A new study presenting a hypothesis for the function of the hypertrophied dorsal neural spines of Spinosaurus aegyptiacus was published this year by Gimsa et al. (2015). 


"The first fossils of the gigantic, semiaquatic predatory dinosaur Spinosaurus aegyptiacus, which lived during the Cretaceous period c. 97 million years ago and was larger than Tyrannosaurus rex, were described by Stromer in 1915. He was perplexed by the presence of many large predators such as crocodilia, despite the lack of any major terrestrial herbivores (Martill et al.2011). Unfortunately, the remains collected by Stromer were lost during the Second World War. Based on newly collected items, a fascinating account of Spinosaurus was recently given by Ibrahim et al. (2014) who have provided arguments for Spinosaurus’ skill in swimming and aquatic hunting, including the adapted anatomical proportions, a centre of body mass towards the front, high-density bones, flattened pedal unguals, a longirostral piscivorous snout and a small nostril located far back on the head. Their findings, together with our knowledge of the presence of large river systems in North Africa at the time of Spinosaurus, point to Spinosaurus’ semiaquatic life and its aquatic prey.
Nevertheless, after solving Stromer's old riddle, Ibrahim et al. (2014) were confronted by another inexplicable puzzle: the dorsal ‘sail’, supported by major spinal processes. The authors discussed three hypotheses. The first, that the sail was a thermoregulatory structure, was dismissed because of the lack of canals for blood vessels in the spinal processes. A second, that the spines supported a muscle or fat-lined hump (Bailey, 1997) was dismissed in favour of Stromer's (1915) hypothesis of convergent evolution with the skin-covered neural spines of the crested chameleon. Based on the idea that the sail was tightly enveloped in skin, the authors proposed that it was used largely for display on land and in water to deter foes and competitors or to impress potential sexual partners, and that it would have remained visible while swimming.
We would like to extend this hypothesis. In water, Spinosaurus’ semiaquatic life would always have risked submerging the sail by accident. The sail would unleash completely different forces under water than when used for display in air. Accordingly, the anatomical characteristics of the animal must have taken such occurrences into account. Given these features, the question of why the animal would not have taken advantage of a hydrodynamically stable sail must be asked.
Spinosaurus’ sail was almost rectangular, unlike the rounded, arc-shaped sails of related non-aquatic spinosaurids (see Ibrahim et al.2014). The size and shape of the spinal sail relative to Spinosaurus’ body roughly resembles the anatomical geometry of the dorsal fins of sailfish (Domenici et al.2014), which in our view suggests homologous functions." 

Gimsa, J., Sleigh, R., Gimsa, U., (2015) : "The riddle of Spinosaurus aegyptiacus ' dorsal sail". University of Rostock, Chair for Biophysics, Gertrudenstr. 11A, 18057 Rostock, Germany
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