US scientists have announced the discovery of a new species of dinosaur. Its fossils offer further clues to how the dinosaurs became extinct 66 million years ago.
Anzu wyliei is a strange, bird-like creature that has a bony crest on top of a beaky head and a long tail like a lizard.
The animal was identified from the partial remains of three skeletons collected in North and South Dakota.
It is reported in PLoS ONE journal.
"We had inklings that there might be such a creature out there, but now with these bones we have 80% of the skeleton and can really look in detail at the structure of this animal and make inferences about its biology," says Hans Sues, curator of vertebrate palaeontology in the department of palaeobiology at the Smithsonian's National Museum of Natural History in Washington DC.
"Anzu is really bizarre, even by dinosaur standards.
"The skull has this extraordinarily tall and thin crest with a snout and a huge beak with sharp edges and a strange sliding jaw joint," that could be used to cut up vegetation and meat, he says.
The size of a small car, the dinosaur also had claws and feathers on its upper arms. It belongs to a group of dinosaurs known as Oviraptorosauria. Most evidence of their existence comes from fossils discovered in Central and East Asia.
The Anzu bones are the first detailed evidence that oviraptorosaurs also lived in North America.
The specimens were found in a geological formation known as Hell Creek, which has been extensively explored and is the source of many dinosaur fossils discovered in North America.
Scientists have nicknamed it "the chicken from Hell" because of its appearance and where it was found.
The site is important because it was formed in the last two million years of the Cretaceous Period, just before dinosaurs were wiped out by an asteroid strike.
Many researchers have argued that the dinosaurs were already in decline because of climate change. But according to Dr Sues and his team, the discovery of Anzu offers further proof that many species were still evolving and dinosaur communities were diverse and flourishing.
"This is consistent with the idea that a mass extinction was caused by the great asteroid impact 66 million years ago. It's clear that dinosaurs were still quite diverse until the very end," says Dr Sues.
A thousand species of dinosaurs have been discovered so far. Scientists believe there are many thousands more waiting to be identified, even in heavily excavated formations such as Hell Creek.
The discovery of another species from the site was announced in December last year - a small raptor called Acheroraptor temertyorum. And scientists have only begun to explore potential dinosaur graves in Central Asia.
Bones in ground
But there's no magic formula for discovery, according to Tyler Lyson, who found one of the Anzu skeletons in 2009 on his uncle's ranch in North Dakota.
"We were just walking along when we saw some dinosaur bones poking out of the ground," he recalls.
"I knew right away that they belonged to a meat-eating dinosaur because meat-eating dinosaurs have hollow bones, and these bones were hollow. We carefully made a plaster jacket to wrap the specimen and get it back to the lab. After several hours cleaning it, we knew we had found something new. It was unlike anything else we had ever seen before."
Dr Lyson is part of the Smithsonian team and the founder of the Marmarth Research Foundation, which promotes the study of fossils. He discovered his first dinosaur bone when he was just six years old. The other two Anzu skeletons, which include a skull, were discovered by private collectors.
"You just have to spend time out there," he says. "The bones come to the surface and then break up into little pieces. You get a trail of broken bits of bone which you follow, and if you're lucky you'll see bones sticking out of the side of the hill - and that's exactly what we found here."
Although the Anzu skeletons were discovered several years ago, scientists work in "deep time", says Dr Lyson. Bones have to be catalogued and compared with other specimens while scientific evidence has to be peer-reviewed. It can take a decade before any official announcement is made.
All three Anzu skeletons are housed at the Carnegie Museum of Natural History in Pittsburgh, which collaborated with the Smithsonian to identify the new species.