Famous swooping depictions of “gentle giant” dinosaurs, such as the diplodocus, are almost certainly wrong, a study suggests.

The largest group of dinosaurs, known as sauropods, are usually portrayed in films, television and museums as having a low-necked, loping posture. In reality these dinosaurs were no slouchers, according to a study carried out by scientists at the University of Portsmouth.

They generally held their necks aloft like giraffes, making them up to 15m (almost 50ft) tall, concludes the study, published in the journal Acta Palaeontologica Polonica.

Mike Benton, a palaeontologist at the University of Bristol, said: “This new work provides plausible evidence that sauropods held their necks elevated, rather than horizontally, as had been assumed.”

The sauropod group of dinosaurs lived about 150 million years ago and included the diplodocus, the brachiosaurus and the apatosaurus. They were herbivores with small heads and big bodies, and they tended to have long necks and tails. The diplodocus could grow to 54m long.

Scientists studied the X-rays of ten species alive today. They found that while the neck slopes forward gently in salamanders, turtles, lizards and crocodiles, it is vertical in mammals and birds — the only modern groups that share the upright position of dinosaurs.

Mike Taylor, a palaeobiologist who led the study, said: “Like the animals we have with us today, they would have spent most of their time with their necks elevated, except when drinking or browsing at low levels.”

Scientists had argued against the vertical posture in sauropods, saying that it would require an implausibly steep bend at the base of the neck. However, very sharp bends were seen in ostriches and giraffes, proving that such a physique is possible.

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