A Penguin-Like Shape May Have Helped This Dinosaur Dive

The only group of dinosaurs still alive today, birds have evolved to feel at home almost everywhere, from digging underground burrows to flying over the oceans for thousands of miles without stopping. And then there are the penguins, who plunge a quarter of a mile deep into icy waters in search of food.

Now it appears that modern birds aren’t the only group of dinosaurs to adopt the dive-and-hunt lifestyle. A team of researchers say they have found the oldest example of an extinct dinosaur with a streamlined body for diving. They described the discovery of the dinosaur in the magazine. Communications Biology Thursday.

The new duck-sized dinosaur was initially overlooked. In 2008, Robin Sissons, then a master’s student at the University of Alberta, was digging in Mongolia’s Gobi desert when he noticed a “string of white shards sticking out of the rock.” After sharing a photo of what he had seen with his colleagues over lunch, they went to the site and eventually transported the fossil to South Korea for further study. (The fossil has since been returned to the Mongolian Academy of Sciences.)

“After preparation in Korea, a complete skeleton came to light,” said Yuong-Nam Lee of Seoul National University, who led the expedition and is an author of the new study. “The specimen was so delicate but beautifully preserved. Instantly, we knew it was something important.”

The researchers found that the fossil had a mouth filled with more than 100 small, sharp teeth and a long, thin neck. These features pointed to a dinosaur that spent time by the water, Dr. Lee said, with teeth clustered at the front of its snout, capable of clinging to slippery, flailing fish.

Similar fossils have also been found in Mongolia, including one called Halszkaraptor escuilliei, and scientists have debated if it was semi-aquatic. But the new dinosaur before Dr. Lee was better preserved than Halszkaraptor, especially in one key area: its ribs, which were slightly flattened and pointed toward the animal’s tail.

“Although the ribcage was not fully preserved, the orientation and shape of the ribs clearly indicate that this animal had a streamlined body, like that of penguins,” Dr. Lee said. This made the ribcage the irrefutable proof: This dinosaur not only ate fish, the researchers said, but also had a sleek body perfect for diving. The name they chose reflects this statement: Natovenator polydontus, “many-toothed swimming hunter.”

If the researchers are right, Natovenator was among the first dinosaurs (other than birds) to dive for their dinner. But some other paleontologists are skeptical.

“I’m always looking for largely aquatic dinosaurs, because I’m the guy on record who says, ‘I think there are more out there,'” said Nizar Ibrahim, a paleontologist at the University of Portsmouth in England who has argued that the giant predator Spinosaurus hunted underwater. But when it comes to adding Natovenator to the ranks of swimming dinosaurs, Dr. Ibrahim said, “I’m still not completely convinced.”

Dr Ibrahim said he could be convinced with “stronger evidence”, such as an in-depth study of the Natovenator’s biomechanical abilities and further comparisons with other aquatic animals, including traits such as bone density. Dr. Ibrahim helped show that it correlates with a animal’s ability to dive.

Lindsay Zanno, a paleontologist at the North Carolina Museum of Natural Sciences, said she’s also interested in seeing more studies. “The time is right, now that we have several animals that we think might be semi-aquatic, to take a closer look at the anatomical correlations with this lifestyle in living and extinct taxa,” she said.

Dr. Zanno noted that a potentially diving dinosaur helps develop scientists’ understanding of how theropod dinosaurs, from Natovenator to T. rex to the pigeon outside your window, became so incredibly diverse, that it is “the question of million”. Answering it may mean undoing paleontology’s tendency to assume that dinosaurs were landlubbers.

“Finding semi-aquatic dinosaurs means that ecological diversity was very high in dinosaurs and could change our prejudice about dinosaur lifestyles,” Dr. Lee said. “More than 30 different lineages of tetrapods have independently invaded aquatic ecosystems. Why not for dinosaurs?