Scientists have discovered signs of unusual muscles on the arms of a new species of dinosaur that once fished in the swamps and bayous of what is now southern Alberta.

The dinosaur, named Apatoraptor pennatus, didn't use those muscles for brawling or lifting heavy weights, but for moving its feathers, University of Alberta paleontologists report in a new paper published in the Journal of Vertebrate Paleontology.

Apatoraptor was a bird-like dinosaur that walked on two legs and had a long neck and a long tail. It was related to other dinosaurs that were known to have feathers all over their bodies, including long tail feathers and head crests.

It was already known that the dinosaurs had feathers on their arms and most of their bodies. "But what we didn't know was that they actually could move those feathers," said Greg Funston, the lead author of the study.

What wasn't bird-like was the dinosaur's size — it was about two metres long and 180 kilograms, making it quite a bit larger than a human.

"'Cause these animals are way too big to fly, they're probably using their feathers for display," added Funston, who studied the dinosaur as part of his PhD.

Those displays would likely have been used to impress potential mates.

Funston said Apatoraptor lived about 70 million years ago — four million years before dinosaurs went extinct — in a swampy, coastal environment that would have looked something like the Mississippi River Delta in Louisiana.

The area was home to a variety of dinosaurs, including duck-billed dinosaurs like Edmontosaurs, tyrannosaurs, horned dinosaurs like triceratops, and relatives of velociraptor.

Apatoraptor likely would have waded through the water like a heron, catching small prey with its beak and fingers. It may have also grazed on aquatic plants.

The Apatoraptor's skeleton, which was very complete and almost fully assembled or "articulated" when it was found, was collected near Drumheller in 1993. At that time, it was mistaken for a common dinosaur called an ornithomimid.

By the time Funston started studying the skeleton, it had been re-identified as another type of dinosaur called Epichirostenotes — a dinosaur he had already studied and was familiar with. But this specimen didn't seem to match up.

"Some of the proportions of the finger bones were off and some shapes of the finger bones were different." There were also some features of its lower jaw that "really just didn't look right."

Funston realized it was a new species. Further studies showed that it was related to Epichirostenotes, but its closest known relatives actually lived in Mongolia.

He discovered the telltale signs of its unusual arm muscles after CT scanning the dinosaur's arm bones. They had tiny scars like those left by the feather-moving muscles of birds.

Funston named the new dinosaur for its mistaken identity and its feathers — Apato means "deceptive" and pennatus means "winged."