Therizinosaurs weighed six tons, had a giraffe-length neck, claws like scimitars -- and feathers. Alaska once had a lot of them, said paleontologist Anthony Fiorillo.
Fiorillo, curator of earth sciences at the Perot Museum of Nature and Science in Dallas, and his colleague Thomas Adams published a paper in the June online edition of the scholarly journal Palaios identifying a single track found in Denali National Park as belonging to the odd plant-eating dinosaur related to Tyrannosaurus rex and the modern chickadee.
In an interview last week, Fiorillo said several more therizinosaur tracks were found in the park over the course of this summer.
Scientists have been baffled by this family of dinosaurs, originally thinking they were something like turtles. The first incomplete specimens came from Mongolia. More fossils were then found in China and North America. The Denali tracks are the first evidence that they were among the ancient reptiles that lived in high latitudes, sometimes called "polar dinosaurs."
The biggest Therizinosaurus is estimated to have stretched 40 feet. Even the smallest member of the family was 7 feet from the tip of its tail to its tiny head. They walked on their hind legs -- very well, as it turns out. Despite those ferocious front-limb talons, which could be a yard long, they seem to have come from a meat-eating species that turned vegetarian.
The purpose of the Edward Scissorhands-like claws is something of a mystery. They could have been used to scythe leaves or stalks, Fiorillo said. Therizinosaur is Greek for "reaping lizard."
Likewise, "The feathers are the source of a lot of speculation," Fiorillo said. Perhaps they were for display. Perhaps they helped the animal control its body temperature.
For purposes of identifying the Alaska animal as a therizinosaur, however, the important feature is its feet.
In their paper, Fiorillo and Adams document the configuration of the original Denali track, saying, "Four-toed theropod tracks are decidedly uncommon." Theropods are the suborder of dinosaurs to which therizinosaurs belonged. Only two groups have all four toes facing forward, the researchers note as they meticulously eliminate other possibilities.
In the case of therizinosaurs, all four toes bear some weight. It's a design not unlike the human foot, built for walking rather then springing, perching, swimming or other things feet do.
And walk they did. The Palaios article includes a map showing a possible therizinosaur highway from Lake Baikal in Central Asia to central Canada by way of the Bering Land Bridge. It likely took generations for the clan to travel that far.
The Denali region, on the other hand, may have been a major seasonal migration corridor, similar to those used by the birds and fish that swarm into Alaska during the summer nowadays. It was a place where different species fed and mingled; the therizinosaur tracks were found in the same layer of "bedding plains" as a number of duck-billed dinosaur tracks. It was a family place; different-size prints show adults and juveniles traveling together.
The abundant signs of various dinosaurs and prehistoric birds in Denali suggest to Fiorillo a scene not unlike the African savannah, with the Cretaceous versions of wildebeest and zebra herds eying each other across the plain.
"Alaska is the best place on the planet to study a high latitude ecosystem in deeper geologic time," he said. "We have something we can contribute to the discussion of what a warming Arctic might look like."
"There's so much (paleontological) potential in this state," Fiorillo said. "You never know what's around the corner."
The Denali footprints supply something like a photograph of what was happening in the far North 70 million years ago or more, he said. "We don't have bones. But the tracks give us a component to the biodiversity of the area that we didn't have before."
The therizinosaur isn't the first feathered dinosaur found in Alaska. That honor probably belongs to the small, wide-eyed, big-brained carnivore Troodon, fossils of which were previously found on the North Slope.