Was Spinosaurus a Swimmer?  Watery Habitat for Largest Theropod

Scientists have found evidence of flying dinosaurs, there is evidence to suggest that some dinosaurs were at home in the trees, some lived in burrows, but a dinosaur that lived an aquatic existence?  That surely must be a step too far for a group of reptiles whose suitability for life on land and simple hinge-like ankle joints gave them a significant advantage over other land animals in the post Permian extinction race for dominance of the Earth.

Swimming Spinosaurs

However, a team of French scientists have concluded that at least one type of dinosaur – the spinosaurids may have spent a considerable amount of time in water, in fact about as much time in the water as extant species of crocodiles do today.

Romain Amoit from the University of Lyon (France) and his colleagues studied the amount of an isotope of oxygen within the fossilised teeth of a number of dinosaurs.  They then compared their findings to known aquatic animals from the fossil record, turtles and crocodiles and from these results they have concluded that the spinosaurs probably spent most of their time in a watery environment.

The spinosaurids, most famously Spinosaurus, the dinosaur villain in the movie Jurassic Park III were a widespread Cretaceous theropod dinosaur.  Remains of such creatures have been discovered in England (Baryonyx), Africa, South America, China, Thailand and Japan.  These dinosaurs were characterised by having long, narrow jaws armed with many sharp, straight teeth, in most cases resembling the teeth of crocodiles.  Most of this group of large, carnivores had humps or elaborate sails along their backs.

Spinosaurus aegyptiacus, was named and described by the great German palaeontologist/geologist Ernst Stromer von Reichenbach in 1915.  He had led a number of fossil hunting expeditions to North Africa between 1911 and the outbreak of World War I.  Unfortunately, very little spinosaur fossil material or information is left from these expeditions as the Munich museum in which these items were stored was utterly destroyed by an American bombing raid during World War II.

However, based on the notes and illustrations from Stromer’s time plus more recent Spinosaurus remains from Morocco, some scientists have claimed that this type of spinosaur is the largest known predatory dinosaur in the fossil record with some size estimates putting Spinosaurus at more than 17 metres long, much larger than the biggest Tyrannosaurus rex, or Giganotosaurus.

An Illustration of a Spinosaurus

The “classic” Papo Spinosaurus dinosaur model.

Picture credit: Everything Dinosaur/Papo

The picture shows a typical model of a Spinosaurus (2010), the distinctive big claw on the thumb can clearly be seen.  Evidence of these types of creatures being fish eaters has been found before, for example, the remains of fish scales (Lepidotes) found in association with the Surrey Baryonyx skeleton, but the French team go further, they claim that as well as eating fish, these animals spent much of their time in the water.

To view the Papo Spinosaurus model and other replica figures in the Papo “Les Dinosaures” range: Papo Dinosaur and Prehistoric Animal Models.

In a report published in the scientific journal “Nature News”, the French team provide evidence that spinosaurs avoided competition with the other large, carnivorous dinosaurs that shared their environment, by specialising as fish-eaters and by being very much at home in their watery habitats.  Such areas would have been avoided by the other types of predatory dinosaurs, the allosauroids for example, after all, there is plenty of evidence to suggest such large and heavy creatures were prone to becoming stuck in soft mud and sand in pursuit of a herbivore that had become mired.

Unable to extricate themselves from what would turn out to be their muddy grave (predator traps) many fossils of allosaurs have been found under these circumstances.

Amiot and his team took a look at oxygen isotopes locked away inside the preserved enamel of the spinosaur’s teeth and compared them to the oxygen isotopes found in the teeth of crocodiles and other dinosaurs, and turtle-shell fragments from the same geological period – the Cretaceous.

It is known that animals that spend a lot of time in a dry environment lose water through breathing and through evaporation from skin.  As one type of oxygen isotope; oxygen-16 is lighter than another isotope – oxygen-18, it is more readily released with water vapour. As a result, oxygen-18 becomes more concentrated in tissues and when tooth enamel is formed.  Analysing the types of oxygen isotopes preserved in the fossil fragments could provide an indication as to how close to water, or at least how close to a humid, watery environment did extinct animals live.

The researchers reasoned that if spinosaurs were aquatic, then the concentration of oxygen-18 in their tissues would closely match aquatic animals such as crocodiles and turtles and be lower than the isotope values of other dinosaurs, as the other dinosaurs would be presumed to be more terrestrial in their habits.  Sort of “land lubbers” to the spinosaurs being “jolly jack tars”.

The University of Lyon team studied the oxygen isotopes of a total of 133 Cretaceous specimens, a mixture of spinosaurs, other dinosaurs and animals known for their nektonic behaviour (active swimmers), such as crocodiles and turtles.  Their study showed that the spinosaurs had oxygen-18 levels that were significantly lower than those found in other more terrestrial dinosaurs.  Interestingly, the oxygen-18 values in crocodiles and spinosaurs were not significantly different.  The French team have postulated that this is evidence that spinosaurs were dwelling in aquatic habitats, perhaps occupying a specific niche in the Cretaceous food chain.

Spinosaurs in Aquatic Habitats

The evidence from the French scientists does suggest that these particular theropods may have spent a considerable amount of time in the water.  The large thumb-claw, a characteristic of the Spinosauridae would have made an effective fish hook, spearing fish as the Spinosaurus stood on the bank fishing, like a modern Grizzly Bear, but equally, it could have been used to help steady this large animal and retain its grip on the muddy bottom of a riverbed.

We think this debate is going to run and run, for example, although fish scales are associated with the Baryonyx fossils found in a Surrey clay pit, so are the remains of a small iguanodontid.  Other spinosaur fossils have been associated with the fossilised remains of their last meal, a flying reptile.  Just like most crocodiles, these animals may have been unfussy eaters, devouring anything and everything that they came across.  The bones of these creatures do not show many adaptations for an aquatic lifestyle.  For instance, there is little evidence for a deepened tail to help with swimming.

Elements such as any webbing between the toes would not necessarilly have been preserved, so the jury is still out as to just how much time this dinosaur spent swimming.

It is known that dinosaurs could swim and many took to the water, traces of their tracks as they crossed lakes and rivers have been found in various parts of the world.  In one remarkable example from northern Spain, the fossilised trackway of a theropod dinosaur as it swam across a body of water has been preserved.

To read more about this remarkable trace fossil: Swimming Dinosaurs, Evidence from Spain.

The sandstone, in which the occasional claw impressions and prints have been preserved as this animal swam across the water, occasionally touching the bottom and pushing itself off again, is approximately 125 million years old.  This date coincides to the time when the spinosaurs were around, could this be evidence of a swimming spinosaur?