ALBERTA, CANADA (NYTIMES) - Alberta is a tyrannosaur graveyard. There rest great carnivores of the Cretaceous, such as Albertosaurus, Gorgosaurus, Daspletosaurus and, of course, the Tyrannosaurus Rex (T-Rex).
Now, palaeontologists in the province have announced the discovery of Canada's oldest known tyrannosaur: Thanatotheristes degrootorum, or "the Reaper of Death".
With its razor-sharp teeth and formidable two-tonne frame, the newly discovered species terrorised the region some 79.5 million years ago.
Though smaller than T-Rex, it still measured about 9m long and about 2.4m tall. The new species was at least 2.5 million years older than its closest relatives, which may provide insight into when tyrannosaurs grew from small carnivores into the apex predators that perished 66 million years ago.
"Prior to the discovery, we knew all the most famous tyrannosaurs like T-Rex, Albertosaurus, Gorgosaurus, were all coming from the last 10 or so million years of the Cretaceous," said Francois Therrien, a palaeontologist at the Royal Tyrrell Museum of Paleontology in Drumheller, Alberta, and an author on the paper. "Now, with the new species, we've actually pushed back the record of tyrannosaurs."
The finding was published in January in the journal Cretaceous Research, based on bones discovered by a pair of paleo-enthusiasts.
In 2008, Sandra and John De Groot were walking in southern Alberta along the shore where Oldman River meets the Bow River. They spotted something in the ice.
"We looked down and I kind of said jokingly 'Hey it looks like a dinosaur jaw!'" Mr John De Groot said. As he bent over to grab it, his heart started pounding. "We looked a little closer and it certainly was."
The couple, who own a farm in nearby Hays, had previously collected ammonites and tiny bones while hiking through the short-grass prairie. But never before had they made such an exciting discovery.
"It was just kind of this 'Wow' moment of 'Holy cow! You actually did find some teeth laying here on the ground,'" said Mrs Sandra De Groot. They recovered three large brown chunks of a dinosaur jaw.
Two years later, Mr Donald Henderson, a palaeontologist at the museum, gave a talk at the school where Mrs De Groot is a substitute teacher.
"I said 'Oh hey, we have a jaw at our house, you should come see it,'" she said. "And he said 'What?! "
The De Groots donated the specimen to the museum and went with a team of palaeontologists to the riverbank and uncovered more skull pieces. The scientists knew it was a tyrannosaur, but not what kind.
For nearly a decade, the bones sat in a museum drawer until Mr Jared Voris, a graduate student at the University of Calgary, began examining them. The long and deep snout was similar to Daspletosaurus, another tyrannosaur group, suggesting the two were closely related.
He also noticed interesting vertical ridges that lined the dinosaur's upper jaw, in addition to a battle scar on its face.
"The ridges were things that we had not seen before in another tyrannosaur, especially not another tyrannosaur from Alberta," he said.
Mr Voris also observed other features that set the tyrannosaur apart from its relatives, such as its oval-shaped cheekbone and aspects of its skull.
Those differences, along with the tyrannosaur's old age, led the team to classify it as a new species, for which they honoured the De Groots, and also as a new genus, named basically for the Grim Reaper.
Ms Lindsay Zanno, a palaeontologist from North Carolina State University who was not involved in the study, said the pronounced ridges were "intriguing".
"Despite its scrappy remains, it is an important new animal that borders a vexing gap in the tyrannosaur fossil record," Ms Zanno said.
But some palaeontologists took issue with calling it a distinct genus.
Mr Thomas Carr, a palaeontologist at Carthage College in Wisconsin, agreed the specimen had enough unique features to call it a new species, but he would have preferred to name it "Daspletosaurus degrootorum".
"A new genus name strikes me as unnecessary, given that the fossils are nearly identical to its closest relative, Daspletosaurus," he said.
One of the paper's co-authors, Ms Darla Zelenitsky, a palaeontologist at the University of Calgary, defended the classification, saying the ridges on the tyrannosaur's upper jaws and its distinct cheekbones warranted the new name.
"Because of all the differences we found, it is very difficult for us to justify this animal as a species of Daspletosaurus," she said.
As the team uncovers more specimens, she added, they will likely find more differences between Thanatotheristes degrootorum and the other tyrannosaurs.
Enough, perhaps, to forever etch the reaper's name on its tombstone.