For decades, paleontologists had defaulted to thinking of dinosaurs as land-lubbers, with other groups of ancient reptiles taking to the water. But increasingly, new fossil evidence has reopened the question of whether some ancient dinosaurs could thrive on both land and water in some form or fashion, like many modern creatures do. Whether dinosaurs were some degree of “semiaquatic” has continued to vex researchers, as so few dinosaur fossils have skeletal features that unambiguously show evolution sculpting the body 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 90° angles, but in modern diving birds, such as penguins and cormorants, the ribs angle down toward the animals’ tail ends. Angling the ribs can narrow animals’ midsections as measured from the back to the belly. 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 living dinosaurs—we know them as birds—thrive at and beyond the water’s edge. Some of their ancient forebears did too, such as the marine proto-birds Ichthyornis and Hesperornis. However, birds are just one branch of the dinosaur family tree, and evidence for non-avian dinosaurs dipping into lakes and rivers has been scarce. Finding dinosaurs such as Natovenator that have skeletons built for swimming means that dinosaurs’ habitats and lifestyles were more diverse than once 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. These predators clearly were more aquatically inclined than other predatory dinosaurs were, with multiple lines of anatomical and chemical evidence suggesting that they at least occasionally stalked shorelines and went after fish.
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. Though the creature was a dromaeosaur—the dino-subgroup that includes Velociraptor—it had a long, decidedly 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.
At the time, the team that first described the fossil proposed that its strange features possibly made it a semiaquatic swimmer like a modern goose or duck. 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. Though she didn’t know what the skeleton was, she coated it in protective plaster for a trip back 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. As preparators carefully scraped away excess rock from the fossil, a beautifully preserved dinosaur began to emerge: one with a very long neck and a skull packed with tiny teeth. “Instantly, we realized it was something important,” Lee says.
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. But paleontologist Kiersten Formoso, a Ph.D. candidate at the University of Southern California, isn’t so sure. In her view, Natovenator’s small size makes its semiaquatic traits an easier sell than those seen in the gigantic Spinosaurus. For one, she says, “when you’re tinier, you’re not diving with huge lungs full of air.”
The announcement of Natovenator is only the beginning of the work to place it in the dinosaur world and increase our understanding of swimming dinosaurs. 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.”