Dinosaur tracks in northern China, once hailed as evidence of large dinosaurs swimming, have been reinterpreted.
Bones and teeth can only tell us so much about extinct animals' behaviors. Sometimes, however, we get lucky with marks indicating how some long-gone species walked or climbed. Dinosaur trackways in particular have often proved exceptionally revealing.
In Gansu Province, China, a 120-million-year-old set of sauropod tracks have inspired controversy. The tracks come from an unknown sauropod species that was roughly 1.35 meters (4.4 feet) high at the hip – no Brontosaurus or Argentinosaurus, but a possible guide to the behavior of their larger relatives. Even at this modest size, these beasts were too big to have been walking on their hind feet, which makes it puzzling that only the hind prints have been preserved. One interpretation is that the great beasts were taking a swim.
Lida Xing, a Ph.D. student at China University of Geosciences, has rejected this theory. "Nobody would say these huge dinosaurs could stagger along on their hind legs alone – they would fall over," Xing said in a statement. "However, we can prove they were walking because the prints are the same as in more usual tracks consisting of all four feet, it's just that here, we don't see the hand prints. If they had been swimming, with the hind legs dangling down, some of the foot prints would be scratch marks, as the foot scrabbled backwards."
Photo and outline drawing of the best preserved of the footprints. Xing et al, Scientific Reports
Instead, Xing proposed in Scientific Reports that when the tracks were made, the ground had a firm top layer with softer material beneath. The sauropod's hind feet, carrying most of the weight, broke through into the sand, while the front feet left marks too shallow to be preserved.
Evidence for this theory comes from the way the claws penetrated more deeply than the hind feet, unlike sauropod tracks found elsewhere. Similarly, footprints in wet sand are deeper at the toes than the heel.
A comparison of one of the dinosaur prints, with color indicating depth. The footprint was made by a woman walking in wet sand. Xing et al, Scientific Reports
From the distance between prints, the authors calculate the maker's speed at 2.7 to 4.2 kilometers per hour (1.7 to 2.6 miles per hour), nearly typical of tracks from similarly sized sauropods elsewhere, while swimmers would be much slower.
Similar debates have occurred elsewhere. Lark Quarry in Queensland was hailed as preserving a dinosaur stampede, but more recent research suggests that many of the dinosaurs involved were swimming, while others were either wading or in water shallow enough to run.
The dinosaurs at Lark Quarry were not sauropods, however, and the paper argues that “none of these incomplete trackways has been entirely convincing.”
“Presently, there is no convincing evidence of swimming sauropods from their trackways,” the paper concludes, before adding, “Which is not to say that sauropods did not swim at all.” The authors observe that elephants have been known to swim for large distances. Seldom, however, do they leave traces for scientists to find when they do.