THE FIRST FEATHER
25 years after the discovery of the first plumed dinosaur, scientists still haven’t found the origin of feathers. Controversial discoveries suggest that dinosaurs were not the first feathered creatures: there may have been a common feathered ancestor.
New discoveries have pushed the first feather back down the evolutionary time-scale. But were they developed for flight, or simply to keep their earliest owners warm?
Asmall striped creature the size of a turkey is uncovered in a fossilised lake bed in China. The dinosaur, which has spent 125 million years on the the lake bed before being discovered in 1996, is quite a surprise to scientists − a predatory dinosaur with thick plumage and a patterned face like that of a racoon. And importantly, Sinosauropteryx is the first dinosaur with feather-like plumage – but without wings.
The discovery of Sinosauropteryx in China offered clear evidence against the theory that feathers originated to allow birds to take flight. The first plumed ancestors of today’s 10,000+ modern bird species, including about 26 billion chickens, apparently had no ambitions to soar above their habitat, as palaeontologists and ornithologists had long believed. So what, then, was the purpose of feathers? The discovery of two fossil pterosaurs has now brought scientists back to a discussion of the first feather – and has shaken up the dinosaur family tree.
The first fossil feather was discovered in 1861 in limestone near Solnhofen, Germany. It was 150 million years old and probably belonged to a nearby fossil of Archaeopteryx, a bird-like dinosaur. For more than 100 years, that feather and Archaeopteryx formed the basis of research into feather origins, in which feathers and flying were considered inseparable. But that was a false trail.
With the discovery of Sinosauropteryx in China, it became clear that feathers served other purposes. Sinosauropteryx had no flight feathers; it was covered in a kind of plumed fur known as ‘dino fuzz’. The purpose of such protofeathers – small, primitive, hair-like feathers – was probably to function as insulation, to help the creature keep warm.
Many scientists were already becoming convinced that birds had descended from dinosaurs, and feather origin research gained momentum from many discoveries over the past 10 to 20 years, including prehistoric birds and at least five different predatory dinosaurs which appear to support a surprising variation of feathers and of feather-like outgrowths.
The new discoveries have been made thanks to improved access to China’s fossil record for young energetic palaeontologists, who bring new technology – such as LSF (laser-stimulated fluorescence), by which lasers make otherwise invisible soft tissue (such as skin and feathers) light up. The discoveries are important, because feathers do more than tell us about evolutionary connections between different animals.They are also evidence of specific physiologies, living conditions, and behaviours.
For example, thick plumage as evident on Sinosauropteryx could mean that the creature was able to maintain a stable body temperature, more like birds and mammals than the reptiles they are. Also, analyses of melanosomes – small pigment pockets in the feathers – reveal that Sinosauropteryx had a reddish colour, with light stripes on its tail. This type of camouflage, with the dark back and white abdomen, is a distinct pattern that fits life in an open landscape.
Animals that live in a open plain tend to have a clear distinction between light and dark high up the abdomen, compared with forest-dwelling animals that have a more diffuse distinction further down their bodies. The distinction high up the abdomen blurs some of the shadow that sunlight casts on the body, making the animals seem ‘flat’. Predators would find it harder to spot Sinosauropteryx against the landscape.
The plumed Sinosauropteryx belonged to the same Coelurosauria sub-group of predatory dinosaurs that includes both T. rex and the small elegant carnivores known as ‘raptors’ from the Jurassic Park films. So there seemed to be every indication that feathers originated in a subgroup of predatory dinosaurs. But just as this idea found acceptance as confirming the origin of feathers, a small horned creature overturned the theory.
PAUL BARRETT PROFESSOR AND DINOSAUR EVOLUTION EXPERT It is still possible that feathers played an important role in the history of dinosaurs.
Herbivores raised their bristles
Psittacosaurus is a herbivore, belonging to the group of horned dinosaurs that also includes Triceratops. It is located far away from the predatory dinosaurs on the family tree. Yet a Psittacosaurus fossil revealed that it had bristles protruding from its tail that were hollow, inflexible, and very reminiscent of primitive feathers.
In 2014, another herbivore with similar growths turned up. Kulindadromeus was discovered in Kulinda, in eastern Siberia. The dinosaur was the size of a medium dog, and the fossil was extremely well-preserved.
It was wearing a very special ‘overcoat’ which consisted of three different kinds of scales, also short feather-like fur, and clusters of fibre structures resembling small curled ribbons. Moreover, Kulindadromeus had a few hair-like bristles on its head, neck and body, while its upper arms and thighs included small, feather-like ‘bouquets’ of 5 to 7 bristles. These last bouquets are particularly interesting, as something similar had been observed in both predatory dinosaurs and modern birds. Yet here the location of the fossil dates it to some 168 million years ago, making Kulindadromeus the oldest dinosaur yet to include feather-like growths.
Several scientists have interpreted the find as an indication that there must be a common ancestor of all plumed dinosaurs, as they consider it unlikely that such complex plumage as that of the carnivore Sinosauropteryx and the herbivore Kulindadromeus could have originated independently in each of these two main groups of dinosaurs. But the hypothesis is controversial, particularly because feathers are still very rare among herbivorous dinosaurs. Some scientists prefer to suggest that the feathers originated randomly several times on different branches of the family tree.
Four kinds of pterosaur feather
Those looking for a common ancestor have examined the pterosaurs that coexisted with dinosaurs from 230 to 66 million years ago. These pterosaurs have their own branch of the dino-family tree, so linking back to a common original form.
Scientists headed by palaeontologist Mike Benton have been taking a close look at 160-million-year-old pterosaur fossils from China, and have found structures that they think originate from four different types of head, neck, body and wing feathers.
MIKE BENTON
PALAEONTOLOGIST The structures are the same as the ones we observe in the feathers of birds and dinosaurs, moving the origin of feathers from 160 to 240 million years ago.
“The structures are the same as the ones we observe in the feathers of birds and dinosaurs, moving the origin of feathers from 160 to 240 million years ago,” Mike Benton says. That dating fits into the period following the Permian-Triassic extinction, when some 95% of all animal and plant species were driven to extinction after severe volcanic eruptions.
After that, life developed new forms. The first dinosaurs and mammals arrived, while reptiles became marine animals, or developed wings to take flight. According to the scientists behind the study, a common ancestor of dinosaurs and pterosaurs developed protofeathers as insulation 240 million years ago, with the other functions of feathers not developing until much later. They cite genetic studies that indicate a common origin for scales, hair, and feathers across different species.
This means that all dinosaur species had an opportunity to develop feathers, but environmental conditions and mutations would then have determined if and how the genes found expression successful enough to carry on through the generations. Some never developed feathers; others had them for a period of time only to lose them again.
Just fraying and decayed fibres?
This interpretation is not, however, yet widely shared. Various palaeontologists still disagree with Benton’s conclusions – and they have supercomputers on their side.