This timeline of plesiosaur research is a chronologically ordered list of important fossil discoveries, controversies of interpretation, taxonomic revisions, and cultural portrayals of plesiosaurs, an order of marine reptiles that flourished during the Mesozoic Era. The first scientifically documented plesiosaur fossils were discovered during the early 19th century by Mary Anning. Plesiosaurs were actually discovered and described before dinosaurs. They were also among the first animals to be featured in artistic reconstructions of the ancient world, and therefore among the earliest prehistoric creatures to attract the attention of the lay public. Plesiosaurs were originally thought to be a kind of primitive transitional form between marine life and terrestrial reptiles. However, now plesiosaurs are recognized as highly derived marine reptiles descended from terrestrial ancestors.

Early researchers thought that plesiosaurs laid eggs like most reptiles. They commonly imagined plesiosaurs crawling up beaches and burying eggs like turtles. However, later opinion shifted towards the idea that plesiosaurs gave live birth and never went on dry land: the biggest genera may have been too heavy to go on land at all, but smaller genera could've been capable. Plesiosaur locomotion has been a source of continuous controversy among paleontologists. The earliest speculations on the subject during the 19th century saw plesiosaur swimming as analogous to the paddling of modern sea turtles. During the 1920s opinion shifted to the idea that plesiosaurs swam with a rowing motion. However, a paper published in 1975 that once more found support for sea turtle-like swimming in plesiosaurs. This conclusion reignited the controversy regarding plesiosaur locomotion through the late 20th century. In 2011, F. Robin O'Keefe and Luis M. Chiappe concluded the debate on plesiosaur reproduction, reporting the discovery of a gravid female plesiosaur with a single large embryo preserved inside her.

Prescientific Associated remains of plesiosaurs and animals like the diving bird Hesperornis or the pterosaur Pteranodon may have inspired legends about conflict between Thunder Birds and Water Monsters told by the Native Americans of Kansas and Nebraska.

18th century

1719 William Stukeley described the first partial skeleton of a plesiosaur, brought to his attention by the great-grandfather of Charles Darwin, Robert Darwin of Elston.

19th century

1810s

1811 Mary Anning discovered some plesiosaur fossils in England.

1820s

1821 Henry De la Beche and William Conybeare are the first to name a plesiosaurian species: Plesiosaurus dolichodeirus.

1822 Parkinson coined the name Plesiosaurus priscus for some of the remains used by de la Beche and Conybeare as the basis for Plesiosaurus. This species is currently regarded as of dubious taxonomic value.

1823 December Mary Anning discovered a nearly complete Plesiosaurus skeleton near Lyme Regis. This specimen would later be catalogued as BMNH 22656.

c. December Around the same time as the discovery of BMNH 22656, another Plesiosaurus specimen was discovered at the same site. The specimen was donated to the Oxford University museum and is probably the specimen known today as OXFUM J.10304.

1824 Conybeare described the new species name Plesiosaurus dolichodeirus for the Plesiosaurus discovered by Anning. As the first species name given to a distinctive and well preserved Plesiosaurus skeleton it has come to be regarded as both the type specimen of Plesiosaurus dolichodeirus specifically and of the genus Plesiosaurus as a whole.

1829 Mary Anning collected the Plesiosaurus dolichodeirus specimen now known as BMNH R.1313.

1830s

1837 Buckland in Conybeare described the new species Plesiosaurus macrocephalus.

1839 De la Beche illustrated a work titled "Duria Antiquior", meaning "A More Ancient Dorset" for fossil hunter Mary Anning. This work, which prominently features plesiosaurs, has been regarded as the first attempt to accurately reconstruct the Mesozoic world through an artistic medium. von Meyer described Pistosaurus. Pistosaurus is believed to be a transitional form linking plesiosaurs to their basal sauropterygian forebears.

1840s

1840 Owen described the species now known as Colymbosaurus trochanterius, Eurycleidus arcuatus, and Thalassiodracon hawkinsii.

1841 Owen described the new species Pliosaurus brachydeirus, Pliosaurus brachyspondylus. and Polyptychodon interruptus.

1842 Sir Richard Owen formally named the pliosaurs.

1846 Stutchbury described the species now known as Atychodracon megacephalus.

1848 The trustees of the British Museum of Natural History bought the type specimen of Plesiosaurus from the estate of the first duke of Buckingham, Richard Glenville. The museum catalogued the specimen as BMNH 22656.

1860s

1863 Carte and Bailey described the species now known as Rhomaleosaurus cramptoni.

1864 Jules Verne depicted a relict plesiosaur's defeat in combat against an ichthyosaur in Journey to the Center of the Earth.

1865 Owen described the species now known as Eretmosaurus rugosus and Microcleidus homalospondylus. Seeley described the species now known as Microcleidus macropterus. Owen described the species now known as Archaeonectrus rostratus.

1867 An army surgeon named Dr. Theophilus Turner discovered the fossils of a large animal in the Pierre Shale of Kansas, USA. The remains represented the first nearly complete plesiosaur specimen from North America. Turner gave some of its vertebrae to a member of the Union Pacific Railroad's survey named John LeConte. LeConte sent the vertebrae to Edward Drinker Cope for study. Cope recognized the find as a significant plesiosaur discovery and wrote to Turner asking him to excavate and ship the fossils to him.

March, mid

Cope erected the new genus and species Elasmosaurus platyurus for the fossils sent by Turner in a rushed descriptive manuscript written within two weeks of obtaining them.

March 24 Cope presented his findings regarding Elasmosaurus to a meeting of the Academy of Natural Sciences in Philadelphia, Pennsylvania.

1868 Cope described the new species Elasmosaurus platyurus. Cope's description of Elasmosaurus was formally published.

September William E. Webb and others collected and shipped a plesiosaur specimen to Cope.

1869 Seeley described the species now known as Liopleurodon pachydeirus and Peloneustes philarchus. Cope named the plesiosaur specimen collected by Webb Polycotylus latipinnis.

August

Cope prepared a preprint for the Transactions of the American Philosophical Society of his Elasmosaurus description, including reconstruction of the animal with a short neck and very long tail. The manuscript was then distributed to other scholars.

1870s

1870 March 8

Cope's mentor Joseph Leidy gave a presentation reporting his recent discovery that Cope's reconstruction of Elasmosaurus positioned the skull at the end of the tail rather than the end of the neck. Leidy's discovery embarrassed Cope, who began spreading notice of an unspecified error in his Elasmosaurus description with an offer to replace it with a corrected version and its second volume.

November O. C. Marsh collected a better an additional specimen of Polycotylus in Kansas that was better preserved than the type described by Cope. The specimen is now catalogued as YPM 1125.

1871 Phillips described the species now known as Pliosaurus macromerus. Phillips described the species now known as Cryptoclidus eurymerus. Cope described the species now known as Hydralmosaurus serpentinus. Cope inaccurately referred to Polycotylus as "the first true plesiosauroid found in America."

1872 Cope imagined elasmosaurs feeding by craning their necks above the water and striking downward at fish long distances from their bodies. B. F. Mudge discovered ten articulated vertebrae in the Fairport Chalk of Kansas that he mistook for ichthyosaur remains. These fossils are now catalogued as KUVP 1325.

1873 Sauvage described the new species Liopleurodon ferox. Joseph Savage discovered a second, better preserved Trinacromerum "anonymum" in Kansas.

1874 Hector described the new species Mauisaurus haasti. Seeley described the new species Muraenosaurus leedsi. B. F. Mudge discovered fragments of a large elasmosaur skeleton in the Fort Hays Limestone of Kansas. Mudge and Williston excavated the remains another large Kansan plesiosaur, this one from the Smoky Hill Chalk. The specimen may be a Styxosaurus snowii and is currently catalogued as YPM 1644. It was the first plesiosaur Mudge had ever found with gastroliths and the first plesiosaur encountered by Williston in general. Hector reported the presence of elasmosaur remains in New Zealand. Seeley published a paper intended to help improve the state of science's understanding of plesiosaur shoulder girdle anatomy, which had been muddled by the poor preservation of the fossils many early paleontologists had to rely on for their observations.

1875 Cope once more portrayed elasmosaurs as feeding by fishing from a distance with heads held above the waterline.

1876 Blake in Tate and Blake described the new species Eretmosaurus dubius and Plesiosaurus longirostris.

1877 Lydekker described the species now known as Simolestes indicus. Mudge discussed the gastroliths of YPM 1644 in a scientific publication. He concluded that plesiosaur exploited gastroliths to assist in breaking down food the way many modern birds and reptiles do.

1879 Lydekker described the species now known as Cryptoclidus richardsoni.

1880s

1880 Oxford acquired the Misses Philpot collection, which included the type specimen of Plesiosaurus macromus. The museum catalogued this specimen as OXFUM J.28587.

1881 Sollas described the species now known as Attenborosaurus conybeari.

1882 The British Museum of Natural History purchased the Edgerton collection, which included the complete Plesiosaurus dolichodeirus jaw now known as BMNH R.255.

1884 Spring The Smithsonian bought a partial plesiosaur skeleton from Charles Sternberg. The specimen is now catalogued as USNM 4989 and would later serve as the type specimen of the new genus and species Brachauchenius lucasi.

1888 Harry Seeley mistakenly claimed to have discovered several fossil plesiosaur embryos. F. W. Cragin named the genus and species Trinacromerum bentonianum from Kansas.

1890s

1890 In an article published in the New York Herald, Marsh brought up Cope's anatomic reversal of Elasmosaurus. E. P. West excavated a skull and partial neck belonging to the elasmosaur that would come to be named Styxosaurus snowii. The specimen is now catalogued as KUVP 1301. Williston described the species now known as Styxosaurus snowii.

1892 Seeley described the species now known as Muraenosaurus beloclis.

1893 Marsh described the new species Pantosaurus striatus. Charles H. Sternberg obtained two large elasmosaur vertebrae that would later serve as the type specimen of Elasmosaurus sternbergi. The specimen is now catalogued as KUVP 1312. F. W. Cragin discovered a partial plesiosaur skeleton and associated gastroliths in what is now recognized as the Kiowa Shale. This specimen is now catalogued as KUVP 1305 and would later be named Plesiosaurus mudgei. Williston argued that plesiosaurs ingested gastroliths only accidentally or to relieve "food craving[s]". However, he also observed that the rocks used as gastroliths were more similar to rocks 400–500 miles away in Iowa or the Black Hills of South Dakota than those of the local geology.

1894 Cragin described the new species Plesiosaurus mudgei for KUVP 1305.

1895 Dames described the species now known as Seeleysaurus guilelmiimperatoris.

1897