Everything we’ve learned about dinosaurs essentially comes from fossils. But million-year-old rocks and bones have left a few hulking gaps in our understanding of the prehistoric world. Dinosaur Mysteries digs into the more secretive side of the “terrible lizards,” and all the questions that keep paleontologists up at night.

DINOSAURS DOMINATED EARTH. We all know the trope. The stupendous reptiles were so numerous and unique that they claimed a 150-million-year-long chunk of Earth’s history as the Age of Dinosaurs

But talking about a single group of organisms “dominating” the planet is silly. For one thing, the only dinosaurs bobbing in the ocean waves were carcasses, washed out by coastal storms.

Oceans have covered the vast majority of our planet for billions of years and contain more than 96 percent of Earth’s water at present. Dinosaurs, so far as we can tell, never made the sea their home. And paleontologists still don’t know why.

If there’s anything more challenging than understanding why a species evolved a particular way, it’s trying to backtrack on the evolutionary roads it didn’t take. Nature is full of invisible barriers and bottlenecks that open and close based on previous change. We usually don’t perceive these biological constraints until we run into a “Why not?” question. And even then, it can be difficult to distinguish between what’s actually impossible and what simply didn’t happen due to coincidence. In the case of the dinosaurs, though, we have a few clues as to why the seas remained beyond their domain.

For the most part, dinosaurs were atrocious swimmers. But it took decades for paleontologists to figure this out as they waited for the right fossil tracks, analyses of dinosaur bone structure, and computer methods capable of estimating the buoyancy of dinosaurs. During much of the 20th century, when experts insulted living reptiles and dinosaurs alike by characterizing the extinct saurians as dimwitted slowpokes, some paleontologists thought long-necked sauropods like Brachiosaurus could only support their weight in water. They also posited that the “duck-billed” dinosaurs, or hadrosaurids, plunged into lakes when tyrannosaurs stalked too near—the only defense herbivores that weren’t covered in armor or horns could have, apparently. Starting in the 1970s, paleontologists realized that fossilized tracks and other clues about the sauropods and duck-bills indicated they lived in terrestrial environments and weren’t adept in water. Not only that, but the relatively few trace fossils made by swimming dinosaurs—scrapes in the sediment from when they kicked their feet—were created by carnivorous dinosaurs, undercutting the idea that water was a refuge for plant eaters. 

A key dinosaurian trait may have prevented the reptiles from getting cozy in the water. The bony respiratory systems of sauropods and theropods show evidence of a unique set of air sacs connected to the lungs and other parts of the respiratory system. These soft-tissue pockets allowed the creatures to breathe more efficiently than mammals by keeping new air constantly flowing instead of relying on distinct inhales and exhales. (Birds have the same feature, with the added benefit that it keeps their skeletons light by filling bony spaces with air.) But when modeling how these air pockets would have affected dinosaurs’ swimming ability, paleontologists found that even large species would have acted like inflatable pool toys—too light for their size to be stable in the water. Adaptations to a life aquatic usually involve denser bones as a form of natural ballast—too much internal air would make dinosaurs work too hard to stay submerged. So much like us, while some dinosaurs could swim, they certainly weren’t diving neck and neck with the prehistoric sea turtles and plesiosaurs.

The same problem comes up for dinosaurs that were once considered skilled swimmers. The sail-backed, roughly 50-foot-long Spinosaurus has a few anatomical hallmarks associated with dipping and diving: Some of its bones seem extra dense, like those of other semiaquatic animals, and its tail is long and eel-esque, like a giant hitched-on paddle. But recent studies have found that Spinosaurus’ airy skeletal structure would have made it unstable in water too, and that the huge sail would have hampered the dinosaur’s ability to chase after prey while submerged. It’s more likely that the creature, once heralded as the world’s first swimming dinosaur, was more of a wader that plodded through the shallows as it tried to ambush fish. While additional evidence might alter the picture, especially because no one has found anything close to a complete Spinosaurus skeleton, for now the dinosaur most closely associated with the water was less aquatic than an alligator.

In all, after more than two centuries of searching, paleontologists have not identified a single dinosaur fossil that definitely spent most of its life at sea. The few specimens dug up from marine sediments—like the beautifully preserved armored Borealopelta from Alberta—represent dinosaurs that perished inland or along the coasts and were washed out to sea by storms or local flooding. Some became food for sharks and marine reptiles; some formed temporary reefs; and some quickly got buried under rock and soil, preserving their scales in place. But there were plenty of other reptiles in the sea—fish-like ichthyosaurs, long-necked plesiosaurs, and mosasaurs that were the ocean equivalent of Komodo dragons—that prove the dominion of dinosaurs was exaggerated. 

Of course, we know that dinosaurs eventually did wander into the water. For example, about 5 million years after the asteroid impact that ended the Cretaceous, the first ancestors of penguins took the plunge. Today, these water-savvy birds “fly” by flapping their wings underwater and sport a variety of adaptations, from hydrophobic feathers to salt-excreting vessels in their bills, that allow them to spend a great deal of their time in the ocean. But they still reproduce on land, shedding yet another clue to why extinct dinosaurs never hit the deep blue.

So far as we know, all dinosaurs laid eggs—from the very first terrible lizard (“dinosaur” translated into Greek) 243 million years ago to the chickadees bouncing around on the sidewalk in the present. Whereas other marine reptiles repeatedly evolved ways to give birth, likely starting with the soft-shelled eggs that some snakes and lizards retain today, dinosaurs don’t seem to have ever evolved a different capability. Or perhaps they did but were so late to the party that the seas were already full of nimble, sharp-toothed reptiles ready to munch on any awkward dino-paddlers. The ancient world of the dinosaurs was one that ended at the shoreline, leaving plenty of space for other creatures to rule the water.

We hope you enjoyed Riley Black’s column, Dinosaur Mysteries. Check back on PopSci+ in May for the next article.