Why Paleontologists Can’t Stop Fighting About Spinosaurus

Dino lovers, rejoice! A brand new Walking with Dinosaurs just launched on PBS, 25 years after the original series. Head to the link in the description to learn more. Dinosaur paleontology has historically been built on disagreement. We had Othniel Charles Marsh and Edward Drinker Cope duking it out in the Bone Wars of the late 1800s. More like the Bonehead Wars, those guys were such jerks. Then we got Luis and Walter Alvarez dropping their asteroid impact hypothesis into the debate about the extinction at the end of the Cretaceous period in 1980. And now we have, literally, how wet did Spinosaurus get? While it's definitely more of a niche disagreement than those famous fossil beefs, it points to something fundamental to the process of science, how evidence does, or does not, stack up to support a hypothesis. So, initially, Spinosaurus was thought to be basically a T-Rex with a sail. But over the last century-ish, it's undergone a radical transformation as scientists realized it was probably not just living on land. Instead, it was likely spending its time around water, making it semi-aquatic. And that's where disagreement comes into the picture, and where our old Spinosaurus episode left off. What does it mean to be a semi-aquatic dinosaur? Like, was it wading around in the shallows like a heron or a crane? Or could it actually have been a skilled swimmer, gliding through the water like a crocodile or an otter? Each scenario paints a very different picture of Spinosaurus, shaping everything from how it moved to how it caught prey. And the discovery of new fossils has paleontologists rethinking just how weird and watery this dinosaur was all over again. Ever since Spinosaurus was first named in 1915, it's been a bit of a mystery. The few fossils that paleontologist Ernst Stromer had to go on painted the picture of a weird, carnivorous dinosaur that looked kind of like T-Rex, but also kind of not. It was huge, with some estimates suggesting up to 15 meters long, making it one of the biggest carnivores ever found. But this giant of the Cretaceous period was also… different. The skull of Spinosaurus, for example, was long and crocodile-like compared to most large theropods. And overall, it was just an awkward creature that no one could fully make sense of. And it didn't help that the early fossils that Stromer described were lost during the Allied bombings of Munich in 1944. So, for a period, Spinosaurus was kind of an enigma, lost to time. But then, beginning in the 1980s, other pieces of the puzzle started to fall into place when fossils of its close relatives were uncovered. Like Baryonyx walkeri, found in the UK, followed by fragments of other Spinosaurids throughout North Africa. These new fossils suggested that Spinosaurids had convergently evolved similar traits to crocodiles. And the researchers that worked on them concluded that, like crocodiles, Spinosaurids must have been eating fish. In 1997, this idea gained more support when fish scales were found during the reanalysis of the stomach contents of a Baryonyx specimen. Then, in the early 2000s, scientists turned their attention to using stable isotopes to infer the diet and ecology of Spinosaurs in general. An isotope is a version of an element that has a different number of neutrons in its nucleus. And isotopes get incorporated into an animal's bones and teeth through its diet. And the ratios of different isotopes can help researchers figure out what an animal was eating and drinking. Basically, you are what you eat, elementally speaking. Looking at oxygen isotopes from Spinosaur fossils, paleontologists discovered that these animals had chemical signatures more similar to crocodiles and turtles than other theropods. And a later study using calcium isotopes from carnivorous dinosaurs in North Africa found evidence that Spinosaurs, like Suchomimus, were feeding in aquatic environments, while other big theropods stuck to terrestrial ones. The picture these close relatives painted was that the Spinosaurids were kind of like mega-herons that waited in the shallows along riverbanks and ate fish. Extreme emphasis here on the mega. Given its size, Spinosaurus has occasionally been called a heron from hell. But that was hardly the end of the debate. See, there still weren't a ton of fossils, especially not of Spinosaurus. And the fossils that scientists did have weren't all that complete. So, when in 2014, a team of paleontologists led by Nizar Ibrahim published new Spinosaurus fossils found in rocks in Morocco called the Kemkem Group, it was a big deal. Dating to the Late Cretaceous, around 97 million years ago, these new finds included pieces of the skull, vertebrae, pelvis, and limbs of a young adult. And these fossils seemed to suggest to the team that Spinosaurus was spending more time in the For instance, its skull had a series of tiny pits that lined the front of the snout, much like those seen in crocodiles, which are pressure receptors that detect movement in the water. And both its nostrils and eyes appeared to be positioned higher than most theropods, which would have allowed it to breathe while partially submerged, the team argued. This hinted at an animal that was not just a riverside hunter, but was actually a skilled swimmer, snaring fish while moving through the water like an otter or a croc. The back half of its skeleton seemed to confirm this, too. Spinosaurus' hind limbs and pelvis were smaller than expected, featuring a proportionally short femur. A shorter femur, Ibrahim's team pointed out, is a feature we see in living swimming mammals that use their hind limbs for paddling. The researchers even thought that Spinosaurus' feet were potentially webbed, which would have also benefited an animal hunting fish in North Africa's river system. But not everyone was convinced that Spinosaurus was doing more than just splashing around in the shallows. Some scientists argued that the nostril position still would have required the animal to raise much of its head above the water to breathe, for example. Others declared that Spinosaurus' body shape suggested it was unsinkable and therefore couldn't dive to chase fish. But then, in 2020, Ibrahim and a team of researchers announced that they had discovered the tail of that same Spinosaurus individual. The tail was estimated to be about 80% complete, and it was entirely different from what people had expected, because it was unlike the tail from any other theropod dinosaur. Previously, paleontologists had assumed that the tail of Spinosaurus was thick, muscular, stiff, and a good counterbalance, like its close relatives. But this tail had odd features that gave it an almost paddle-like appearance, like elongated neural spines. It was also very long, and it didn't have features that would generally stiffen and stabilize it, meaning it had a ton of flexibility. And all of these traits would be beneficial to a swimmer that propelled itself through the water with its tail. So, Ibrahim's team created models that compared two terrestrial theropod dinosaurs and two aquatic tetrapods, a crocodile and a newt, to show that the shape of the tail generated enough thrust for an active swimmer. They concluded that Spinosaurus wasn't just a wading dinosaur, but instead a swimmer that pursued its prey in deeper waters, like a crocodile. So, under this hypothesis, the dinosaur would actively dive into rivers, swimming in the water to chase lungfishes, coelacanths, and other fish found in the chem chem group. So that's it, right? One complete tail, and the arc of the wild Spinosaurus saga is complete. No, that's actually not what happened. The idea of a submerged swimming dinosaur hasn't been universally accepted. Other paleontologists stuck with the original hypothesis. For these researchers, the animal's anatomy didn't point to an efficient swimmer. Like, those toe claws that had been interpreted as potentially webbed? Maybe they weren't actually that different from the claws of other terrestrial theropods, after all. Then, in 2022, a team working with Ibrahim turned to Spinosaurus's bone structure as another way to test the swimming hypothesis. They looked at bone density, which can vary across species depending on how much buoyancy an animal needs when diving or foraging. And they found a strong correlation between foraging underwater and bone density. Species who moved underwater to feed had denser bones than those who did not. And Spinosaurus seemed to have this type of dense bone, suggesting that it was likely an underwater forager. In comparison, another Spinosaurid relative for North Africa, Suchomimus, had much less dense bones and was therefore probably more terrestrial and confined to the water's edge. This, of course, has not been the last word on Spinosaurus's lifestyle. Now look, we can keep citing studies back and forth like some kind of paleontological ping-pong match, with each side of the debate either presenting new evidence for their hypothesis or disputing the claims of the other. Like, we've all seen that movie before, right? But ultimately, without more fossils or new methods, we're unlikely to solve the mystery of Spinosaurus. Which is unsatisfying, I know, I get it, I'm there with you. But that's just what being in the middle of the process of science is like. It seems that most researchers now agree that Spinosaurus was some version of semi-aquatic. But there's a lot of disagreement about what exactly that term means. What's really exciting is that its adaptations show that dinosaurs radiated into many niches during the Mesozoic era, including at least to the water's edge. And these are niches that their descendants, aka birds, still occupy today. So whether you think the evidence supports Spinosaurus as a wader or a swimmer, the next time you watch a heron spear a fish in your local pond, you can appreciate watching a relative of one of the weirdest dinosaurs that ever lived. For more dino drama, head to the PBS app or pbs.org to watch Walking with Dinosaurs. 25 years ago, the original Walking with Dinosaurs showed us what life looked like for these incredible animals. And the new Walking with Dinosaurs is doing it again. They used state-of-the-art visual effects and the latest evidence to bring those long-lost creatures back to life. Check out the link in the description to see their stories unfold. And there's no debate about thanking this month's Eontologists. By becoming an Eonite at patreon.com slash eons, you can get fun perks like access to exclusive polls and videos from the Eons team, including Yo. And as always, thanks for joining me in the Ken Barnes studio. Subscribe at youtube.com slash eons for more Mesozoic memoirs. 25 years ago, the original Walking with Dinosaurs showed us what life... I was just going to make a remark about how that show really shaped my childhood, but I was 30 years old. It really shaped that phase of my life when I was having my second child.