Could dinosaurs swim? A new fossil revives an age-old debate.

For decades, paleontologists had defaulted to thinking of dinosaurs as land-lubbers, with other groups of ancient reptiles taking to the water. New fossil evidence is reopening the question of whether ancient dinosaurs could live on both land or water. This is similar to how many modern creatures do. Researchers have been unable to determine whether dinosaurs were “semiaquatic”. There are very few fossils of dinosaurs that clearly show evolution creating bodies for water.

Now, a new study offers another clue: a toothy relative of Velociraptor more than 68 million years old that shows signs of the streamlining seen in the rib cages of diving birds. Unveiled in the journal Communications Biology, the southern Mongolian fossil–known as Natovenator polydontus after the Latin and Greek for “many-toothed swimming hunter”–marks the first time that this trait has ever been seen in dinosaurs outside of birds and their closest extinct relatives.

In many creatures, the ribs jut out from the backbone at roughly 90deg angles, but in modern diving birds, such as penguins and cormorants, the ribs angle down toward the animals’ tail ends. The ribs can be angled to narrow the animals’ midsections, measured from the belly to the back. This trait is thought to help streamline the animals as they swim through the water. And it’s this kind of adaptation that scientists see in the fossil of Natovenator, which includes a well-preserved partial ribcage.

Many of today’s dinosaurs, which we know as birds, thrive at or beyond the water’s edge. Some of their ancient forebears did too, such as the marine proto-birds Ichthyornis and Hesperornis. Birds are only one branch of the dinosaur family tree. There is little evidence that non-avian dinosaurs have ever been seen in rivers and lakes. Natovenator is one example of a dinosaur that has skeletons that can swim. This shows that dinosaurs lived in more diverse habitats than previously thought.

In this vein, Natovenator sheds light on “how diverse dinosaurs’ ecological position was,” says study coauthor Yuong-Nam Lee, a paleontologist at South Korea’s Seoul National University.

Swimmers, waders, and everything in between

Over the past decade, scientists have grown to suspect that at least one group of dinosaurs had close ties to the water: the spinosaurids. Multiple lines of anatomical evidence and chemical evidence suggest that these predators were more aquatically inclined to fish than other predatory dinosaurs.

Some studies have gone so far as to argue that the biggest member of this family, the school bus-sized Spinosaurus, was a “river monster” that spent much of its time in the water. These claims have been challenged over the years, most recently in a study published yesterday in the journal eLife argues that Spinosaurus was more of a wading ambush predator than a swimming fish-chaser.

A 2017 study described another potential swimmer: Halszkaraptor, an oddball skeleton from Mongolia. Although the creature was a dino-subgroup including Velociraptor, it had a long, clearly goose-like neck. What’s more, its snout–which was riddled with holes for nerves–suggested that it had a great facial sense of touch like modern crocodiles, which can detect prey’s movement in water.

The original team that described the fossil suggested that it might have been a semiaquatic swimmer, similar to a modern duck or goose. Over the years, though, some scientists have pushed back on whether Halszkaraptor was semiaquatic, in part because the fossilized rib cage was not preserved well enough to discern the crucial traits.

The new study of Natovenator helps clear up the Halszkaraptor picture, finding that the two creatures were close relatives. As a result, it seems likely that the ribcage of Halszkaraptor was similarly streamlined, which strengthens the claim that the strange little dinosaur really did have strong ties to the water.

Halszkaraptor “certainly looked like a raptor trying to be a goose … but that was a sample size of one,” says University of Maryland paleontologist Tom Holtz, who reviewed the new Natovenator study before publication. “Obviously, more data points would be better–and here comes another.”

A long-necked, toothy hunter

Natovenator came to light during the 2008 Korean-Mongolian International Dinosaur Expedition, a 27-person international team that found nearly 200 fossils across Mongolia. On August 26 of that year, team member Robin Sissons saw bones poking out of the rocks of Hermiin Tsav, a fossil-rich cliffside in southern Mongolia that’s roughly 68 million to 75 million years old. Although she didn’t know the name of the skeleton, she covered it with protective plaster so that she could return to the lab.

Sissons’s find and the expedition’s other fossils were shipped to South Korea for preparation and were later returned to Mongolia. Preparators carefully removed any excess rock from the fossil and revealed a beautifully preserved dinosaur: one with a very long neck, and a skull filled with tiny teeth. Lee says, “Instantly we realized it was important.”

The fossil of Natovenator was one of hundreds that Lee’s team had collected in Mongolia over the years, so preparation took a long time, off-and-on. Then a major announcement helped make sense of the slowly emerging fossil: the 2017 unveiling of Halszkaraptor.

Deeper dives ahead

If the energetic debates over Spinosaurus are any precedent, it’s possible that Natovenator will face its share of controversy. Paleontologist Kiersten Formoso is a University of Southern California Ph.D. candidate. In her view, Natovenator‘s small size makes its semiaquatic traits an easier sell than those seen in the gigantic Spinosaurus. One, she said that Natovenator’s small size makes it easier to sell than the semiaquatic traits of the huge Spinosaurus. Paleontologist Matteo Fabbri, a researcher at the Field Museum of Natural History in Chicago, recommends that future studies look at the bone density of Natovenator, given that its ribcage resembles those of modern diving birds such as penguins and cormorants. In a March study, a team led by Fabbri showed that penguins, hippos, and other modern animals that often forage underwater tend to have dense bones that act as ballast. That study also found that Halszkaraptor had hollow bones unlike penguins–and that Spinosaurus and its cousin Baryonyx had dense bones consistent with spending a lot of time in the water.

What is already clear, though, is that is that in eking out a living by the water, Natovenator followed a different evolutionary path than many of its relatives. For Holtz, the University of Maryland paleontologist, Natovenator serves as a powerful reminder that closely related species followed wildly different paths, just as today’s bone-crunching spotted hyenas are close relatives of the insect-munching aardwolves. “Just because you’re a dromaeosaur doesn’t mean that you are committed to being a Velociraptor-like predator,” he says. “They’re all part of the same big family–but [pursuing] adaptive trends on different lines.”