Spinosaurs did not swim

Digital palaeontology posits that this fish-eating dinosaur was happier on shore than in the water...
15 December 2022

Interview with 

Paul Sereno, University of Chicago


An artist's rendition of a spinosaurus.


The era of digital palaeontology means that guesswork and speculation about the size, speed, habits and preferred habitats of dinosaurs and other ancient animals is rapidly being replaced by objective computer-based reconstructions; and if you're really ready to become the next Indiana Jones, you can even do all of that during your expedition, as Chris Smith hears from Paul Sereno...

Paul - My name is Paul Sereno. I'm a palaeontologist at the University of Chicago. I've just come back from an expedition to the Sahara, one of my favourite haunts. We found 35 tons of wondrous things from the past.

Chris - Did I hear that correctly? 35 tonnes!

Paul - That's correct: 35 tonnes. It's an enormous quantity of material because some of the dinosaurs are large. And so we went to five different time horizons in the country of Niger, in the heart of the Sahara as part of the country, and they have the best and the deepest record of dinosaurs. And among our finds is the subject of today's podcast: Spinosaurus. We found new materials - very exciting material - from Spinosaurus.

Chris - Describe then if I were wandering around and I wouldn't be, but if I could go back in time and wander around at the time of Spinosaurus, A) how far back would I have to go and B) how would I spot one?

Paul - Well, <laugh>, you hope you spot it before it spots you. But you'd go back about 90 million years that time, which is what we call the late Cretaceous period. This part of Africa was a lot wetter than it is today. So we found petrified wood and giant fish. There were rivers and forests. There'd be a lush habitat and there'd be this triumvirate of predators that you'd want to keep an eye out for it because each one of them could do you in. The biggest of them is Spinosaurus not far behind is Carcharodontosaurus, blade shaped teeth on that one. Spinosaurus, a fish eating dinosaur. But there's a third one rarer, but dangerous and a little bit smaller. And that's an Abelisaur. And so these three predators, we find them for millions of years in Africa. We have nothing really like them living in the same period of time in North America. This, this is T-Rex territory in North America. Africa is a totally different dinosaur world.

Chris - So were they like T-Rex then, these spinosaurs?

Paul - No, Spinosaurs were not like T-Rex other than they could do you in, they really had a fish eating skull. But they, if they wanted to go after something else, they'd do it with their hands. Their forelimbs are among the strongest of any bipedal dinosaur. They are four feet long or five feet long with sickle shaped claws. They had a snout that was really designed for going into mud, going into the shallow waters after a wide variety of large fish. That was clearly their mainstay, their specialty, but they were big enough to take anything down.

Chris - So they walked around on two legs, but were pretty good with these forelimbs, which they could use for fishing, but they could also use their face and snout for that.

Paul - Absolutely. And the eLife paper was, I think, settling the question of whether they were four legged on land or two legged. We actually spent a lot of time on that. We tried to work on where the centre of mass was with a better reconstruction of the skeleton. And we find that it's not a fast moving dinosaur on land, but it is like all the other theropods on land bipedal or two-legged.

Chris - How did you work that out then? Because, presumably, what you've got to go on are skeletal remains - fossilised remains? (Exactly.) How did you then turn that into something that tells us this was a two-legged creature and this is how it moved. This is what it would've done?

Paul - Look, this is the age of digital palaeontology. And for all the young folks out there that are listening to this, this is your time to get into palaeontology. This is a digital age, and so even in the field, we were able to scan the fossils by stereophotographing and put them together while we're sitting in our expedition tent! Literally a few weeks ago, we were able to look at and, and, and move our Spinosaurus bones around. So what we did in the paper was, yeah, we start with the fossils and then we reconstructed Spinosaurus better than ever before by connecting the bones into the skeletons, making sure they we're all pieced together correctly. Then we looked at modern animals, CT scans of modern animals to understand how much flesh we should put on this dinosaur. And we're surprised that we had made mistakes before and papers had made mistakes.

Paul - The tail needs a lot more muscle on it than had been previously put on it because we looked at alligators, large lizards, they have a lot of muscle on the tail. It goes beyond the bone. And so when we did that, and then we checked out digitally, where was the centre of mass? Where would it balance over its time limbs or would it need its forelimbs to progress the land? The answer was, was definitive. It balanced over its hind limbs. Then we put it in water to see how and how well it could swim, what the forces were against the body with a big sail. This was an awkward animal to be swimming in water. And we proved that by putting it in water digitally, seeing where and when it would float, and then what it would do underwater, how fast it could swim.

Chris - When you say it's got an awkward sail, do you mean sort of on its head, on its tail? Where was the sail tissue?

Paul - So people are very familiar with Spinosaurus and the sail. It's actually not one sail. It has three sails. There's one on the head, a small one in the midline, then there's one on the neck that sort of pops out. We discovered after reconstructing it and then it goes down again. Then the big one is on the back. And these are bony crests covered probably with colourful flesh, but they were stiff bony crest. Now you don't want something like that in water and nothing that swims in water has anything like that. A sailfish, for example, which has a sail, it's retractable and when it wants to swim, and it's one of the fastest swimmers in water, it pulls that down. When it wants to break, it pulls it up because it operates as a break. It, it, there's nothing in water that looks like a Spinosaur for good reason the lizards that look closest to Spinosaurus with the big back sail. They don't use it in swimming and they're not aquatic swimmers. And so we think we've, we, we've fairly tracked down every conceivable line of evidence to test this idea of whether it was an underwater diving, marine predator or whether it was, was in fact a shoreline semi-aquatic animal that liked the shoreline like crocodilians and also was capable of walking on land on two feet.

Chris - So had people previously kind of cherry picked the evidence a bit and made it fit together to make a nice story. This thing's got these sail structures. They looked like they'd work in water. It's got a fish eating head. Therefore it was probably an aquatic dinosaur that occasionally went on the land and, and that was the story they built. But you are saying if you do the digital palaeontology and put it in water, it just doesn't float. It does float too much. It doesn't swim. It doesn't work!

Paul - You're, you're right. The idea, I, I believe that we have formally sunk the idea with this paper, but we're open to challenges. That's what science is about. Look, science is about arguments. And when the first bones of Spinosaurus came to light, they were, they were controversial. And the German scientists who described these a century go from, from Egypt, they were all destroyed in the, in, in the world wars that we've had. And we have refound their bones. When he described it, he described it as a fish eating dinosaur, and he was very careful and conservative and accurate, but he didn't have enough of the skeleton. And when we found the skeleton, I made the argument that it was semi-aquatic and then subsequently a tail was found and the tail had a big sail on it as well. And so the, the back sail actually continues on the, the tail.

Paul - And we find now that the predecessors of Spinosaurus, one that I found in, in Niger earlier in time Suchomimus, they also have this sail on the back and it continues to the tail. And the idea came, well, maybe he was using it as a paddle. And this got to be very popularised in the last few years: it was an aquatic dinosaur diving and swimming with the tail. And so we put that to the test and and the test says, "sorry, its sail was for advertising like a billboard on land and it could wade into water". It can go into six feet of water. We show on the paper before it floats. But float this boat was gonna happen, because it's lighter than water. It has a lot of air inside its vertebrae and it's very clumsy in water. So while it could - most dinosaurs and even humans can swim - we, we are not aquatic animals. And it is not definitely an aquatic predator. When you're an aquatic predator, underwater you have to - especially if you're big - you have to be good and you look like a whale, you look like an ichthyosaur you do not look like a dinosaur. And so we put this to the test.


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