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After a case of mistaken identity, the forelimb of a 155-million-year-old dinosaur has been identified as belonging to a new species of sauropod.
Now known as Rhomaleopakhus turpanensis, or 'robust forelimb from the Turpan Basin', the dinosaur was much chunkier than any other sauropod, which has implications for what scientists think these giant herbivores were doing.
The front arm and hand, or forelimb, of the dinosaur were originally uncovered almost 30 years ago in the Xinjiang Autonomous Region in the far north-west of China.
Found in the same areas as other fossils of sauropod dinosaurs, including vertebrae and teeth, the bones were originally all lumped together and assumed to belong to the same species. But a closer analysis of the large, two-metre-long forelimb that was unearthed showed a number of distinct characteristics not found in any other dinosaur.
These included the sheer bulkiness of the bones, which are more heavy-set than would normally be expected, distinctive scars for large muscle attachments and an odd-shaped funny bone.
'This new species was formally described as a specimen of another similar dinosaur called Hudiesaurus, but it has a number of features that are not seen in any other sauropod,' explains Paul. 'For example, it has perhaps the stockiest arms of any known sauropod of this age.
'That means it would have been really heavily-set in comparison to the other sauropods it lived alongside. If it was a rugby player, it would be a prop forward.'
Sauropod dinosaurs are characterised by their really long necks and tails, including members such as Diplodocus. But this new species belonged to a group of sauropods which took this to an extreme.
Known as the mamenchisaurs, they were a group of sauropods found mainly in what is now China, including species such as Klamelisaurus, Xinjiangtitan and Chuanjiesaurus, although at least one species is also known from Africa.
There were unusual in that they had wildly long necks even in comparison to other sauropods.
'These dinosaurs added extra vertebrae to their necks to elongate them, and in addition to this made each individual neck vertebra longer,' says Paul. 'We don't know why they did this, but we presume it was either a feeding adaptation or sexual selection.
'The best-known member of the group is Mamenchisaurus itself which is known from a couple of relatively complete skeletons.'
At the time these dinosaurs were wandering about, the land that would become China was a large continental island. This is one of the reasons why some palaeontologists think that the dinosaurs living in this region were so distinct when compared to those found in the rest of the world at this time.
This new species would have likely to have been very similar to Mamenchisaurus and the others within this group. With a forelimb measuring a whopping two metres long, Paul suspects that, overall, Rhomaleopakhus would have measured between 20 and 25 metres in length.
What is so unusual about this new species, however, is not the length of the bones but their stockiness.
'It is that chunkiness of the arm which gives the animal its name, which means 'robust forearm',' explains Paul. 'There are also ridges reflecting strong muscle attachments.'
In addition to this, the forearm also shows a curiously large 'funny bone'. This part of the forearm, or ulna, was surprisingly pronounced, which would have had implications for how the large dinosaur held its feet as it moved.
'We think that an unusually large chunky projection at the top of its funny bone is associated with a more strongly flexed forelimb, so that the forelimb is held habitually in a slightly bent, rather than straight pose,' says Paul. 'We think this means that the forelimb was not just propping the large animal up, but that they might have been doing something interesting with it.'
Coupled with its stocky build and the fact that the animal's centre of mass may have moved slightly forward when compared to other sauropods, Paul and his colleagues think that these dinosaurs might have been improving their adaptations for walking.
The changes suggest that these large animals may have been sacrificing their ability to rear up on their hind legs (which some sauropods did to reach leaves) to improve their efficiency when walking, perhaps because they had to migrate between widely-spaced patches of food.
The new sauropod helps to expand our understanding of dinosaur fauna from China during the Late Jurassic at a time when they may have been cut off from the rest of the world.