An artist's conception of how a swimming dinosaur may have left tracks behind. (Nathan Rogers / University of Alberta) A series of strange, 100-million-year-old footprints suggest that at least some dinosaurs were decent swimmers, a University of Alberta paleontologist has found.
The unusual tracks consist of eight groupings of scratch marks imprinted over 15 metres in an ancient riverbed in China's Szechuan province. The prints are the right size and distance apart to belong to a two-legged, wolf-sized carnivorous dinosaur similar to tyrannousaurus rex.
But they don't look anything like the usual tracks for that kind of dinosaur, said Scott Persons, the University of Alberta Ph.D. student who was part of the international team that analyzed them.
"Usually they leave behind a beautiful imprint, where you can see all of the different pads of the toes, et cetera," he said in an interview Tuesday.
"These aren't like that. Instead, they seem to preserve just the very, very tippy toes of a dinosaur just scratching the very bottom of this riverbed."
The scratch marks are similar to those left behind by ducks as they're swimming in shallow water, Persons said. And similar dinosaur tracks at other sites have been interpreted as swimming tracks as well.
However, this is the first time such a long series of swimming tracks has been discovered.
"It tells us the dinosaur was certainly comfortable in the water, that it does not appear to have any reservations about going across the river," Person said.
He noted that ripples left in the riverbed suggest there was such a strong current, and the water is thought to have been quite deep.
'Dinosaur superhighway'
The swimming tracks are among dozens of dinosaur tracks from at least 20 individuals of several different species imprinted in the riverbed, which was wet at some times of the year and dry during other periods.
They were likely left around 100 million years ago over a period of a year, Persons said. He suggested that the environment at the time may have been similar to the savannahs of Africa, with wet and dry seasons that forced animals to cope by migrating annually.
The tracks left by the front three toes of the dinosaur were among dozens of footprints from multiple dinosaur species at asite in China's Szechuan province. (Scott Persons / CBC) "It's not unusual for animals to use old riverbeds as a migration route," Persons added, describing this particular riverbed as a kind of "dinosaur superhighway."
The tracks include walking tracks for carnivorous dinosaurs that appear to belong to the same species as those that left the swimming tracks.
Typically, such "theropod" dinosaurs have three toes pointing forward, like birds, and leave very distinctive footprints.
Because they were meat eaters, they had much sharper claws than plant-eating dinosaurs, which are obvious in the swimming tracks. Based on the size of both the swimming and walking tracks, Persons estimates that the dinosaur was about three metres long, including its tail.
He thinks it would have paddled in the water with its hind feet, like a duck, using its tail to steer, as a crocodile does. Like birds, he said, dinosaurs would have had lots of air sacs that would help them afloat.
All the tracks are described in a paper published this week in the journal China Science Bulletin.
In addition to the theropod tracks, there seem to be footprints from giant, four-legged, long-necked plant-eating dinosaurs called sauropods, some armoured dinosaurs, and a group of eight smaller plant eaters called ornithopods. The ornithopods were all headed in the same direction, Persons said, suggesting that they may have been moving as a herd.
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