What Was Spinosaurus? The Dinosaur That Went Into the Water

Ernst Stromer, a Bavarian palaeontologist, collected the first Spinosaurus from the Bahariya oasis in the Western Desert of Egypt between 1910 and 1914: a lower jaw, vertebrae with spines up to 1.65 metres tall, and fragments, from rocks about 95 million years old. He named it Spinosaurus aegyptiacus in 1915 and reconstructed a large theropod with a sail. The bones went into the Bavarian State Collection in Munich, whose director refused Stromer's request to move them to safety during the war, and on the night of 24 to 25 April 1944 the building was destroyed. Stromer's drawings and a few photographs are all that remain, and until this century they were nearly all that was known.

Fragments turned up in Morocco's Kem Kem beds through the 1990s and 2000s, and in 2014 a team led by Nizar Ibrahim published a partial skeleton from a single individual, including hind limbs and pelvis, and a reconstruction that startled everyone: the hind legs were short and the pelvis small, so that the animal seemed poorly built for walking on two legs, and its bones were dense like a penguin's or a manatee's, an adaptation for controlling buoyancy. A tail found at the same site and described in 2020 was tall and paddle-shaped from root to tip, unlike any other dinosaur's, and in a flume it produced far more thrust than a normal theropod tail. The features assembled from Egypt and Morocco:

• •Length of 14 to 15 metres, longer than Tyrannosaurus though probably lighter
• •A long, narrow, crocodile-like snout with conical teeth for gripping fish rather than blades for cutting flesh, and pits on the snout that in crocodiles house pressure sensors
• •Nostrils set well back on the head, as in animals that keep the snout in water
• •A sail of neural spines up to two metres tall along the back, its purpose unknown, display and thermoregulation being the guesses
• •Short hind legs, dense limb bones, and a fin-like tail
• •Isotopes in the teeth showing a diet with much aquatic prey, and a habitat of rivers and deltas full of giant fish, crocodiles and sawfish

Ibrahim's team argued in 2020 that Spinosaurus was a pursuit predator that swam after prey in the rivers of Cretaceous North Africa, propelled by its tail, the first known aquatic dinosaur. Other palaeontologists have tested the claim against the physics and disagreed. A 2022 analysis of bone density across many animals found Spinosaurus in the range of divers; a 2022 study of the body's shape and buoyancy found it would have floated too high to dive, been unstable in the water and a slow swimmer, and concluded it was a wader that stood in shallows and struck like a heron, its tail perhaps for display. The two camps have exchanged papers since, and the most likely picture is between them: an animal that lived in and beside water, waded and swam as a crocodile does, ate fish and whatever else it could catch, and did not chase prey through deep river channels like a seal.

Spinosaurus belongs to a family of long-snouted theropods, the spinosaurids, found across the Cretaceous world: Baryonyx from a Surrey clay pit, described in 1986 with fish scales in its stomach and a great hooked claw on its thumb, Suchomimus from Niger, and Irritator from Brazil. All had the crocodile snout and the conical teeth, and all lived by water; Spinosaurus took the trend furthest. The family shows that the theropods, usually pictured as the lions and wolves of their age, also produced fish-eaters and semi-aquatic hunters, and that the ecology of a Cretaceous river was as crowded as a modern one, with a fifteen-metre dinosaur competing with crocodiles the size of buses for fish the size of cars.

Many people first met Spinosaurus in the film Jurassic Park III, where it fights and kills a Tyrannosaurus. The two could never have met. Spinosaurus lived in North Africa roughly 95 million years ago, while Tyrannosaurus rex lived in western North America at the very end of the Cretaceous, about 68 to 66 million years ago. That is a gap of roughly thirty million years, longer than the time separating the last non-bird dinosaurs from the early ancestors of horses. The film's animal also has the long legs and upright posture of the older reconstructions, before the Moroccan finds changed the picture.

The sail has its own history of reinterpretation. Because the tall spines were known only from Stromer's lost bones and drawings, researchers have debated what covered them. In the 1990s one palaeontologist argued that the spines were more like those of a bison and supported a fleshy hump, perhaps for storing fat, rather than a thin sail of skin. Most researchers today favour a sail, partly because the spines are relatively thin and flat, but the case is a good example of how much palaeontologists have to infer about soft tissue from bone alone, and why reconstructions change as new fossils arrive.

The rocks that hold Spinosaurus present a puzzle that Stromer noticed in Egypt and that later researchers named after him. In both the Bahariya beds and the Kem Kem beds of Morocco, fossils of large meat-eating dinosaurs are surprisingly common, while those of the plant-eaters they should have been hunting are rare. Alongside Spinosaurus lived Carcharodontosaurus, a predator about as long as Tyrannosaurus, whose Egyptian bones Stromer also described and also lost in the 1944 raid, as well as other sizeable theropods. In a normal ecosystem plant-eaters far outnumber predators, so the balance looks upside down.

Several explanations probably combine. The Kem Kem rivers were full of enormous fish, including sawfish, coelacanths and lungfish, so many predators may have lived largely on fish rather than on dinosaurs, sharing the food out as herons, crocodiles and otters do today. River deposits also gather bones from different times and places, and predators' teeth, which were shed throughout life and survive well, are over-represented. The fossil trade adds a further complication: many Kem Kem specimens are dug up by local collectors and sold, often without a record of exactly where they were found, which makes it hard to tell which animals truly lived side by side.

Spinosaurus lived in a river system in what is now North Africa that was full of large fish, which helps explain why a fish-eating giant could thrive there. The same rocks have produced remains of Mawsonia, a coelacanth that could reach several metres long, much larger than the living coelacanth, and of Onchopristis, a sawfish-like animal with a long toothed snout.

There were also huge lungfish and many other fish, along with crocodile relatives that would have competed for the same food. A landscape with such abundant large prey in the water makes a predator specialised for catching fish easier to understand, whatever the exact balance between wading and swimming turns out to have been.

Spinosaurus was a fifteen-metre theropod of Cretaceous North Africa with a crocodile-like snout and teeth, nostrils set back for a snout in water, a two-metre sail, short hind legs, dense bones and a paddle tail, first found in Egypt, destroyed in Munich in 1944 and rebuilt from Moroccan finds since 2014. It ate fish and lived in and beside rivers, and whether it swam after prey or waded and struck from the shallows is the argument that its unique anatomy has not yet settled.