Plesiosaur - History of Discovery

History of Discovery

Skeletal elements of plesiosaurs are among the first fossils of extinct reptiles recognised as such. In 1605, Richard Verstegen of Antwerp illustrated in his A Restitution of Decayed Intelligence plesiosaur vertebrae that he referred to fishes and saw as proof that Great Britain was once connected to the European continent. The Welshman Edward Lhuyd in his Lithophylacii Brittannici Ichnographia from 1699 also included depictions of plesiosaur vertebrae that again were considered fish vertebrae or Ichthyospondyli. Other naturalists during the seventeenth century added plesiosaur remains to their collections, such as John Woodward; these were only much later understood to be of a plesiosaurian nature and are today partly preserved in the Sedgwick Museum.

In 1719, William Stukeley described a partial skeleton of a plesiosaur, which had been brought to his attention by the great-grandfather of Charles Darwin, Robert Darwin of Elston. The stone plate came from a quarry at Fulbeck and had been used, with the fossil at its underside, to reinforce the slope of a watering-hole in Elston. After the strange bones it contained had been discovered, it was displayed in the local vicarage as the remains of a sinner drowned in the Great Flood. Stukely affirmed its "diluvial" nature but understood it represented some sea creature, perhaps a crocodile or dolphin. The specimen is today preserved in the Natural History Museum, its inventory number being BMNH R.1330. It is the earliest discovered more or less complete fossil reptile skeleton in a museum collection. It can perhaps be referred to Plesiosaurus dolichodeirus.

During the eighteenth century the number of English plesiosaur discoveries rapidly increased, although these were all of a more or less fragmentary nature. Important collectors were the reverends William Mounsey and Baptist Noel Turner, active in the Vale of Belvoir, whose collections were in 1795 described by John Nicholls in the first part of his The History and Antiquities of the County of Leicestershire. One of Turner's partial plesiosaur skeletons is still preserved as specimen BMNH R.45 in the British Museum of Natural History; this is today referred to Thalassiodracon.

In the early nineteenth century plesiosaurs were still poorly known and their special build was not understood. No systematic distinction was made with ichthyosaurs so the fossils of one group were sometimes combined with those of the other to obtain a more complete specimen. In 1821, a partial skeleton discovered in the collection of Colonel Thomas James Birch, was described by William Conybeare and Henry Thomas De la Beche, and recognised as representing a distinctive group. A new genus was named, Plesiosaurus. The generic name was derived from the Greek πλήσιος, plèsios, "closer to" and the Latinised saurus, in the meaning of "saurian", to express that Plesiosaurus was in the Chain of Being more closely positioned to the Sauria than Ichthyosaurus which had the form of a more lowly fish. The name should thus be rather read as "approaching the Sauria" than as "near lizard". Parts of the specimen are still present in the Oxford University Museum of Natural History.

Soon afterwards, the morphology became much better known. In 1823, Thomas Clark reported an almost complete skull, probably belonging to Thalassiodracon, which is now preserved by the British Geological Survey as specimen BGS GSM 26035. The same year commercial fossil collector Mary Anning and her family uncovered an almost complete skeleton at in Dorset, England, on what is today called the Jurassic Coast. It was acquired by the Duke of Buckingham who made it available to the geologist William Buckland. He in turn let it be described by Conybeare on 24 February 1824 in a lecture to the Geological Society of London, during the same meeting in which for the first time a dinosaur was named, Megalosaurus. The two finds revealed the unique and bizarre build of the animals, in 1832 by Professor Buckland likened to "a sea serpent run through a turtle". Conybeare in 1824 also provided a specific name to Plesiosaurus: dolichodeirus, "longneck". The skeleton was in 1848 bought by the British Museum of Natural History and catalogued as BMNH 22656. When the lecture was published Conybeare also named a second species: Plesiosaurus giganteus. This was a short-necked form later assigned to the Pliosauroidea.

The majority of plesiosaur finds discovered in the 19th Century were found in the sea cliffs of Lyme Regis. Sir Richard Owen alone named nearly 100 new species. Despite this, plesiosaurs were little known. The majority of the descriptions of new species were based on isolated bones, without sufficient diagnosis to be able to distinguish them from the other species that had previously been described. Many of the new species described at this time have subsequently been invalidated. The genus Plesiosaurus is particularly problematic as the majority of the new species were located in this genus so that it has become a "recyclable" taxon.

The classification of plesiosaurs is further complicated by two factors. Although they have been found on every continent, including Antarctica, nearly all the discoveries come either from the Upper Jurassic Oxford Clay in England or from the Middle Cretaceous Niobrara chalk formation in Kansas in the United States. As only two links of a long evolutionary chain are well known it is difficult to extrapolate the connection between the two. Another problem is that the traditional way of classifying plesiosaurs relates to the thickness of their bodies, but it appears that the same body shape has been developed on multiple occasions in an example of convergent evolution. Recent analysis shows that elasmosaurs, which have long necks, are in fact descended from at least three unrelated lineages, making the taxa polyphyletic. Some pliosaurs could also be more closely related to the long-necked species than to other short-necked species. The four major groups, although convenient, do not appear to be based on real evolutionary relationships.