Were any dinosaurs aquatic?

No dinosaur is known to have been primarily aquatic. However, the idea has been raised a few times before. This post will look at three of the main ones: the sauropods, the hadrosaurs and Compsognathus corallestris.

When Sauropods were first discovered in the early 19th century, palaeontologists were puzzled about the lifestyle of the animals. They assumed that due to their large size, sauropods could not possibly have lived on land and must have lived in water. Indeed, this is even reflected in some of their names such as Cetiosaurus (Whale Lizard) (Owen, 1841). One of the first people to suggest sauropods might have been terrestrial was Elmer Riggs, who noted that sauropods had large hollows in their vertebrae, a feature that no aquatic animal but plenty of terrestrial animals possess (Riggs, 1904). In fact, these hollows had been discovered earlier by Harry Seeley in the sauropod Ornithopsis (Seeley, 1870), although this led him to believe the animal to be a flying pterosaur. In fact, these hollows had been interpreted by Othniel Marsh as weight-reduction features (Marsh, 1877) and by Edward Cope as floats (the dinosaur equivalent of armbands) (Cope, 1878). Furthermore, Riggs found that the limbs and feet of sauropods are ill-suited to an aquatic way of life but were perfect for supporting a large amount of weight. However, Riggs' ideas went unnoticed and the view of sauropods being aquatic prevailed. But in 1951, Kenneth Kermack argued that a submerged sauropod's ribcage would be under immense pressure - pressure it was not designed to handle (Kermack, 1951). And so this became the proof that convinced the scientific community that sauropods were fully terrestrial. End of story, roll the credits, fat ladies singing everywhere. Or so we thought. An interesting wrench was thrown into the works in 2004 when Donald Henderson investigated how well sauropods could float. What he discovered was quite startling - sauropods, even the biggest ones, were less dense than modern alligators. In other words, they could float better than modern aquatic reptiles! (Henderson, 2004) Riggs' evidence that sauropod limbs were better for terrestrial locomotion and not for swimming still stands, however.

Next are the hadrosaurs, better known as the duck-billed dinosaurs. The first to argue for this lifestyle was Joseph Leidy who suggested that Hadrosaurus was fully aquatic - despite also saying that Hadrosaurus walked upright on two legs (Leidy, 1858). Odd, considering all known aquatic animals are quadrupeds. Our good friend Edward Cope also believed the hadrosaurs were aquatic, based on the duck-like bill and the "weak" teeth. Cope supposed that the only food they could possibly eat would be soft water plants (Cope, 1883). However, this argument was refuted by Richard Lull and Nelda Wright who showed that the teeth were not weak but arranged in a dental battery and were continuously replaced. In this way, hadrosaurs had jaws that worked like a grinder, enabling them to break down tough plant material. However, Lull and Wright still believed hadrosaurs were aquatic (Lull and Wright, 1942). The idea of hadrosaurs being aquatic was finally put to rest in 1964 when John Ostrom showed that hadrosaurs were fully terrestrial based on gut contents (some of which had been known since 1922 but Lull and Wright thought were anomalous) and the presence of ossified tendons (tendons containing bone) in their vertebrae. This gave hadrosaurs a stiff but well-supported spine perfect for a large terrestrial reptile but very much a hindrance for an aquatic one (Ostrom, 1964).

(Words cannot describe how inaccurate this image is)

In 1972, a fossil of a small theropod was described from France. Named as Compsognathus corallestris, it made headlines because it was announced as the first fully aquatic dinosaur discovered, based on the imprints of flippers found on the hands (Bidar et al., 1972). But then, things went wrong. Our favourite debunker John Ostrom had a look at the fossil and couldn't work out how Bidar and friends thought it had flippers because there was nothing to suggest there was any webbing of any kind on the forelimbs (Ostrom, 1978). Not only that, but it was later shown to be the same as the only other species in the genus, Compsognathus longipes (Wagner, 1861, Peyer, 2006).

Maybe one day we will find evidence of fully aquatic dinosaurs. But until then, it is just speculation.

See also:
More dinosaurs
More about Richard Owen who named Cetiosaurus
More about Edward Drinker Cope who described the sauropods' floats

References
Bidar, A., Demay, L. and Thomel, G. (1972) 'Compsognathus corallestris, une nouvelle espèce de dinosaurien théropode du Portlandien de Canjuers (Sud-Est de la France)', Annales du Muséum d'Histoire Naturelle de Nice, 1, pp. 9-40

Cope, E. (1878) 'A new species of Amphicoelias', American Naturalist, 12, pp. 563-565

Cope, E. (1883) 'On the characters of the skull in the Hadrosauridae', Proceedings of the National Academy of Sciences of Philadelphia, 35, pp. 97-107

Henderson, D. (2004) 'Tipsy punters: sauropod dinosaur pneumaticity, buoyancy and aquatic habits', Proceedings of the Royal Society of London B, 271, S180-S183

Kermack, K. (1951) 'A note on the habits of sauropods' Annals and Magazine of Natural History, 4, pp. 830-832

Leidy, J. (1858) 'Hadrosaurus foulkii, a new saurian from the Cretaceous of New Jersey', Proceedings of the Academy of Natural Sciences of Philadelphia, 1858, pp. 215-218

Lull, R. and Wright, N. (1942) 'Hadrosaurian Dinosaurs of North America', Geological Society of America Special Paper, 40, pp. 1-272

Marsh, O. (1877) 'Notice of new gigantic dinosaur', American Journal of Science, 14, pp. 87-88

Ostrom, J. (1964) 'A reconsideration of the paleoecology of the hadrosaurian dinosaurs', American Journal of Science, 262 (8), pp. 975-997

Ostrom, J. (1978) 'The osteology of Compsognathus longipes', Zitteliana, 4, pp. 73-114

Owen, R. (1841) 'A description of a portion of the skeleton of the Cetiosaurus, a gigantic extinct saurian reptile occurring in the oolitic formations of different portions of England', Proceedings of the Geological Society of London, 3, pp. 457-462

Peyer, K. (2006) 'A reconsideration of Compsognathus from the Upper Tithonian of Canjuers, Southeastern France', Journal of Vertebrate Paleontology, 26 (4), pp. 879-896

Riggs, E. (1904) 'Structure and relationships of opisthocoelian dinosaurs. II'', Publication Field Columbian Museum Geological Series, 2, pp. 229-248

Seeley, H. (1870) 'On Ornithopsis, a gigantic animal of the pterodactyle kind from the Wealden, Annals and Magazine of Natural History, 5, pp. 279-283

Wagner, A. (1861) 'Neue Beiträge zur Kenntnis der urweltlichen Fauna des lithographischen Schiefers; V. Compsognathus longipes Wagner', Abhandlungen der Bayerischen Akademie der Wissenschaften, 9, pp. 30-38