Were there aquatic dinosaurs roaming the oceans once upon a time? And if there were, what did they look like? In short, they did exist millions of years ago, and thanks to the fossil record, we have a pretty good idea of what they looked like. From toothier versions of today's dolphins to enormous Komodo-dragon-like sea monsters, these fabulous beasts once ruled the seas.

Did Dinosaurs Live in Water?

Before diving into the prehistoric world of aquatic dinosaurs, it's best to lay one myth to rest. Yes, there were dinosaurs that hunted the seas and lakes, but that's because they could, not because they had to. At one time, it was proposed that certain dinosaurs were so large that they needed to immerse themselves in lakes and other large bodies of water to provide enough buoyancy to stay upright. You've probably seen this in school textbooks and other illustrations, where a brachiosaurus or diplodocus peers out of a primeval lake, munching a mouthful of mossy greens.

This isn't the truest depiction, as explained in The Smithsonian Magazine. Examination of fossil records shows that there's good evidence to support the notion that large dinosaurs did not need to stand armpit deep in water to stay upright. For example, clear fossil footprints track ancient creatures that stomped across soft mud at the banks of rivers and lakes. Footprints gradually became more tip-toe as water buoyancy took over, with the reverse taking place on the opposing bank when the ancient beasts lumbered out.

Another pointer is dinosaur bone density. As The Guardian points out, most fully aquatic animals have dense bones, which help them submerge. Aquatic dinosaurs had much lower density, and some even had air pockets in their bones, which means they would have bobbed around on the surface, unable to stand on the lake bed. Although they didn't have inbuilt snorkels, many dinosaurs did have upward-flaring nostrils that would have been handy for semi-submerged life. However, this still doesn't point toward the creatures being fully aquatic to cope with bulk. Rather, it only serves as evidence that dinosaurs, like other animals, fully exploited the various habitats available to them in their search for food.

So, were there aquatic dinosaurs that did live the entirety of their life submerged?

Spinosaurus, the First Water-Loving Dinosaur?

In April 2020, National Geographic reported the discovery of a water-loving dinosaur in the Moroccan Sahara with a snout like a modern crocodile and conical teeth to keep slippery fish from getting away. The fossilized skeleton, found in what would have been one of the abundant river systems that once covered this modern-day desert region, had several features that showed the cretaceous theropod was well equipped for aquatic life 95 to 100 million years ago.

The spinosaur not only had bones dense enough to help with buoyancy, but when paleontologists uncovered its tail, they found bony struts sticking out from the vertebrae. Reconstruction shows that this gave the spinosaur a tail like an oar, which could be used to propel the aquatic dinosaurs through the water. Furthermore, the articulation between the vertebrae showed that the kind of flexion this enabled was similar to that of other animals that swim.

Plesiosaurs, Still With Us Today?

Another recently uncovered Moroccan Sahara fossil shows that plesiosaurs were also aquatic dinosaurs. Science Direct describes how paleontologists found scattered bone and teeth remnants in the ancient river systems showing that these animals could also move between salt water and fresh water. The fossilized teeth indicate that plesiosaurs not only ate armored river fish but also spent a considerable amount of time living and hunting in fresh water. According to the BBC, plesiosaurs died out around 66 million years ago — but then again, maybe the myths about the Loch Ness Monster are true. (Spoiler alert: Probably not).

Mosasaurus, the Film Star

If you remember a particular film scene from a summer blockbuster about dinosaurs —where a giant water monster erupts from below to grab a great white shark before crashing back and showering the crowd with water, that's the cinematic version of a mosasaurus. The real version was around 56 feet long (just over 17 meters), which is similar in size to a megalodon. The mosasaurus lived during the Cretaceous period, around 145.5 to 65.5 million years ago.

As NewsWeek notes, these weren't truly dinosaurs but reptiles and distant relatives of today's iguana. Science Daily describes fossil skeletons found in Morocco as having a short, wide muzzle and looking like a Komodo dragon with flippers. The fossil records of this species show a shark-like fin tail for moving through water, as well as a set of palatal teeth. This secondary row of huge conical teeth helped the Mosasaurus prevent prey from escaping once they were grabbed. Tooth evidence shows that the mosasaurus ate more than just fish, as damage to teeth and healed fractures in jaw bones show that larger animals were munched as prey, and that the mosasaurus lived a violent life as an ocean apex predator.

Ichthyosaur, the Sea Dragon

An ichthyosaur fossil found in a Rutland Water excavation in the UK shows that the 180-million-year-old sea dragon had a 2-meter (around 6 feet) skull that weighed around one metric ton (over 2,000 pounds). The 10-meter (around 33 feet) specimen is the largest, most complete ichthyosaur fossil skeleton ever found, though The Guardian notes that this species could be anywhere from 1 to 25 meters long. This prehistoric reptile, which was alive during the Jurassic period, became extinct around 90 million years ago.

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