First dog-sized dinosaurs, and now a cat-sized stegosaur?
An international team of paleontologists has discovered a single footprint of a tiny stegosaur, left 100 million years ago in what is now the Xinjiang Province in northern China.
“This footprint was made by a herbivorous, armoured dinosaur known broadly as a stegosaur – the family of dinosaurs that includes the famed stegosaurus,” says Anthony Romilio, a palaeontologist from the University of Queensland who was part of the research team.
“Like the stegosaurus, this little dinosaur probably had spikes on its tail and bony plates along its back as an adult.”
The footprint is the smallest known stegosaur print in the world, measuring just under 6 cm in length. The dinosaur it belonged to may have only been 60 cm long; it’s thought to be a juvenile.
Paleontologists have found 10 other ancient track sites in the area, with the first discovered in 2011, showing the footprints of ancient birds and two types of carnivorous dinosaurs.
“One type was about the size of large dogs, albeit stretched to just over two metres long, and the other was a pigeon-sized meat-eater,” Romilio says.
But this new track site is dominated by the footprints of plant-eating dinosaurs (Deltapodus curriei), mostly adults with prints around 30 cm long. There was just one smaller print of a juvenile among the crowded chaos of tracks.
The tiny cat-sized stegosaur footprint shares characteristics with the larger footprints, with three short, wide, round toes – but it wasn’t elongated like others, suggesting that young stegosaurs may have walked differently.
“Stegosaurs typically walked with their heels on the ground, much like humans do, but on all fours which creates long footprints,” says Romilio. “The tiny track shows that this dinosaur had been moving with its heel lifted off the ground, much like a bird or cat does today. We’ve only previously seen shortened tracks like this when dinosaurs walked on two legs.”
The researchers suggest that juveniles may have walked on their toes and gradually transitioned to heel-walking as they grew older.
The footprints (a type of trace fossil) have been given the name Deltapodus curriei; the researchers think that these were created by the species Wuerhasaurus homheni, which is the only stegosaur found in the area and is known from bones (body fossils).
The tracks were made on a muddy surface by the shores of an ancient lake. These impressions were later covered by sand-laden water.
“After eons, the mud turned to mudstone, and the sand became sandstone,” Romilio explains. “The mudstone is much softer than the sandstone and so the vast majority of fossil tracks were found as sandstone infilled casts.”
Other famous dinosaur tracks include those found near Broome in Western Australia, where stegosaur footprints have been found measuring up to 80 cm long.
The study, led by Lida Xing from the China University of Geosciences in Beijing, is published in the journal Palaios.