Temnodontosaurus

Generally estimated at 9 m (30 ft) long, Temnodontosaurus is one of the largest known ichthyosaurs, although not as imposing as some Triassic forms. Specimens assigned to the genus may nevertheless have reached larger measurements. As an ichthyosaur, Temnodontosaurus had flippers for limbs and a fin on the tail. Boasting eye sockets measuring more than 25 cm (9.8 in) wide, Temnodontosaurus quite possibly had the largest eyes known in the entire animal kingdom, rivaling in size those of the colossal squid. The snout appears to be longer than the mandible, being equipped with several sharp teeth (hence its name). On the basis of numerous very complete skeletons, it is estimated that the animal had at least more than 40 presacral vertebrae. Temnodontosaurus is a basal representative of the parvipelvian subgroup of ichthyosaurs, in addition to being its largest representative. A monotypic family, Temnodontosauridae, was even established in 1974 to include the genus. Various phylogenetic analyses as well as diagnostic problems concerning the genus make it, for the moment, a polyphyletic taxon (unnatural grouping), and therefore in need of revision.

Research history

Discovery and identification

Temnodontosaurus is historically the very first ichthyosaur to have been scientifically described. Around 1810, a certain Joseph Anning discovered the first skull of the taxon on the cliffs of Black Ven, between the town of Lyme Regis and the village of Charmouth, two localities located in the county of Dorset, in the south of England. The remaining skeleton was later discovered by his sister, the now famous Mary Anning, in 1812. Although other ichthyosaur skeletons had been discovered locally and elsewhere, this particular specimen was the first to attract attention of the scientific community. After the discovery was announced in the press, the specimen was purchased by the lord of a local manor, Henry Hoste Henley, for a price of £23. Subsequently, Henley passed the fossils on to the naturalist William Bullock, who put them on display in the collections of his museum in London. In 1819, Bullock's own collection was sold to the Natural History Museum in London for a price of around £47. The specimen, now cataloged as NHMUK PV R 1158, is still currently housed at this museum, although the postcranial remains have since been lost.

Beginning in 1814, Sir Everard Home wrote a series of six papers for the Royal Society describing the specimen, initially identifying it as a crocodile. Perplexed as to the real nature of the fossil, Home kept changing his mind about its classification, thinking that it would be a fish, then as an animal sharing affinities with the platypus, which was then recently described at that time. Finally, in 1819, he thought that the fossil represented an animal that embodied an intermediate form between salamanders and lizards, which led him to erect the genus name Proteosaurus (originally written as Proteo-Saurus). In 1821, Henry De la Beche and his colleague William Daniel Conybeare made the very first scientific description of Ichthyosaurus, but did not name any species. Although being initially a nomen nudum, this generic name was already proposed in 1818 by Charles Konig, but was thus chosen as the definitive scientific name of this genus, Proteosaurus having since become a nomen oblitum. In their article, De la Beche and Conybeare refer several additional fossils discovered at Black Ven to this genus, also including the specimen originally described by Home, and finally identify it as a marine reptile. It was in 1822 that De la Beche named three species of Ichthyosaurus on the basis of several anatomical differences distinguishing the specimens, one of them being I. platyodon. He nevertheless announces that future descriptions will be done with the help of Conybeare. However, it was Conybeare himself who described the fossils the same year, attributing the largest specimens to I. platyodon, The specific name platyodon comes from the Ancient Greek πλατύς (platús, "flat", "broad"), and ὀδούς (odoús, "tooth"), all meaning "flat teeth", in reference to the rather distinctive dentition of this species.

In 1889, Henry Alleyne Nicholson and Richard Lydekker published a two-volume work that served as an introduction to the rules of paleontology for students. However, it is in the second volume that the two paleontologists gave a very detailed description of numerous prehistoric vertebrates, and during which the taxonomy of I. platyodon took another direction. Indeed, in his correction notes, Lydekker noticed that the teeth of I. platyodon had great differences from those of other previously recognized species of Ichthyosaurus and suggest that the latter could be the type species of a completely new genus of ichthyosaurs, which he named Temnodontosaurus. This generic name is formed from the Ancient Greek τέμνω (temnō, "to cut"), ὀδούς (odoús, "tooth"), and σαῦρος (saûros, "lizard"), to give "cutting-tooth lizard". In a broad review of fossil vertebrates published in 1902, Oliver Perry Hay suggested that because the name Proteosaurus technically took precedence over Ichthyosaurus, he then displaced I. platyodon as the type species of that genus, then renamed Proteosaurus platyodon. In 1972, Christopher McGowan again used this combination proposed by Hay (although not mentioned), but the latter revised his judgment two years later, in 1974, in which he moved this species to Temnodontosaurus, as originally proposed by Lydekker. The holotype of Temnodontosaurus platyodon consisted of a single tooth which was preserved by the Geological Society of London. As the latter has since been noted as lost in 1960, McGowan designated specimen NHMUK PV OR 2003* as the neotype of this taxon. This specimen, already mentioned as a representative of the species by Richard Owen in 1881, was originally discovered and partly collected by Mary Anning in July 1832 in Lyme Regis. After the discovery, she sold the find to Thomas Hawkins, who himself sold the specimen to the Natural History Museum in London in 1834 for a price of £210.

Other species

Recognized species

In 1843, Carl von Theodori described a new species of Ichthyosaurus, I. trigonodon, which he described as "colossal", based on an imposing specimen comprising a complete skull and a partial postcranial skeleton discovered in the town of Holzmaden in the state of Baden-Württemberg, Germany. The specific name comes from Ancient Greek τρίγωνον (trígônon, "triangle") and ὀδούς (odoús, "tooth"), in reference to the dental crown which is visibly triangular in this species. In 1854, von Theodori made a much more in-depth description of the holotype specimen. In 1889, only some time before he established the genus Temnodontosaurus, Lydekker noted that the dentition of I. trigonodon was quite similar to that of I. platyodon. Based on these dental characteristics, he moved this species to the genus Temnodontosaurus the following year, consequently being renamed T. trigonodon. In 1931, Friedrich von Huene transferred this species to the genus Leptopterygius. In 1998, Michael W. Maisch moved the species again to the genus Temnodontosaurus, and attributed to this taxon other, mostly very complete, specimens having been discovered in Germany and France. This classification has since been retained in subsequent works, to the point that a large specimen discovered in England in 2021, nicknamed as the 'Rutland Sea Dragon', was even considered as the first probable representative within this country. However, a 2023 morphological study conducted by Rebecca F. Bennion and colleagues shows that the holotype specimen differs in cranial and dental traits from other specimens since assigned to the species. The authors therefore suggest that a future re-evaluation is necessary for better a diagnosis of T. trigonodon.

In 1857, an almost complete skeleton of a large ichthyosaur was discovered north of Whitby, Yorkshire. The latter was also found near another skeleton, that of a pliosaur, which is today recognized as the holotype specimen of Rhomaleosaurus cramptoni. Shortly after its discovery, the ichthyosaurian skeleton was subsequently sent to the Yorkshire Museum, where it was cataloged as YORYM 497. The following year, 1858, Owen examined the specimen and classified it as distinct from I. platyodon, attributing it to a completely new species which he named I. crassimanus. However, the latter was never scientifically described by Owen, although it is briefly mentioned in a work by John Phillips and Robert Etheridge published in 1875. It was in 1876 that John Frederick Blake made the first scientific description of the animal, although he did so only very briefly. In 1889, Lydekker considered this species as a potential junior synonym of I. trigonodon, an opinion which was followed by numerous authors until around the beginning of the 20th century. In 1930, Sidney Melmore made the first in-depth description of I. crassimanus based on the holotype specimen, restoring the distinct status of the species. In his revision published in 1974, McGowan synonymized I. crassimanus with the proposed taxon Stenopterygius acutirostis, also attributing other specimens discovered in the original locality. In 2003, McGowan and Ryosuke Motani suggested that all specimens historically attributed to I. crassimanus appeared sufficiently different from T. platyodon and T. trigonodon to belong to a distinct species of the genus Temnodontosaurus, being renamed T. crassimanus. However, they note that further research could question its validity. Long remaining an under-analyzed taxon, it was in 2020 that Emily J. Swaby wrote a thesis considerably revising it. Contrary to the suggestion previously made by McGowan and Motani, Swaby maintains the attribution of this species to Temnodontosaurus. The descriptions of this same thesis were finally published the following year in a study co-authored with Daniel R. Lomax.

In 1880, Harry Govier Seeley described the species I. zetlandicus on the basis of a well-preserved skull loaned by an Earl of Shetland (hence its name) around an unspecified date to the Sedgwick Museum of Earth Sciences in Cambridge. This skull, cataloged as CAMSM J35176, was discovered in the coasts of Whitby, near the locality where T. crassimanus was already discovered. In 1922, von Huene moved the species within Stenopterygius. In 1974, McGowan considered S. zetlandicus as a synonym of S. acutirostris, before this species was itself synonymized with T. acutirostris from 1997. In 2022, Antoine Laboury and colleagues reestablished the validity of the species by redescribing CAMSM J35176, but moved it to the genus Temnodontosaurus, being renamed T. zetlandicus. In their description, they attribute another specimen to the taxon, cataloged as MNHNL TU885, a partial skull which was originally discovered in Schouweiler, southern Luxembourg.

In 1931, von Huene described a new species of the genus Leptopterygius, L. nürtingensis, based on a skull and some postcranial remains of a single specimen discovered in a quarry in the town of Nürtingen (hence its name), Baden-Württemberg, Germany. This specimen, cataloged as SMNS 13488, is mentioned for the first time in a work by Eberhard Fraas published posthumously in 1919, in which the author considers it to be the representative of an undetermined species of Ichthyosaurus. In another work also published posthumously in 1926, Fraas attributed this specimen to a proposed new species which he named I. bellicosus. Fraas was initially expected to carry out the first scientific description of this taxon, but the latter's premature death in 1915 prevented this project from being achieved. Thus, in the absence of a scientific description, the name I. bellicosus is seen as a nomen nudum, and therefore does not have priority over L. nürtingensis. Although L. nürtingensis was only officially described in 1931 by von Huene, the taxon was already mentioned a year earlier by the same author in an article concerning the ribs of the holotype specimen, which have since been noted as lost. In 1939, Oskar Kuhn assimilated an incomplete specimen discovered in Aue-Fallstein, Lower Saxony, to this species. However, Kuhn did not present sufficient evidence to confirm his claims, and the specimen has since been viewed as indeterminate. In 1979, McGowan carried out a large revision of the ichthyosaurs known from Germany, in which he classified L. nürtingensis as a nomen dubium. In 1997, Maisch and Axel Hungerbühler formally criticized McGowan's view, given that the holotype specimen is preserved in an excellent state of conservation and is easily diagnosable. He then redescribed this specimen and considered it to be attributable to Temnodontosaurus. In their analysis, the authors change the typography of the species nürtingensis to nuertingensis, due to rule 32.C of the ICZN requiring it. The species is again considered a nomen dubium by McGowan and Motani in 2003, but its validity as well as its belonging to this genus is maintained in subsequent studies.

Dubious species

In 1881, Owen attributed a large isolated skull discovered at Lyme Regis, cataloged as NHMUK PV R1157, to the newly erected species of the genus Ichthyosaurus, I. breviceps. In 1922, von Huene moved this species to the genus Eurypterygius, a taxon which is itself recognized as a junior synonym of Ichthyosaurus. Although I. breviceps is still recognized as belonging to this genus, the large skull historically attributed to the species has large differences with the holotype specimen. Noting this, McGowan redescribed this specimen in more detail and made it the holotype of an entirely new species of Temnodontosaurus, T. eurycephalus. The specific name comes from the Ancient Greek ευρύς (eurús, "broad"), and κεφαλή (kephalế, "head"), all meaning "broad head", in reference to the cranial morphology of the taxon.

In 1984, an almost complete skeleton of a large ichthyosaur was discovered in the Lafarge quarries in the French commune of Belmont-d'Azergues, located near Lyon. Although the specimen is mentioned in a detailed biostratigraphic analysis of the Lafarge quarries published in 1991, it was in 2012 when the fossil, uncatalogued but stored in the Saint-Pierre-la-Palud local mining museum, was officially designated as the holotype of the new species T. azerguensis by Jeremy E. Martin and his colleagues. The specific name comes from the Azergues, a river located near the site of the discovery.

In 2014, British paleontologist Darren Naish expressed doubts in a blog in the journal Scientific American about the attribution of these two species to Temnodontosaurus, noting their large anatomical differences highlighting the need for a taxonomic revision of this genus. A similar observation is shared in the study describing T. zetlandicus in 2022, with the authors mentioning these two species as too phylogenetically unstable to be included in a monophyletic grouping of Temnodontosaurus.

Formerly assigned species

In 1840, Owen named the species I. acutirostris on the basis of a partial skeleton discovered near Whitby, now numbered NHMUK PV OR 14553. The holotype specimen was for a long time noted as lost, but this was only in 2002 when it was officially found, although some anatomical parts such as the snout are missing. Even before the specimen was found, some studies classified this species within the genus Temnodontosaurus, as Maisch and Hungerbühler did in 1997. However, Maisch reversed his decision in 2010, citing that numerous cranial features prove it is not part of the genus. Therefore, the author withdraws his attribution of this specimen to Temnodontosaurus and instead suggests that the latter would be the representative of a completely new genus. In 2022, Laboury and colleagues share the same conclusions and considers "I." acutirostris as a species inquirenda.

In 1892, Albert Gaudry officially described a new species of Ichthyosaurus, I. burgundiae, on the basis of a specimen discovered in the quarries of the town of Sainte-Colombe, in Yonne, France. Even before the taxon was described by Gaudry, the specimen, being one of the largest ichthyosaurs known at the time, led to it being presented at the 1889 Paris Exposition, the same exhibition for which the Eiffel Tower was built. After the end of the exhibition, the specimen was subsequently donated to the National Museum of Natural History in Paris, joining its collection on November 12, 1889, where it is still exhibited to this day. Gaudry already proposed the name of I. burgundiae at the French Academy of Sciences in 1891, but it was not until the following year that he published the first formal description of the taxon. In 1996, McGowan attributed a number of specimens discovered in Germany to this species, but moved it there to the genus Temnodontosaurus. In 1998, Maisch compared these specimens to the holotype of T. trigonodon, and suggested synonymizing T. burgundiae with the latter. Maisch's opinion is followed by McGowan and Motani in 2003, considering T. burgundiae as a junior synonym of T. trigonodon, despite slight osteological differences. The synonymy is however based only on German specimens, a new examination of the holotype specimen discovered in Sainte-Colombe having never been carried out due to its questionable state of conservation.

In 1974, McGowan described an additional species of Temnodontosaurus, T. risor, based on three skulls discovered at Lyme Regis, designating NHMUK PV R43971 as the holotype specimen. The specific name of this taxon comes from the Latin Risor, meaning "mocker". In his description, he justifies the distinction of this species via the larger orbits, the smaller maxillae and the curved snout. In 1995, the same author carried out a more in-depth revision of the three specimens attributed to this taxon. He then discovered that the characteristics he had previously judged to be distinctive were in fact stages of growth, the three specimens representing juveniles of T. platyodon.

Early depictions

One of the earliest representations of Temnodontosaurus in paleoart is a life-size concrete sculpture created by Benjamin Waterhouse Hawkins between 1852 and 1854, as part of the collection of sculptures of prehistoric animals on display at the Crystal Palace Park in London. It is one of the three ichthyosaur sculptures exhibited in the park, the other two representing Ichthyosaurus and Leptonectes. Although the park is known for its obsolete or even false reconstructions of extinct animals, the sculptures depicting ichthyosaurs have the most elements still recognized as valid. These recognized features include smooth, scaleless skin, a fin at the end of the tail, and large eyes. Hawkins also reconstructed the facial features of these three ichthyosaurs based on those of whales and dolphins, which is still recognized as reasonable given their strong morphological similarities. Discoveries and reconstructions subsequent to those at the Crystal Palace add to this the presence of a dorsal fin, a caudal fin with two crescent-shaped lobes as well as a reconstruction of the skin on the basis of most well-preserved fossils.

Many elements of these reconstructions still remain obsolete, such as the eyes and the flippers which are reconstructed by the shape of their bones, namely the scleral rings and the phalanges. Although Owen suggested the still viable hypothesis that the scleral rings served to protect the eye, it is highly unlikely that the eyes of ichthyosaurs would have looked as shown in the carvings, given that the scleral rings are located under the eyelids. The flippers were faithfully reconstructed by Hawkins based on Owen's misinterpretation of the phalanges as scales. The park's ichthyosaurs are depicted as crawling in shallow water, reflecting the ancient hypothesis that they came to the shores to sleep or to breed. Additionally, their tails are shown to be eel-like and having a great degree of flexibility. However, the three ichthyosaurs actually had a fairly variable degree of flexibility. Two of the three taxa shown, i. e. Temnodontosaurus and Leptonectes, were found to have much more flexible tails than that of Ichthyosaurus, the latter having a tuna-like morphology. This way of reconstructing the tail of ichthyosaurs as similar to those of eels is not an error specific to Hawkins, being the norm in reconstructions dating from the 19th century.