A predatory dinosaur that was bigger than Tyrannosaurus rex may have been able to use its tail to swim through water, allowing it to hunt aquatic prey.
鈥淚t opens up an entire new world of ecological possibilities for dinosaurs,鈥� says Nizar Ibrahim at the University of Detroit Mercy.
Ibrahim and his colleagues analysed a 95 million-year-old fossilised tail of Spinosaurus aegyptiacus, which they first unearthed in the Kem Kem beds in south-eastern Morocco in 2018.
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They discovered that the dinosaur鈥檚 tail had an unusual paddle-like shape, with long spines near the tip, and was far more flexible than previously thought. 鈥淚t came as a massive surprise, because nothing like this has been documented in any other dinosaur,鈥� says Ibrahim.
This flexible tail indicates the dinosaur would have been able to propel itself in the water. This suggests it was actively hunting for prey in deeper waters, says Ibrahim.
Previous studies have shown that Spinosaurus had adaptations for fish-eating. 鈥淏ut it was generally considered a wader, possibly waiting for fish to swim by, while standing in shallow waters,鈥� says Ibrahim.
鈥淚t may mean some of these [types of] dinosaurs had distinct behaviours, entering the water in a more significant way than we have previously thought,鈥� says Steven Jasinski at the State Museum of Pennsylvania.
The new tail bones and shape of the tail add to evidence from other parts of its anatomy, such as its retracted nostrils and solid limb bones, that indicate this predator was adapted to swimming and feeding in water, says Paul Sereno at the University of Chicago in Illinois.
鈥淚 don鈥檛 think it was very fast or agile in water,鈥� says Sereno. 鈥淢any aspects of its body form and feet are not suitable for aquatic speed or agility,鈥� he says. Spinosaurus was around 15 metres long, 3 metres longer than T. rex, making it likely the largest carnivorous dinosaur. It also had a large sail on its back.
It most likely took advantage of its long arms and neck to catch relatively large prey at close range, says Sereno, but he thinks it is unlikely that these animals were primarily aquatic. 鈥淔ossils of Spinosaurus are known from localities more than a thousand miles from the ocean.鈥�
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