Remains referred to Pelorosaurus date from the Early Cretaceous period, about 140-125 million years ago, and have been found in England and Portugal.

The taxonomic history of Pelorosaurus and Cetiosaurus, as noted by reviewers including Michael P. Taylor and Darren Naish, is highly confusing.

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.

He also emended the specific name (honouring William Conybeare) to conybearei, but under the present rules of the ICZN, 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, NHMUK PV OR 28626, which he assumed to have been of the same individual, being discovered only a few metres away from the vertebrae.

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.

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.

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.

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.

That group being still very incompletely known however, it proved difficult to determine its more precise affinities, with the Atlantosauridae, Cardiodontidae, Cetiosauridae and Morosauridae being suggested until in 1927 von Huene understood the possible link with Brachiosaurus, placing Pelorosaurus in the Brachiosauridae, a placement followed by subsequent authors until the early 21st century.

The uncertainties about whether the qualities of the vertebrae or the humerus should be analysed, both specimens not necessarily belonging to the same taxon, prevents any firm conclusion to be reached, however.