Old views on dull-witted semi-aquatic dinosaurs correct after all…

A new study suggests dinosaurs were aquatic... too heavy to move on land.

For more than half a century, dinosaurs were depicted as slow-moving creatures living in swampy environments. Giants such as the 35 tons Brachiosaurus were deemed too heavy on land to be able to support their own weight and the prominent view for most of the 20th century was that they were amphibious animals that spent most of their time half-submerged in water, grazing on soft aquatic plants. Dinosaurs were also thought to be incredibly stupid in view of their very small braincase. For instance, the plated Stegosaurus had a brain the size of a walnut which is indeed minuscule for an animal reaching 9 m in length. This view on dinosaurs drastically changed starting in the 1960s (the so-called “dinosaur renaissance”) when new discoveries such as the numerous fossils of the duck-billed dinosaur Maiasaura found at “Egg Mountain” in Montana, and encompassing individuals of all ages from hatchlings to adults, purportedly showed that dinosaurs raised their youngs and thus had intelligence matching those of modern mammals and birds. Today, dinosaurs are ubiquitously portrayed as highly active fully terrestrial creatures capable of a wide range of social behavior such as pack hunting and parental care.  

The old view of dinosaurs as dull-witted amphibious beasts has been totally abandoned and these animals are now often seen as one of the prime evolutionary successes rather than as a failed nature experiment doomed for extinction. However, in recent years, scattered discoveries indicated that at least some dinosaurs were aquatic: in the 1970s, a new specimen of the little theropod Compsognathus  found in southern France was described as having webbed feet indicative of a semi-aquatic lifestyle.  In 2010, the analysis of the oxygen isotope ratios in Spinosaurus teeth showed that it was close to those of crocodiles and aquatic turtles, proving that the giant sailed theropod spent lots of time in water. In 2011, a new ceratopsian dinosaur, Koreaceratops, was described as having a tail adapted for swimming. Now, a new study published in the April 2014 issue of the Journal of the Australian Society of Vertebrate Palaeontology, shows that the aquatic lifestyle of dinosaurs was actually widespread. The study applied the same isotopic analysis technique used for Spinosaurus, on the teeth of some 20 species of sauropods (the long-necked, long-tailed giants such as Diplodocus and Brachiosaurus) and ornithopods (including the duck-billed dinosaurs such as Parasaurolophus and  Edmontosaurus) from the Late Jurassic and Late Cretaceous of North America. The results show for all teeth a level of oxygen isotopes compatible with life in water. “This technique has until now only been applied to Spinosaurus because a fish-eating diet was long suspected for this theropod due to its crocodile-like snout. Nobody thought to check on dinosaurs such as Brachiosaurus or Edmontosaurus as the general consensus was that these were fully terrestrial” says lead author Dr. Avril Zierste from the Palaeontology department of the University of Sao Paulo. “The results came quite as a shock because the current views on these ancient creatures will have to take a 180 degree turn”, she adds. Many observed dinosaurian features that were left unexplained as land animals, make perfect sense if they were water dwelling gentle giants. For instance, the flattened tail of duck-billed dinosaurs rigidified by ossified tendons, were superbly adapted for swimming. Moreover the often complex nasal apparatus of many of these same duck-billed dinosaurs were probably used as snorkels to breathe while underwater, as initially hypothesized well before the “dinosaur renaissance". Sauropod bones are well known to show high degree of pneumaticity, which are seen as evidence of the presence of air sacs. This purpose of this becomes clear if these animals were aquatic as air sacs will help with buoyancy and act as floating devices. It is probably not a coincidence that fossils of dinosaurs around the world were predominantly found in regions that were very close to water and often among remains of marine or freshwater animals such as fish and crocodiles.

Old depiction of Brachiosaurus depicted as aquatic animals by Czech painter Z. Burian (1905-1981). New study suggests early paleontologists were right from the beginning about the lifestyle of dinosaurs.

Is it possible that the current view of terrestrial dinosaurs with high metabolism portrayed after  the “dinosaur renaissance” was more based on wishful thinking than hard evidences? The eminent scholar Josh Mothorn certainly thinks so: “We were so keen to believe that no group of animals could have dominated the earth for 165 millions years without sharing at least some of the qualities of our kind, the mammals, such as warm-bloodedness and intelligence, that any circumstances tending to indicate that they were something else than slow moving idiotic monsters, were branded as evidence of complex behavioral habit. The fact is that movies such as “Jurassic Park” would never have been successful if the heroes were confronted not by incredibly fast and skillful killing machines but by clumsy sluggish creatures that could be easily fooled and outsmarted”.  “The dinosaur renaissance is dead” concludes Art Kerbebrok after hearing the conclusions of the study “it is high time that we stop imagining them as nothing more than stupid giants too heavy to support their own weight. People were quick to dismiss the obvious: their brain to body size ratios were among the smallest in any tetrapods. Dinosaurs were slow, gigantic and idiotic. Mammals have outwitted them, that’s why dinosaurs are extinct”.  In light of the new study by Dr. Zierste and co-workers, it seems that the old prevalent view of dinosaur as slow-witted and placid semi-aquatic giants was correct after all.

Reference:

Avril Z., Peter S. G., Robert T. B., Ines L. P., Lambert J. B., Stewart M.T., Fernando E. N., Oviedo T. F., Oscar J., Lewis C. C. (April 1, 2014) “Oxygen isotope analysis and brain-body ratio measurements of 20 species of Sauropoda and Ornithopoda as evidence for semi-aquatic lifestyle and low I.Q. for the clade Dinosauria” J. of Australian Society of Vertebrate Palaeontology, 90., 1-12.