Pelorosaurus ("monstrous lizard") is the generic name of a sauropod dinosaur. Pelorosaurus was one of the first sauropod names ever. Remains referred to Pelorosaurus date from the Early Cretaceous period, about 140-125 million years ago and have been found in England and Portugal.
Many species have been named in the genus but today these are largely seen as belonging to other genera. The first named species of Pelorosaurus, P. conybeari, is a junior synonym of Cetiosaurus brevis.
Pelorosaurus was the first sauropod to be identified as a dinosaur, although it was not the first to be discovered. Richard Owen had discovered Cetiosaurus in 1841 but had incorrectly identified it as a gigantic sea-going crocodile-like reptile. Mantell identified Pelorosaurus as a dinosaur, living on land.
The taxonomic history of Pelorosaurus and Cetiosaurus, as noted by reviewers including Taylor and Naish, is confusing to the extreme. In 1842 Owen named several species of Cetiosaurus. Among them was Cetiosaurus brevis, based on several specimens from the early Cretaceous Period. Some of these, four caudal vertebrae, BMNH R2544–2547, and three chevrons, BMNH R2548–2550, found around 1825 by John Kingdon near Cuckfield in the Tunbridge Wells Sand Formation of the Hastings Beds, belonged to sauropods. Others however, among them perhaps the present specimen BMNH R10390 found near Sandown Bay on the Isle of Wight and BMNH R2133 and R2115 found near Hastings, actually belonged to some member of the Iguanodontidae. Noticing Owen's mistake in assigning iguanodont bones to Cetiosaurus, in 1849 comparative anatomist Alexander Melville renamed the sauropod bones Cetiosaurus conybeari.
Gideon Mantell in 1850 decided that C. conybeari was so different from Cetiosaurus that it needed a new genus, so he reclassified it under the new name Pelorosaurus Conybearei. Mantell had originally, in November 1849, intended to use the name "Colossosaurus", but upon discovering that kolossos was Greek for "statue" and not "giant", he changed his mind. The generic name is derived from the Greek pelor, "monster". He also emended the specific name, honouring William Conybeare, to conybearei but under present rules the original conybeari, today written without a capital, has priority. Mantell not only used the sauropod material of C. brevis as the type of Pelorosaurus conybeari but also a large humerus found by miller Peter Fuller at the same site, BMNH 28626, which he assumed to have been of the same individual, being discovered only a few metres away from the vertebrae. Mantell acquired the bone for ₤8. The humerus, clearly shaped to vertically support the weight of the body and presumed to possess a medullary cavity, showed that Pelorosaurus was a land animal. This was a main motive in naming a separate genus; shortly afterwards, however, by studying the sacral vertebrae of Cetiosaurus Mantell established that it too lived on land.
Owen was highly piqued by Melville's and Mantell's attempts to "suppress" his Cetiosaurus brevis. By a publication in 1853 he tried to set matters straight, as he saw it, while avoiding having to openly admit his original mistake. First he suggested that Melville's main motivation for the name change was the presumed inaccuracy of the epithet brevis, "short", because the total length of the animal could not be deduced from such limited remains. Owen pointed out that anyone being acquainted with taxonomy would have understood that "short" referred to the vertebrae themselves, not to the animal as a whole. On a subsequent page, apparently separate from this issue, Owen in covert terms implied that his 1842 publication was not descriptive enough, thus merely having resulted in a nomen nudum, to which he now assigned the sauropod material, making Cetiosaurus brevis a valid name. This still left the problem of it having been named a new genus by Mantell. Owen resolved it by simply presenting the humerus as the sole holotype of Pelorosaurus conybeari. Remarkably, in 1859 he repeated his mistake by again referring iguanodontid vertebrae, specimens BMNH R1010 and R28635, to C. brevis. The last of these he had in 1853 proposed to belong to Pelorosaurus together with a number of other iguanodontid vertebrae because Mantell had once labelled them as such in his collection; Owen suggested it had been by a mere mistake that the name Pelorosaurus had been connected with the C. brevis material instead of with these finds.
Owen's interpretation was commonly accepted until well into the twentieth century. By 1970 however, both John Ostrom and Rodney Steel understood that Owen's claim that C. brevis in 1842 was still a nomen nudum should be rejected as a transparent attempt to change the type specimen, inadmissible by present standards. By those same standards though, Melville's name change was also incorrect: as the name Cetiosaurus brevis was still "available" he should simply have made the sauropod bones the lectotype, removing the iguanodontid remains from the syntype series. The sauropod bones, not the iguanodont bones, would then have retained the name C. brevis. Therefore, Cetiosaurus conybeari is a junior objective synonym of C. brevis, that is, C. brevis is not only an older name, but one based on exactly the same fossils as the younger, invalid name. C. conybeari is thus a junior objective synonym.
After 1850, more specimens continued to be assigned to both Pelorosaurus and Cetiosaurus, and both were studied and reported on extensively in the scientific literature. Slowly a tendency developed to subsume fragmentary sauropod material from the Jurassic of England under the designation Cetiosaurus, while assigning incomplete European Cretaceous sauropod finds to Pelorosaurus. Pelorosaurus thus came to be a typical wastebasket taxon for any European sauropod of this period. However, in recent years much work has been done to rectify the confusion.