A buoyancy, balance and stability challenge to the hypothesis of a semi-aquatic Spinosaurus Stromer, 1915 (Dinosauria: Theropoda)

A recent interpretation of the fossil remains of the enigmatic, large predatory dinosaur Spinosaurus aegyptiacus Stromer 1915 proposed that it was specially adapted for a semi-aquatic mode of life-a first for any predatory dinosaur. To test some aspects of this suggestion, a three-dimensional, digital model of the animal that incorporates regional density variations, lungs and air sacs was generated, and the flotation potential of the model was investigated using specially written software. It was found that Spinosaurus would have been able to float with its head clear of the water surface, although it was laterally unstable and would tend to roll onto its side. Similarly detailed models of another spinosaurid Baryonyx (Suchomimus) tenerensis Sereno et al. 1998, along with models of the more distantly related Tyrannosaurus rex Osborn 1905, Allosaurus fragilis Marsh 1877, Struthiomimus altus Lambe 1902, and Coelophysis bauri Cope 1887 were also able to float in positions that enabled the animals to breathe freely, showing that there is nothing exceptional about a floating Spinosaurus. Validation of the modelling methods was done with floated models of an alligator and an emperor penguin. The software also showed that the center of mass of Spinosaurus was much closer to the hips than previously estimated, similar to that observed in other theropods, implying that this dinosaur would still have been a competent walker on land. With its pneumatised skeleton and a system of air sacs (modelled after birds), the Spinosaurus model was found to be unsinkable, even with its lungs deflated by 75%, and this would greatly hinder a semi-aquatic, pursuit predator. The conclusion is that Spinosaurus may have been specialized for a shoreline or shallow water mode of life, but would still have been a competent terrestrial animal.