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There's some controversy over whether all dinosaurs were feathered or not.
Although there is strong evidence that the majority of theropod dinosaurs had feathers, the dinosaurs in the new Jurassic World film are all portrayed with scaly skin, and some scientists say that all dinosaurs should have them. So were feathers the norm for the group as a whole?
No, says Paul Barrett, dinosaur expert at the Natural History Museum, in a paper published this week. ‘Feathers and feather-like structures did not characterise all dinosaurs,’ he said. ‘They were found only in one group - the theropod group closest to birds, the Coelurosauria’.
Barrett and his colleagues studied the fossil skins of 80 non-avian dinosaurs from the theropod, ornithischian and sauropod groups.
Evidence for feathers was found in Coelurosauria, which is a major group that, as well as including modern birds, also includes the tyrannosauroids (featuring the famous T.rex), ornithomimids, dromaeosaurids, troodontids and many others.
Hundreds of coelurosaur species lived during the age of the dinosaurs, which is called the Mesozoic Era and that lasted from 250 to 66 million years ago.
Most of the Mesozoic dinosaur species with direct evidence of feathers have been discovered in the last 20 years and the vast majority of these are from China, such as Sinosauropteryx (named in 1996), Sinornithosaurus (named in 1999), and Caudipteryx (named in 1999).
The largest dinosaur with direct evidence of feathers is the 1.4-tonne Yutyrannus huali (named in 2012) meaning 'beautiful feathered tyrant'.
And one of the earliest discovered, found in Germany in 1861, is Archaeopteryx, which provided the first evidence that birds evolved from dinosaurs.
The team found no firm evidence of feathers in non-coelurosaurs, which make up the majority of all dinosaurs. Some feather-like structures that have been proposed in some other dinosaur groups, such as the ornithischians, may have been misidentified, they say.
'It might be that the "quills" seen in dinosaurs like Psittacosaurus might represent highly modified scales rather than feathers,' said Barrett.
Preserving feathers is difficult because they easily rot away. Barrett's team found that finding fossil feathers depends upon the type of rock in which the animal was preserved.
So it is possible that if a feathered dinosaur skeleton was preserved in the ‘wrong’ type of rock, there would be no evidence of the feathers ever being there.
Barrett says 'It's likely that we're missing a large part of the feather fossil record due to incomplete preservation, but for the time being a direct reading of the dinosaur fossil record doesn't support the presence of feathers outside of theropods.
'It could be that new discoveries, particularly of early dinosaurs, will have a major effect on this pattern and hopefully we'll find new sites in the next few years that will shed more light on the early evolution of feathers.'