Was Spinosaurus a Swimmer? The Sail-Backed Giant and Palaeontology’s Biggest Debate

With a sail on its back taller than a person, a snout like a crocodile and a body longer than a city bus, Spinosaurus is one of the strangest dinosaurs ever discovered. It starred as the villain in Jurassic Park III, beating T. rex in a famous fight scene. But in real science, Spinosaurus is famous for a different battle: an ongoing debate about whether this giant predator was a swimmer, the first dinosaur adapted to life in water, or a wading hunter that stalked fish in the shallows.

A lost dinosaur

The story begins in Egypt. In 1912, fossil hunters working for the German palaeontologist Ernst Stromer found bones of a large predator with extraordinarily long spines on its back. In 1915, Stromer described it as Spinosaurus aegyptiacus, the “Egyptian spine lizard”.

Then disaster struck. In 1944, during the Second World War, an Allied bombing raid destroyed the museum in Munich where the fossils were stored. Spinosaurus vanished. For the rest of the 20th century, scientists had to rely on Stromer’s drawings, notes and a few photographs, together with scraps of related animals.

The giant from the river

Newer finds, especially from the Kem Kem beds of Morocco, have filled in much of the picture. Spinosaurus lived around 100 to 95 million years ago in the Cretaceous period, when North Africa was crossed by enormous river systems teeming with fish, some of them several metres long.

2014: the first semi-aquatic dinosaur?

In 2014, a team led by palaeontologist Nizar Ibrahim published a study in Science based on a new partial skeleton from Morocco. They reconstructed Spinosaurus with short hind legs, a long body and dense limb bones, and they argued that it was semi-aquatic, spending much of its time in water, rather like a crocodile.

This was a radical claim. Dinosaurs were long thought to be strictly land animals; the marine reptiles and pterosaurs that are often mistaken for dinosaurs belong to separate groups.

2020: a tail built for swimming

In 2020, the same team reported in Nature that they had found much of the tail. It was surprising: tall spines and bony rods along the top and bottom made it deep and flat from side to side, like a paddle or the tail of a newt or crocodile. Using a robotic model in a water tank, they found that this tail produced much more forward thrust than the tails of other meat-eating dinosaurs. They concluded that Spinosaurus used its tail to swim in pursuit of prey.

2022: dense bones and divers

In 2022, a study led by Matteo Fabbri in Nature looked at bone density in more than 250 living and extinct animals. Animals that dive underwater for food, such as penguins and hippos, tend to have very dense bones, which act like ballast. Spinosaurus and its relative Baryonyx had such dense bones, suggesting to the authors that they hunted while submerged.

The counter-argument: a giant heron?

The same year, a team led by Paul Sereno published a study in eLife with a blunt title: “Spinosaurus is not an aquatic dinosaur”. Using a detailed digital model of the whole animal, they calculated how it would behave in water. Their findings:

• Spinosaurus would have floated easily and struggled to dive, despite its dense bones.
• Its tall sail would have made it unstable, tipping it onto its side in deep water.
• As a swimmer, it would have been slow, compared with the fish it was chasing.
• Its tail was not stiff or powerful enough to be an efficient swimming organ.

Instead, they suggested Spinosaurus was a “semiaquatic bipedal ambush piscivore”: a two-legged hunter that waded in shallow water and along riverbanks, snapping up fish, much like a giant heron or stork.

Why the debate is so hard to settle

Spinosaurus is known only from incomplete skeletons, often of different individuals, and the original type specimen was destroyed. Small differences in reconstructions, such as how long the legs were, how big the sail was or how dense the bones were, make big differences to calculations about buoyancy and balance. Interpreting bone density is also tricky, since some land animals have dense bones too.

What the two sides largely agree on is important: Spinosaurus was a giant predator specialised for catching fish and lived around water. The disagreement is about how much of its life it spent in the water and whether it actually swam after prey.

What was the sail for?

The great sail remains a mystery in its own right. Its spines could exceed 1.5 metres. The most widely supported idea is display: a big, possibly colourful sail would have signalled size and health to rivals and potential mates. Earlier ideas, such as regulating body temperature or storing fat like a camel’s hump, have less support today.

In short

• Spinosaurus lived 100–95 million years ago in North Africa and was about 14 m long.
• Its original fossils were destroyed in 1944; new finds in Morocco revived research.
• A narrow crocodile-like snout, retracted nostrils and dense bones link it to water.
• Ibrahim’s team (2014, 2020) argued it was semi-aquatic and swam with a paddle tail.
• Sereno’s team (2022) argued it was too buoyant and unstable and hunted while wading.
• Both sides agree it was a fish-eating giant that lived by rivers; the details are still debated.