Paleontologists at the University of Alberta have found evidence inside the stomachs of two fossilized dinosaurs of the species Sinocalliopteryx gigas that suggests the raptor-like predator did not shy away from feeding on its own kind, hunting small flying dinosaurs for food.

The Sinocalliopteryx gigas holotype, the specimen used to name a particular species, in this case a dinosaur found in the Yixian Formation in northeastern China. (PLoS One)

The researchers found the fossilized remains of two undigested dinosaurs of the species Confuciusornis sanctus and a third herbivorous, bird-like dinosaur inside the stomach of a Sinocalliopteryx, a feathered, non-flying carnivorous dinosaur about the size of a wolf that could reach lengths of up to 2.3 metres.

Confuciusornis were some of the earliest examples of birds, with skeletons and muscles that crudely resembled those of modern-day birds.

"The fact that this Sinocalliopteryx had not one but three undigested birds in its stomach indicates it was a voracious eater and a very active hunter," said University of Alberta paleontology student Scott Persons in a press release.

'This is the first direct evidence of a raptor becoming another predatory dinosaur's meal.'— Scott Persons, University of Alberta paleontologist

Persons is a co-author of a paper on the find published this week in the open-access journal PLoS One.

The abdominal cavity of a second Sinocalliopteryx was found to contain a partial leg of a bird-like dinosaur called Sinornithosaurus that the researchers estimate was roughly one-metre long in total.

"Sinornithosaurus is a relative of Velociraptor, which means this is the first direct evidence of a raptor becoming another predatory dinosaur's meal," Persons said in the release.

The researchers write in their paper that although they can't say definitively whether the metre-long bird ended up in the Sinocalliopteryx's stomach as a result of being actively hunted or passively scavenged, "if the Sinornithosaurus was predated upon (rather than scavenged), this would imply Sinocalliopteryx was capable of tackling carnivorous prey more than a third its own size."

High metabolism, feather-like fuzz

Sinocalliopteryx dinosaurs lived in the Early Cretaceous period (which roughly spans from 145 million to 100 million years ago) and were first described by a group of paleontologists from the Chinese Academy of Geological Sciences in 2007. One of the Sinocalliopteryx specimens whose stomach contents the Alberta researchers examined was the one originally described by the Chinese scientists.

A close-up of the stomach contents of a Sinocalliopteryx specimen. (PLoS One)

The Sinocalliopteryx did not have wings or the physical characteristics to be able to climb trees, but they were covered with a feather-like fuzz that kept their body temperature warm and their metabolism high, meaning they required a lot of food to survive and had to be stealthy hunters.

The researchers suggest in their paper that evidence of the Sinocalliopteryx's high metabolic rate can be seen in the fossilized stomach contents they examined.

The delicate bones of small bird-like dinosaurs like the Confuciusornis don't take long to digest so the fact that the stomach of one dinosaur contained the bones of three such creatures — all undigested — suggests the birds were consumed "in fairly rapid succession," the researchers write.

Likely hunted, not scavenged

Part of a right hind limb of a Sinornithosaurus within the abdominal cavity of a Sinocalliopteryx. (PLoS One)

The rapid consumption of the birds also suggests they were actively hunted rather than simply scavenged, the researchers say.

The skeletons of the two dinosaurs were found in Liaoning province in northeastern China and analyzed with the co-operation of researchers from Alberta's Pipestone Creek Dinosaur Initiative and the Chinese Academy of Geological Sciences.

The region where the fossils were found, called the Lower Cretaceous Yixian Formation, is known for having some of the best-preserved dinosaur remains in the world owing to its rich volcanic sedimentary deposits.