Liopleurodon (; meaning 'smooth-sided teeth') is an extinct genus of carnivorous pliosaurid plesiosaurs that lived from the Callovian stage of the Middle Jurassic to the Kimmeridgian stage of the Late Jurassic period (approximately 166 to 155 million years ago). The type species is L. ferox, which is probably the only valid species. Although some studies still recognize L. pachydeirus, it is generally considered as a junior synonym of the type species due to the lack of a clear diagnosis. Fossils attributed to Liopleurodon, including some skeletons, are mainly known from Europe, with one occurrence reported in Mexico. As the holotype specimen of L. ferox consists of a single tooth with questionable distinguishing features, several recent studies recommend the designation a more complete neotype to preserve its validity. Additional species were proposed, but these are currently viewed as belonging to other pliosaurid genera.
Like other plesiosaurs, Liopleurodon has four flippers, a short tail, and, like most pliosaurids, a long head and a short neck. In 1999, its size was greatly exaggerated in the BBC documentary series Walking with Dinosaurs, where it was depicted as reaching in length. However, the different attributed specimens show that the animal could reach a size ranging from long. Various studies show that Liopleurodon would have been an ambush predator, feeding on fish, cephalopods and other marine reptiles.
Research history
Even before Liopleurodon was named, material likely belonging to it was described in scientific literature. In 1838, Hermann von Meyer applied the name Ischyrodon meriani to a large tooth from possibly Callovian-aged rocks in Fricktal, Switzerland. This tooth lacks identifying characteristics, and therefore it is not clear what it belonged to, although Lambert Beverly Tarlo noted the possibility of it pertaining to Liopleurodon in 1960. A 2022 study by Daniel Madzia and colleagues noted that while the tooth likely came from Liopleurodon or a similar species, there was too little information available to make a confident assignment, so they treated Ischyrodon as a nomen dubium. In 1841, von Meyer named the species Thaumatosaurus oolithicus based on a fragmentary specimen consisting of partial teeth, skull elements, vertebrae, and ribs from deposits in Württemberg, Germany, possibly dating to the Oxfordian. However, this material is nondiagnostic, lacking distinguishing features. Johann Andreas Wagner published a description of a large plesiosaur tooth from Bavaria, Germany, in 1852, assigning it to a new species that he named Pliosaurus giganteus. However, in 1824, William Conybeare had named a species of Plesiosaurus, Plesiosaurus giganteus, and this species was later viewed as a synonym of either Pliosaurus brachydeirus or Pliosaurus brachyspondylus by following authors. Since the name Pliosaurus giganteus had been used prior to Wagner's publication, Wagner's name is invalid due to preoccupation. In 1860, Hermann Trautschold assigned the name Pliosaurus giganteus to a small tooth that was discovered in the Moscow Basin of Russia, dating from the Oxfordian. This tooth is now thought to pertain to Liopleurodon, but the name Pliosaurus giganteus having already been used twice by this point, Trautschold's name is also invalid.
The genus name Liopleurodon was coined by Henri Émile Sauvage in 1873. Sauvage named three species which he assigned to this genus, each based on a single tooth. The holotype tooth of Liopleurodon ferox was discovered in Le Wast, near Boulogne-sur-Mer, and is since numbered as BHN 3R 197 and stored in the Lille Natural History Museum. More precisely, the tooth consists of a crown measuring long, coming from sedimentary layers dating from the Callovian. Another tooth from Charly, since preserved at the Paris School of Mines, embodies the holotype of L. grossouvrei. This tooth is long, with the crown measuring . The third, discovered near Caen, France, was originally attributed to Poikilopleuron bucklandi by Eudes Deslongchamps. While the tooth could have come from the megalosaur, Sauvage considered this identity unsubstantiated, and assigned it to the species Liopleurodon bucklandi. Sauvage did not ascribe the genus to any particular group of reptiles in his description. The genus name Liopleurodon derives from Ancient Greek , "smooth"; , "side" or "rib"; and , "tooth", all meaning "smooth-sided tooth", in reference to the very contrasting dentition of the animal. The specific name derives from Latin ferox, meaning "fierce", in reference to the large size of the teeth.
However, in 1880, Sauvage synonymized Liopleurodon with Polyptychodon, noting that it was similar to this genus, but distinct from Plesiosaurus and Pliosaurus. In 1888, Richard Lydekker, after studying some teeth attributable to Liopleurodon ferox in the Leeds Collection, concluded that they were so similar to those of Pliosaurus that they should be placed in that genus. These teeth had been collected by Alfred Leeds from the Oxford Clay Formation, near Peterborough, England. In 1869, Harry Govier Seeley had applied the name Pliosaurus pachydeirus to a series of cervical (neck) vertebrae representing the first 17 in the neck from the Oxford Clay Formation near Great Gransden. Other than its large size, Seeley provided no distinguishing characteristics. The specific name pachydeirus means "stout neck", due to its vertebral morphology. Lydekker stated in 1888 that the neck described by Seeley probably belonged to Pliosaurus ferox. Walerian Kiprijanoff named Thaumatosaurus mosquensis in 1883 based on remains including teeth, vertebrae, and limb bones from Oxfordian-aged rocks of the Moscow Basin. However, in 1889, Lydekker considered this species to be a probable junior synonym of P. ferox.
In 1905, John Frederick Blake described two teeth from Rushden, England, similar to those of other Liopleurodon ferox specimens, though from older strata than those from Peterborough. He noted that the teeth were quite different from those of Pliosaurus, while the bones were dissimilar to those of Polyptychodon. Since the species couldn't be assigned to either genus, he recommended reinstating the name Liopleurodon. After considering Liopleurodon to be a subgenus of Pliosaurus, N. Bogolubov also listed the two genera as distinct in 1912. When Lydekker had first visited the collection of Alfred Leeds (known as the Leeds Collection), the only remains of Liopleurodon in his collection were teeth. However, since then, Alfred Leeds, as well as his brother Charles Edward Leeds, had collected many more specimens of Liopleurodon, including skulls and much of the postcranial skeleton. Charles William Andrews described the anatomy of the marine reptile specimens of the Leeds Collection acquired by the British Museum of Natural History in two volumes, the first published in 1910 and the second in 1913. He described the Liopleurodon specimens in the second volume, though considered them to belong to Pliosaurus.
Hermann Linder also described specimens of Liopleurodon ferox in 1913. One of these was a poorly preserved partial skeleton excavated from the Oxford Clay of Fletton, England, housed in Institut für Geowissenschaften, University of Tübingen. The skeleton was mounted and missing regions were restored with material from other Liopleurodon specimens. Like Andrews, Linder also considered L. ferox to be a species of Pliosaurus. Additionally, Linder described some skulls from Fletton housed at both the University of Tübingen and the State Museum of Natural History Stuttgart as specimens of Pliosaurus grandis. Linder also assigned a nearly complete paddle to Pliosaurus sp. All of these specimens have since been assigned to Liopleurodon with varying degrees of confidence, though the skull Linder attributed to Pliosaurus grandis that was housed in Stuttgart was destroyed during World War II. In 1934, Friedrich von Huene described a partial skeleton from Swabia, Germany, in the Ornatenton Formation. As Linder, he also used the name Pliosaurus ferox instead of Liopleurodon ferox. In 1938, Alexandre Bigot used Pliosaurus ferox as well, assigning some teeth known from Calvados, in the north of France.
Lambert Beverly Halstead, then known as Tarlo, published a review of Upper Jurassic pliosaurid taxonomy in 1960. He considered Liopleurodon to be distinct from Pliosaurus, noting major differences between the mandibles of the two genera. In addition to the type species L. ferox, Tarlo also considered Pliosaurus pachydeirus to be a valid species within Liopleurodon, L. pachydeirus, noting that the two species had differences in their teeth and cervical vertebrae. L. grossouvrei was not considered valid, though it was tentatively retained for teeth from the Kellaways Formation. In 1971, Halstead published another paper about Jurassic pliosaurids, this time focusing on Pliosaurus rossicus, a species he was formerly unwilling to consider valid, due to a lack of information. After reviewing its anatomy, he considered it valid, though assigned it to Liopleurodon instead, based on its short mandibular symphysis. Halstead also considered Pliosaurus macromerus, which he had previously considered to belong to its own genus, Stretosaurus, to instead be a species of Liopleurodon, despite its irregularly-shaped scapula (although this was later discovered to be an ilium).
In a 2001 dissertation, Leslie F. Noè argued that L. pachydeirus was not diagnostic, and that L. ferox was the only valid species of Liopleurodon. The teeth of mounted skeleton in Tübingen, which Tarlo had attributed to L. pachydeirus, showed distinctive characteristics of L. ferox, indicating that cervical vertebrae are not useful for differentiating species, as argued by David S. Brown in 1981. While Tarlo had considered differences in tooth morphology to be diagnostic, Noè instead considered it to be individual variation. Noè also removed L. macromerus and L. rossicus from the genus, citing differences in tooth shape and mandibular symphysis length. The former species was tentatively placed back in Pliosaurus, while the latter was thought to warrant a new genus.
Based on Noè's observations, Espen Knutsen reassigned both L. macromerus and L. rossicus to Pliosaurus in his 2012 revision of the genus. The following year, Roger Benson and colleagues agreed with Knutsen's classification, but regarded the species P. macromerus as a nomen dubium due to the absence of a formally designated neotype. In 2015, Jair Israel Barrientos-Lara and colleagues described two pliosaurid fossils found near the town of Tlaxiaco in Oaxaca, Mexico. These fossils were extracted from Kimmeridgian deposits in the Sabinal Formation, and one of them, the partial front end of a snout, was attributable to Liopleurodon, though the researchers considered the remains too fragmentary to provide a species-level identification. L. grossouvrei, although synonymized with "Pliosaurus" andrewsi by most authors, was considered to potentially be a distinct genus in its own right by Davide Foffa and colleagues in 2018, given its differences from "P." andrewsi and Liopleurodon ferox. Madzia and colleagues in 2022 noted that the fact that Liopleurodon was named based on a single tooth of dubious distinctiveness is problematic, and that a more complete neotype may need to be designated to preserve the stability of L. ferox. They also stated that further study of the taxon was needed to confirm that the supposed differences between L. ferox and L. pachydeirus were indeed due to individual variation.