Heavy Bones Helped Some Spinosaurs Swim

Spinosaurs broke all the rules. Where most carnivorous dinosaurs had knife-like, curved teeth, spinosaurs had conical chompers more like those of crocodiles. Even the snouts of these dinosaurs were long, giving them a crocodile-like look, instead of the familiar profiles of Tyrannosaurus and Allosaurus. Some species even had prominent sails growing from their backs, ornate biological billboards that acted as social signals. But that’s hardly all. Some spinosaurs – including the immense Spinosaurus itself—were the only non-avian dinosaurs known to split their time between land and water.

The idea that spinosaurs spend a great deal of their time at the water’s edge isn’t brand new. Since the discovery of Baryonyx in 1986, which included fish in the dinosaur’s gut contents, paleontologists have speculated that spinosaurs lived along the margins of ancient rivers and lakes. Then, starting in 2015, paleontologists began to document broad-splayed feet, a paddle-like tail and other traits of the Cretaceous celebrity Spinosaurus that indicated that this dinosaur was an accomplished swimmer—including dense bones that would help Spinosaurus maintain neutral buoyancy in the freshwater habitats of prehistoric Egypt and Morocco. Now, as reported in the journal Nature, Field Museum of Natural History paleontologist Matteo Fabbri and colleagues have discovered that Spinosaurus wasn’t the only spinosaur that was so adept in the water. A smaller species named Baryonyx was likely a skilled swimmer, too, which opens new questions about how and when these dinosaurs evolved their aquatic abilities.

The inspiration for the study partially came from a museum visit. “I remember I was going around the zoological exhibition of the Museum La Specola in Florence and I noticed a beautifully mounted skeleton of a hippopotamus,” Fabbri says. “I was shocked to see how an animal that we are used to seeing in aquatic environments doesn’t have striking skeletal traits suggesting such ecology,” he notes. That got Fabbri wondering if non-avian dinosaurs, like the spinosaurs, had hidden aquatic traits, too.

Paleontologists have had a difficult time discerning which, if any, dinosaurs were skilled swimmers. During much of the 20th century, experts believed that herbivores such as the long-necked Brachiosaurus and the duckbilled Parasaurolophus wallowed in the water to take refuge from carnivores. But no positive evidence could be found to back up this claim. Later, paleontologists realized that many carnivorous dinosaurs were capable swimmers and even left swim tracks—scratches into sediment that document their strokes as they crossed rivers and lakes. But it’s one thing for a dinosaur to briefly swim and quite another for a dinosaur to have aquatic adaptations, or parts of their anatomy directly related to moving around in the water.

One of the most common aquatic adaptations in creatures that split their time between land and shore is denser bones. While trying to cross a river and lake, being able to easily float is an advantage. But for creatures that want to move or hunt beneath the surface, buoyancy can be a major problem. The more buoyant an animal, the more energy they have to spend getting under the water and swimming below the surface, which shortens the amount of time they can forage. But denser bones act as a sort of natural ballast, making amphibious vertebrates just heavy enough that they can hang in the water column, neutrally buoyant, instead of sinking or struggling to stay below. Anatomists known this condition as osteosclerosis. Many ancient whales that walked along ancient shores had bone ballast, for example, and the connection between bones and a more aquatic way of life has been well-established in other animals.

Spinosaurus was found to have dense bones, too, and so Fabbri and coauthors wondered about whether other spinosaurs might have similar adaptations that paleontologists just hadn’t looked for yet. The researchers sampled the bones of 380 specimens representing both living and extinct animals, including 36 thigh bones and 12 ribs from non-avian dinosaurs ranging from Spinosaurus to the long-necked Alamosaurus.

Consistent with previous results, Spinosaurus had dense bones and was most similar to semi-aquatic animals like early whales and hippos. But two other spinosaurs came out in different places. Baryonyx from prehistoric England had relatively dense bones—not quite as compact as Spinosaurus, but still heavy and suitable for swimming at depth—while Suchomimus from Niger had bones most similar to land-dwelling predators like T. rex. This “crocodile mimic” dinosaur was more comfortable strutting around on land than doing the dino doggy-paddle.

“We were surprised and excited to see that different spinosaurs had different ecologies,” Fabbri says.

The fact that Baryonyx and Suchomimus were of similar size and anatomy but had such divergent bone densities was, University of Maryland paleontologist Thomas Holtz, Jr. says, “a bit of a shock.” And the difference between the species raises questions about the pattern of evolution among these animals. Did spinosaurs initially evolve to be aquatic predators and Suchomimus evolved to do something different, or did spinosaurs became suited to the water over and over again in different times and places? For now, the answer to that puzzle is still unknown. What’s clear is that these dinosaurs weren’t just carbon copies of each other but represent different ways of living even if they superficially look similar. The key to telling the difference is recorded in the dinosaurs’ bones.

“They make a strong case based on a wide selection of modern and fossil animals that osteosclerosis is a trait for animals which spend a fair amount of time submerged,” says Holtz, who was not involved in the new study. The spinosaurs with denser bones had a swimming advantage, spending more energy on moving than trying to stay below. What’s challenging, he notes, is reconciling some other dinosaur traits with their aquatic abilities. “The orientation of their skulls and morphology of their necks is still more consistent with vertical striking,” like a heron fishing from the shallows, Holtz notes, than catching prey underwater like crocodiles do. Being great at swimming doesn’t necessarily mean that these immense dinosaurs were true crocodile mimics.

Nevertheless, the new results indicate that dinosaurs inhabited a broader array of ecological roles than experts knew even fifty years ago. “When we saw the results for the first time,” Fabbri says, “we understood how far we are from actually understanding these animals and how we are probably underestimating the ecological breadth of the non-avian dinosaurs.” After all, Baryonyx was the first well-known spinosaur and was only uncovered less than half a century ago. Experts are just beginning to get to know these enigmatic dinosaurs. “The more we learn about spinosaurs,” Holtz says, “the more puzzling they become.”

The new study may even allow other hidden dinosaur lifestyles to be drawn out. Most dinosaurs are known only from partial skeletons or are known as composites of various elements from the same species. It’s rare to get a look at the complete skeleton of a non-avian dinosaur, and that makes ascertaining a dinosaur’s lifestyle all the more challenging. But in this case, Fabbri notes, bone density seems to be a good indicator of whether a dinosaur primarily lived on land or had swimming adaptations. “Aquatic adaptations can be inferred from fragmentary skeletons or even single bones,” Fabbri says, the secrets knitted inside a dinosaur’s bone speaking volumes to how these terrible lizards truly lived.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.