Footprints long considered to be evidence of the largest Triassic-era carnivorous dinosaur have, on closer examination, turned out to be something a little less... meaty. Nevertheless, the truth still has significance, revealing the presence of large sauropod dinosaurs in Australia long before any bone evidence found.
Queensland hosts several mines and caves with dinosaur footprints on the ceiling. No, dinosaurs "down under" didn't walk upside down. The dinosaurs made indentations on marshy ground that later filled with silt and sand. Both the swamp and filling turned to stone under the pressure of subsequent layers, but the filled-in prints proved harder than the surrounding material and survived when the rest eroded away.
“It must have been quite a sight for the first miners in the 1960s to see big bird-like footprints jutting down from the ceiling," Dr Anthony Romilio of the University of Queensland said in a statement.
When first discovered, the 220 million-year-old prints in the Rhondda colliery were attributed to the meat-eating Eubrontes family. Based on the measured track size the maker was thought to have legs more than 2 meters (7 feet) long, easily the largest Triassic carnivore ever found. “This idea caused a sensation decades ago because no other meat-eating dinosaur in the world approached that size during the Triassic period,” Romilio explained.
However, a new study in Historical Biology led by Romilio has reanalyzed the tracks and found they actually came from a rather smaller herbivorous dinosaur.
Mine closure deprives scientists of access to the footprints, so for decades paleontologists relied on drawings and photographs that didn't show the true contours. However, in 1964 plaster casts were made of the prints, which Romilio obtained. He created 3D models of the casts and emailed them to scientists worldwide.
Romilio and other experts concluded the prints were smaller than previously thought, implying the dinosaur was too. “The people who made the original conclusions were including a geological feature as part of one's heel, as well as marks where the claw dragged through sediment,” Romilio told IFLScience. Removing these, the print size dropped from 46 centimeters (18 inches) to 34 cm (13 in), with estimates of the size of the creature that made them also shrinking in proportion.
Moreover, the toes were found to be quite spread, and the middle toe only a little longer and the walk tiled inward. All of these are consistent with what we know of Triassic herbivores, but not carnivores. The authors place the trackmaker in the Evazoum family, although they cannot identify an exact species.
Indeed, the species is almost certainly one we know nothing about, since Evazoums, or any sauropods, are unknown from Australia at the time – the first quadruped sauropod fossils we have are 50 million years younger.
Romilio told IFLScience he can't say whether Australia's recently discovered sauropods, including a candidate for the world's largest, descended from those like the trackmaker, or from later arrivals from other continents. “A lot can happen in 50 million years,” he said.
Nevertheless, the discovery of a still-impressively sized sauropod will alter our view of Australia's Triassic fauna.
Asked why the assessment had been so wrong for so long Romilio told IFLScience, “It was done by a geologist, not a paleontologist, and at a time when the study of dinosaur tracks was in its infancy. The only point of comparison was North American meat-eaters.” Nevertheless, he is unsure why the conclusion was not challenged as decades passed with no hint of similarly sized Triassic carnivores.
Ironically, Romilio has led the identification of a nearby mine-roof print as actually being from an even larger carnivorous dinosaur, but this was considerably later.