Let me tell you a story. Three time travellers meet in a bar. They decide that they want to journey back to Mid Cretaceous North Africa, around 95 million years ago. They plan to have a dinosaur safari, just like the safaris that are conducted across the modern day Serengeti. However the area is dangerous as there are plenty of dangerous dinosaurs roaming around, some big enough to take out the jeep faster than the time travelling tourists can say “holy cow is that a dinosaur!” So they come up with a plan. Instead of going by jeep, they will use a river boat. “It’s perfect!” they say, “no dinosaur will be able to reach us as we are on the water and because it’s inland there’s no need to fear any sea faring marine reptiles”. After some time planning the time travellers undertake their boat journey. They marvel at the herds of the huge sauropod Paralititan and see an impressively large Carcharodontosaurus watching the herd hungrily. However as they are distracted by what they are seeing they don’t notice a large object moving in the river. Suddenly one of them hears the sound of rushing water and turns round to see a large sail poking out of the water like Jaws’ fin and heading right towards the boat. In five minutes, the time travellers will have seen their last dinosaur….
The unidentified river monster in this story is not a fish, nor a crocodile. It is instead the largest of all theropod dinosaurs: Spinosaurus aegyptiacus (meaning “Egyptian spine lizard”). The first Spinosaurus fossils were discovered in Egypt by German palaeontologist Ernst Stromer in 1912, however sadly these remains were destroyed during allied bombings raids on Munich during World War Two. Spinosaurus has gained more attention in recent years as new research has shown that this theropod had a lifestyle unlike any other dinosaur. The research suggests that Spinosaurus is the first known aquatic dinosaur.
There are certain anatomical features that support this interpretation. One area is the shape and design of the skull. The teeth were long and conical, well suited for gripping and holding on to slippery prey such as fish. The nostrils were placed high on its snout so it could hold it submerged in water while still being able to breath. It’s also thought that holes at the end of Spinosaurus’ snout contained pressure sensors, similar to those seen in modern crocodiles, which are used to detect disturbances in the water. The water based adaptations don’t stop there; one specimen of a Spinosaurus upper jaw has a barb embedded in it that belonged to a species of giant swordfish called Onchopristis. This suggests that Onchopristis was one of Spinosaurus‘ main sources of food. So taking all these adaptations together it suggests that Spinosaurus had a mostly fish based diet (however it is still possible that it ate meat as well).
All these adaptations are well and good, however the biggest evidence for Spinosaurus‘ aquatic lifestyle was detailed in a paper released in 2014, written by a team of palaeontologists led by Spinosaurus expert Nizar Ibrahim. The paper examined and described a new 11.3 metre long specimen (that wasn’t fully grown, Spinosaurus could reach a maximum length of 15 metres) that is the most complete Spinosaurus skeleton ever found. Examination of the hind limb bones and pelvic girdle showed that they were much shorter than previously thought. These hind limbs were so short that Spinosaurus could not have walked on two legs for long periods of time as its legs couldn’t support its massive, front heavy bulk. This meant only one thing, Spinosaurus walked on all fours (at least occasionally). This is certainly very different to the bipedal, T-Rex killing beast that terrorised Alan Grant and the Kirbys in the 2001 film Jurassic Park 3. This new discovery also fits with the theory of a mostly aquatic based Spinosaurus. Short hind legs reduce drag when swimming and diving through water and is something that other aquatic and semi-aquatic animals, such as crocodiles and otters, exhibit today. These hind limbs were also solid and dense, helping with buoyancy control, and it has been speculated that feet would have been webbed to further aid in swimming. This means that Spinosaurus would have been comfortable and manoeuvrable in water, and a somewhat clumsy walker on land!
This reconstruction, which has commonly been dubbed “new Spinosaurus“, has caused quite a stir in the palaeontology community, Not everyone is on board with the idea and a few have even written articles and papers questioning the findings of the Ibrahim et al. 2014 paper. For example palaeontologist Scott Hartman questioned the length measurements of Spinosaurus’ hind limbs and Donald Hendersons 2018 paper questioned its buoyancy. However both these counter papers have their own issues; for example the Henderson paper based their buoyancy calculations on bone density data from other theropods and birds, except Spinosaurs have been shown to have proportionately thicker bones than these animals. The debate is still ongoing to this day, and so it is still not known for certain whether Spinosaurus truly was a quadruped or a biped.
So let us rejoice in the quadrupedal walking, gracefully swimming, humongously sized, sail-backed fish eater that Spinosaurus has become! Proof that during their 170 million year reign there were few habitats that the dinosaurs couldn’t reach.
UPDATE: Spinosaurus reconstructions continue to change with every new paper! A new study was published on the 29th April 2020 in the journal nature. Written by a team led by Nizar Ibrahim, has revealed that Spinosaurus had unusually tall neural spines and elongated chevrons on its tail vertebrae. These special vertebrae supported a flexible, paddle-like tail and it’s theorised that Spinosaurus used it to swim through the water! The image below illustrates what most palaeontologists now think Spinosaurus aegypticus looked like.
EDIT: One key piece of evidence, that I missed when writing this blog (my apologies!), from the Donald Henderson 2018 paper is that Spinosaurus’ centre of mass was located closer to its hips than to its torso. This is further evidence that Spinosaurus was a biped, walking on its short stubby legs (almost like a duck!). At the time of writing this Nizar Ibrahim has yet to publish his latest research on Spinosaurus so the two legged/four legged debate rages on. But currently the common consensus, at time of writing, is that Spinosaurus was a biped.
Ibrahim, N., et al. (2014). “Semiaquatic adaptations in a giant predatory dinosaur.” Science 345(6204): 1613-1616.
Henderson DM. 2018. A buoyancy, balance and stability challenge to the hypothesis of a semi-aquatic Spinosaurus Stromer, 1915 (Dinosauria: Theropoda) PeerJ 6:e5409 https://doi.org/10.7717/peerj.5409
Ibrahim et. al., “Aquatic Spinosaurus – The authors respond”, Scott Hartman’s Skeletal Drawing.com, Sept. 18, 2014, skeletaldrawing.com/home/aquatic-spinosaurus-the-authors-responsd9182014
Smithsonianmag.com, “Cracking the Code of Spinosaurus”, Smithsonian Magazine, Apr. 19, 2017, smithsonianmag.com/sponsored/spinosaurus-lost-dinosaur-paleontology-new-discovery-great-courses-plus-180962953/
• New Ibrahim et. al. 2020 paper on the structure of Spinosaurus’ tail (USED FOR UPDATE)
Ibrahim, N., Maganuco, S., Dal Sasso, C. et al. Tail-propelled aquatic locomotion in a theropod dinosaur. Nature (2020). https://doi.org/10.1038/s41586-020-2190-3