacii Brittannici Ichnographia, sive Lapidum aliorumque Fossilium Brittanicorum singulari figurà insignium, London In 1708, the Swiss naturalist Johann Jakob Scheuchzer described two ichthyosaur vertebrae assuming they belonged to a man drowned in the Universal Deluge.J. J. Scheuchzer, 1708, Piscium Querelae et Vindiciae, Zürich: Gessner In 1766, an ichthyosaur jaw with teeth was found at Weston near Bath. In 1783, this piece was exhibited by the Society for Promoting Natural History as those of a crocodilian. In 1779, ichthyosaur bones were illustrated in John Walcott's Descriptions and Figures of Petrifications.Walcott, John, 1779, Descriptions and Figures of Petrifications Found in the Quarries, Gravel-Pits etc. Near Bath. Collected and Drawn by John Walcott, Esq., S. Hazard, Bath, pp. 51 Towards the end of the eighteenth century, British fossil collections quickly increased in size. Those of the naturalists Ashton Lever and John Hunter were acquired in their totality by museums; later, it was established that they contained dozens of ichthyosaur bones and teeth. The bones had typically been labelled as belonging to fish, dolphins, or crocodiles; the teeth had been seen as those of sea lions.Evans, M., 2010, "The roles played by museums, collections, and collectors in the early history of reptile palaeontology", pp. 5–31 in: Richard Moody, E. Buffetaut, D. Naish, D. M. Martill (eds). Dinosaurs and Other Extinct Saurians: A Historical Perspective. Geological Society of London
The demand by collectors led to more intense commercial digging activities. In the early nineteenth century, this resulted in the discovery of more complete skeletons. In 1804, Edward Donovan at St Donats uncovered a ichthyosaur specimen containing a jaw, vertebrae, ribs, and a shoulder girdle. It was considered to be a giant lizard. In October 1805, a newspaper article reported the find of two additional skeletons, one discovered at Weston by Jacob Wilkinson, the other, at the same village, by Reverend Peter Hawker. In 1807, the last specimen was described by the latter's cousin, Joseph Hawker.Hawker, J., 1807, Gentleman's Magazine, 77: 7–8 This specimen thus gained some fame among geologists as 'Hawker's Crocodile'. In 1810, near Stratford-upon-Avon, an ichthyosaur jaw was found that was combined with plesiosaur bones to obtain a more complete specimen, indicating that the distinctive nature of ichthyosaurs was not yet understood, awaiting the discovery of far better fossils.
The first complete skeletons
In 1811, in Lyme Regis, along the Jurassic Coast of Dorset, the first complete ichthyosaur skull was found by Joseph Anning, the brother of Mary Anning (who in 1812 was still a young girl,) secured the torso of the same specimen. Their mother, Molly Anning, sold the combined piece to squire Henry Henley for £23. Henley lent the fossil to the London Museum of Natural History of William Bullock. When this museum was closed, the British Museum bought the fossil for a price of £47/5s; it still belongs to the collection of the independent Natural History Museum and has the inventory number NHMUK PV R1158 (formerly BMNH R.1158). It has been identified as a specimen of Temnodontosaurus platyodon.
In 1814, the Annings' specimen was described by Professor Everard Home, in the first scientific publication dedicated to an ichthyosaur. Intrigued by the strange animal, Home tried to locate additional specimens in existing collections. In 1816, he described ichthyosaur fossils owned by William Buckland and James Johnson. In 1818, Home published data obtained by corresponding with naturalists all over Britain. In 1819, he wrote two articles about specimens found by Henry Thomas De la Beche and Thomas James Birch. A last publication of 1820 was dedicated to a discovery by Birch at Lyme Regis. The series of articles by Home covered the entire anatomy of ichthyosaurs, but highlighted details only; a systematic description was still lacking.
Home was very uncertain how the animal should be classified. Though most individual skeletal elements looked very reptilian, the anatomy as a whole resembled that of a fish, so he initially assigned the creature to the fishes, as seemed to be confirmed by the flat shape of the vertebrae. 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 , the "Chain of Being" hierarchically connecting all living creatures. In 1818, Home noted some coincidental similarities between the coracoid of ichthyosaurians and the sternum of the platypus. This induced him to emphasize its status as a transitional form, combining, like the platypus, traits of several larger groups. In 1819, he considered it a form between newts, like the olm, and lizards; he then gave a formal generic name: Proteo-Saurus. However, in 1817, Karl Dietrich Eberhard Koenig had already referred to the animal as Ichthyosaurus, "fish saurian" from Greek , , "fish". This name at the time was an invalid and was only published by Koenig in 1825,C. König, 1825, Icones Fossilium Sectiles, London but was adopted by De la Beche in 1819 in a lecture where he named three Ichthyosaurus species. This text would only be published in 1822, just after De la Beche's friend William Conybeare published a description of these species, together with a fourth one. The type species was Ichthyosaurus communis, based on a lost skeleton. Conybeare considered that Ichthyosaurus had priority relative to Proteosaurus. Although this is incorrect by modern standards, the latter name became a "forgotten" . In 1821, De la Beche and Conybeare provided the first systematic description of ichthyosaurs, comparing them to another newly identified marine reptile group, the Plesiosauria. Much of this description reflected the insights of their friend, the anatomist Joseph Pentland.
In 1835, the order Ichthyosauria was named by Henri Marie Ducrotay de Blainville. In 1840, Richard Owen named an order Ichthyopterygia as an alternative concept.
Popularisation during the 19th century
The discovery of a hitherto unsuspected extinct group of large marine reptiles generated much publicity, capturing the imagination of both scientists and the public at large. People were fascinated by the strange build of the animals, especially the large scleral rings in the eye sockets,John Glendening, 2013, Science and Religion in Neo-Victorian Novels: Eye of the Ichthyosaur, Routledge of which it was sometimes erroneously assumed these would have been visible on the living animal. Their bizarre form induced a feeling of alienation, allowing people to realise the immense span of time passed since the era in which the ichthyosaur swam the oceans. Not all were convinced that ichthyosaurs had gone extinct: Reverend George Young found a skeleton in 1819 at Whitby; in his 1821 description, he expressed the hope that living specimens could still be found. 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 1834Hawkins, T. H., 1834, Memoirs on Ichthyosauri and Plesiosauri; Extinct monsters of the ancient Earth, Relfe and Fletcher, London and The Book of the Great Sea-Dragons of 1840.Hawkins, T. H., 1840, The Book of the Great Sea-dragons, Ichthyosauri and Plesiosauri, Gedolim Taninum of Moses. Extinct Monsters of the Ancient Earth, W. Pickering, London The first work was illustrated by mezzotints by John Samuelson Templeton. These publications also contained scientific descriptions and represented the first textbooks of the subject. In the summer of 1834, Hawkins, after a taxation by William Buckland and Gideon Mantell, sold his extensive collection, then the largest of its kind in the world, to the British Museum. However, curator Koenig quickly discovered that the fossils had been heavily restored with plaster, applied by an Italian artist from Lucca; of the most attractive piece, an Ichthyosaurus specimen, almost the entire tail was fake. 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.McGowan, C., 2001, The Dragon Seekers: How an Extraordinary Circle of Fossilists Discovered the Dinosaurs and Paved the Way for Darwin, Basic Books