Dinosaurs had wrists like birds

Wrists and feathers in the lineage that led to birds preceded flight

The flexible wrists of birds that let them fold their wings have now been seen in dinosaurs well before flight, scientists find.

Dinosaurs such as Velociraptor might have partly folded their feathered arms to protect such plumage from harm's way, researchers explained. The wrists and the feathers in the lineage that led to birds then became more extreme, laying the groundwork for flight, they added.

Although birds are most known for their feathers, wings and toothless beaks, another distinctive feature is a wrist joint that is extremely flexible, although only in one direction. A bird can bend its wrist to the point where the side of the hand where the little finger would be can lie closely alongside the forearm, so any fingertips would point back almost towards the elbow, but the wrist cannot bend in the opposite direction, nor even fully straighten. 

This unique joint permits a bird to fold the wing when at rest, and to partly fold the wing during the upstroke in flight, greatly improving the efficiency of their flight. The question then is when and how this wrist evolved.

Evolution evidence

There is overwhelming evidence that birds evolved from predatory dinosaurs , the theropods, which include carnivores such as Velociraptor and giants such as Tyrannosaurus rex.

For instance, these hunters seemed to have possessed a similar way of breathing, and many also may have possessed feathers to keep them warm or to serve as courtship ornaments. The earliest theropods had wrists that were apparently relatively straight and inflexible, raising the questions of when and how the avian-style wrist developed.

A team of Canadian, British and Chinese researchers traced the gradual evolution of this wrist in roughly a dozen species of theropods, analyzing several well-preserved members of this group from 110 to 160 million years ago that included a wide range of body designs. They found that a wrist bone called the radiale slowly and progressively took on a wedge-like shape in the theropods more closely related to birds, such as Velociraptor and Deinondenychus.

As this bone changed its shape, the wrist would have become increasingly bird-like in its range of motion. 

This increase in the ability to fold the wrist took place alongside two other trends — an increase in arm length and an increase in the length of the feathers that adorned the arms of many theropods.

"If the wrist could not fold like this, the long feathers on the hand would drag on the ground, getting dirty or snagging on vegetation," said researcher David Hone, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology of China in Beijing. "Protecting them would have been important for feathered dinosaurs in the past, as it is for birds today."

Cause or effect?
It remains unclear if the fold able wrist allowed dinosaurs to evolve longer feathers or if the evolution of longer feathers drove the need for more flexible wrists .

Still, it is clear "that wing-folding, or at least feathered arm folding, significantly preceded flight," said lead researcher Corwin Sullivan, a vertebrate paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. "This pattern of flexibility originally evolved in a terrestrial context and just happened to be present and available for use when birds took to the air.

This discovery highlights the fact "that some characteristics that biologists used to think of as distinctively avian — feathers and air sacs are other good examples — are actually quite deeply rooted in theropod evolution," Sullivan added.

 

New dinosaur discovery solves evolutionary bird puzzle

A newly discovered fossil has shed light on why a group of dinosaurs looks like birds, say scientists.

Haplocheirus sollers may not be as charismatic as T. rex or as agile as a pterodactyl but it's thought to solve a long standing puzzle.

Researchers believe its short arms and large claw show how bird-like dinosaurs evolved independently of birds.

The 3m-long skeleton, found on an expedition to China's Gobi desert, is described in the journal Science.

The fossil is a member of the Alvarezsauridae family, a group of bird-like dinosaurs. The group shares features with birds, including fused wrist elements and a loosely structured skull.

But the researchers say the new fossil shows the Alvarezsauridae group split from birds much earlier on the evolutionary tree than was thought.

"Haplocheirus is a transitional fossil," Jonah Choiniere from George Washington University told the BBC.

"Previously we thought the Alvarezsauridae were primitive, flightless birds. This discovery shows they're not and that the similarities between them evolved in parallel."

The fossil is of a nearly complete adolescent dinosaur skeleton and was found in orange mudstone beds in the Junggar Basin, Xinjiang, China.

It was spotted when a member of the team noticed the pelvis at the ground's surface. The rest of the skeleton was found only inches down.

The new dinosaur shows an early evolutionary step in the development of the short, powerful arm typical to the Alvarezsauridae group.

"The rest of the members of this group have really short forelimbs with huge muscle attachments, like body-builder arms. The fossil shows the first step in the evolution of this weird arm and claw," said Mr Choiniere.

Varied diet

The researchers believe the fossil shows development of the two diverged in the Late Jurassic period, about 160 million years ago. Until now there was no evidence of this type of dinosaur living at that time.

"It's like finding a great, great grandfather in your family which doubles the age of your family tree," said Mr Choiniere.

Scientists believe that birds descended from theropods or bird footed dinosaurs in the Late Jurassic. Theropods include alvarezsaurs, other bird-like dinosaurs including the well known Velociraptor, meat eaters like T. rex and modern birds.

Haplocheirus sollers means simple, skillful hand. The fossil shows the dinosaur had small teeth and researchers believe the claw may have been used for digging termites.

"It may have had a very general diet, tackling smaller animals like lizards, very small mammals and very small crocodile relatives," explained Mr Choiniere. "It was a lightly built animal and could run very quickly."