September 24, 2012
Paleontologist R.T. Bird inspected many dinosaur trackways while combing Texas for the perfect set to bring back to the American Museum of Natural History. During several field seasons in the late 1930s, Bird poked around in the Early Cretaceous rock in the vicinity of the Paluxy River for a set of sauropod footprints that would fit nicely behind the museum’s famous “Brontosaurus” mount. Bird eventually got what he was after but not before poring over other intriguing dinosaur traces. One of the most spectacular seemed to be made by a swimming dinosaur.
Known as the Mayan Ranch Trackway, the roughly 113-million-year-old slab is almost entirely made up of front foot impressions. The semicircular imprints were undoubtedly left by one of the long-necked sauropod dinosaurs. But towards the end of the trail, where the dinosaur’s path makes an abrupt turn, there was a single, partial impression of a hind foot.
At the time Bird and his crew uncovered this trackway, sauropods were thought to be amphibious dinosaurs. Other than their immense bulk, what defense would they have had but to trundle into the water, where theropods feared to paddle? Under this framework, Bird thought he knew exactly how the Mayan Ranch Trackway was made. “The big fellow had been peacefully dog-paddling along, with his great body afloat, kicking himself forward by walking on the bottom here in the shallows with his front feet,” Bird wrote in his memoir. The great dinosaur then kicked off with one of its hind feet and turned.
With the exception of well-defended dinosaurs such as the ceratopsids and stegosaurs, many herbivorous dinosaurs were thought to be at least semi-aquatic. There seemed to be only two options for Mesozoic prey species–grow defenses or dive into the water. In time, though, paleontologists realized that the sauropods, hadrosaurs and other herbivores didn’t show any adaptations to swimming. Our understanding of the ecology of these dinosaurs was based on false premises and faulty evidence.
In the case of the Mayan Ranch Trackway, for example, there’s no indication that the sauropod that made the trackway was swimming. A more likely scenario has to do with evolutionary changes among sauropods. While the sauropods that dominated the Late Jurassic of North America–such as Diplodocus, Apatosaurus and Barosaurus–carried much of their weight at the hips and left deeper hindfoot impressions, the center of mass shifted among their successors–the titanosaurs–such that more of the weight was carried by the forelimbs. Hence, in some trackways, the deeper impressions made by the forefeet are more likely to stand out than those made by the hindfeet, especially if some of the top layers of the rock are eroded away to leave only “undertracks.” What seemed to be evidence of swimming sauropods instead owes to anatomy and the characteristics of the mucky substrate the dinosaur was walking on.
As far as I’m aware, no one has yet found definitive evidence of swimming sauropods or hadrosaurs–the two groups previously thought to rely on water for safety. Stranger still, paleontologists have recently uncovered good evidence that theropod dinosaurs weren’t as bothered by water as traditionally believed. In 2006, paleontologists Andrew Milner, Martin Lockley and Jim Kirkland described swim tracks made by Early Jurassic theropods at a site that now resides in St. George, Utah. Such traces weren’t the first of their kind ever discovered, but the tracksite was one of the richest ever found.
Small to medium-sized theropods made the St. George swim tracks–think of dinosaurs similar to Megapnosaurus and Dilophosaurus. Even better, the large number of smaller-size swim tracks hints that whatever dinosaurs made these tracks were moving as a group as they struggled against the current in the lake shallows. The larger dinosaurs, on the other hand, were a bit taller and able to wade where their smaller cousins splashed around.
A different team of researchers announced additional evidence for swimming theropods the following year. Paleontologist Rubén Ezquerra and co-authors described dinosaur swim traces from Early Cretaceous rock near La Rioja, Spain. Based on the details of the track and their direction, the theropod was swimming against a current that pushed the dinosaur diagonally. Along with other theropod swim tracks, the researchers noted, the discovery meant that paleontologists would have to revise their ideas about the kind of habitats theropods lived in and what carnivorous species would do. Theropod dinosaurs were not so hydrophobic, after all.
Does this mean that dinosaurs like Dilophosaurus were adapted to an amphibious lifestyle? Not at all. As Ezquerra and co-authors pointed out, the swimming strokes of these dinosaurs were exaggerated walking motions. The way the dinosaurs moved on land allowed them to be adequate swimmers while crossing rivers or lakes, but, compared with semi-aquatic animals such as crocodiles and otters, no known dinosaur shows traits indicative of a primarily waterlogged existence. (And dinosaurs found in marine sediments don’t count as evidence, as these were washed out to sea prior to burial. I can’t imagine ankylosaurs taking to life among the high seas, in any case.) Some dinosaurs could swim, but that doesn’t mean that they made the water their home. Still, thanks to special prehistoric traces, we can imagine packs of Megapnosaurus fighting to get ashore, and Dilophosaurus strutting into the shallows, aiming to snatch whatever fish were foolish enough to swim into the carnivore’s shadow.
Bird, R.T. (1985). Bones for Barnum Brown, edited by Schreiber, V. Forth Worth: Texas Christian University Press. pp. 160-161
Ezquerra, R., Doublet, S., Costeur, L., Galton, P., Pérez-Lorente, F. (2007). Were non-avian theropod dinosaurs able to swim? Supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain) Geology, 40 (10), 507-510 DOI: 10.1130/G23452A.1
Milner, A., Lockley, M., Kirkland, J. (2006). A large collection of well-preserved theropod dinosaur swim tracks from the Lower Jurassic Moenave Formation, St. George, Utah. New Mexico Museum of Natural History and Science Bulletin, 37, 315-328