‘Sea Monsters’ Were Real Millions of Years Ago: New Fossils Tell Their Rise and Fall

Sea monsters really existed 66 million years ago. They were mosasaurs, giant marine iguanas that lived at the same time as the last dinosaurs. The mosasaurs, up to 12 meters long, looked like a Komodo dragon with fins and a shark-like tail. They were also very diverse, developing dozens of species that filled different niches. Some ate fish and squid, others ate shellfish or ammonites.

Now we’ve found a new Mosasaurus that hunts large sea creatures, including Miscellaneous mosasaurs.

The new species, Thalassotitan atrox, was unearthed in the Oulad Abdoun Basin of Khouribga Province, an hour outside of Casablanca in Morocco.

At the end of the Cretaceous, sea levels were high and flooded much of Africa. Ocean currents, driven by the trade winds, drew nutrient-rich groundwater to the surface and created a thriving marine ecosystem. The seas were teeming with fish, attracting predators – the mosasaurs. They brought their own predators, the giant thalassotitan. Nine meters long and with a massive 1.3 meter long head, it was the deadliest animal in the sea.

Most mosasaurs had long jaws and small teeth for catching fish. But thalassotitan was constructed quite differently. It had a short, broad snout and strong jaws shaped like those of a killer whale. The back of the skull was wide to attach large jaw muscles, giving it a powerful bite. Anatomy tells us that this mosasaur was adapted to attack and tear apart large animals.

The massive, conical teeth resemble the teeth of orcas. And the tips of those teeth are chipped, broken and ground down. This severe wear — not found in fish-eating mosasaurs — suggests that thalassotitan damaged its teeth by biting into the bones of marine reptiles such as plesiosaurs, sea turtles, and other mosasaurs.

In the same place, we found what looks like the petrified remains of his victims. The rocks that produce thalassotitan skulls and skeletons are full of partially digested mosasaur and plesiosaur bones. The teeth of these animals, including the half-meter-long skull of a long-necked plesiosaur, were partially eaten away by acid. This suggests they were killed, eaten and digested by a large predator, which then spat out the bones. We can’t prove Thalassotitan ate them, but it fits the killer’s profile and nothing else makes him the prime suspect.

Thalassotitan, being at the top of the food chain, also tells a lot about ancient marine food chains and how they evolved in the Cretaceous period.

development of a killer

The discovery of thalassotitan tells us about marine ecosystems just before the asteroid struck 66 million years ago and ended the age of dinosaurs.

Thalassotitan was just one of a dozen mosasaur species to inhabit the waters off Morocco. Mosasaurs made up a fraction of all the thousands of species that inhabited the oceans, but the fact that predators were so diverse implies that the lower levels of the food chain were also diverse, allowing the oceans to feed them all. This means that the marine ecosystem was not in decline prior to the asteroid’s impact.

Instead, mosasaurs and other animals — plesiosaurs, giant sea turtles, ammonites, myriad species of fish, molluscs, sea urchins, crustaceans — thrived and then suddenly died out when the 10-kilometer-wide asteroid Chicxulub crashed into Earth, hurling dust and soot into the air and blocks the sun. The mosasaur extinction was not the predictable result of gradual environmental changes. It was the unpredictable result of a sudden catastrophe. Like a bolt from the blue, her end came quickly, definitively, unpredictably.

But the evolution of the mosasaurs could also have started with a catastrophe. Curiously, the evolution of the giant carnivorous mosasaurs is similar to that of another family of predators – the tyrannosaurs. The giant T. rex evolved on land around the same time that mosasaurs became the top predators in the seas. Is that a coincidence? Maybe not.

Both mosasaurs and tyrannosaurs begin to diversify and grow simultaneously in size around 90 million years ago in the Turonian stage of the Cretaceous. This followed large land-sea extinctions about 94 million years ago on the Cenomanian-Turonian border.

These extinctions are associated with extreme global warming – a “supergreenhouse” climate – fueled by volcanoes releasing CO2 into the atmosphere. As a result, giant predatory plesiosaurs disappeared from the seas and giant allosaur predators were wiped out on land. As predator niches remained vacant, mosasaurs and tyrannosaurs moved to the top predator alcove. Although wiped out by a mass extinction, Thalassotitan and T. rex only evolved through a mass extinction.

The bigger they are, the harder they fall

Top predators are fascinating because they are large, dangerous animals. But their size and position at the top of the food chain also make them vulnerable. You have fewer animals as you move up the food chain. It takes a lot of small fish to feed a big fish, a lot of big fish to feed a small mosasaur, and a lot of small mosasaurs to feed a giant mosasaur. That means top predators are rare. And apex predators need a lot of food, so they get in trouble when the food supply is cut off.

As the environment deteriorates, dangerous predators can quickly become endangered species.

It’s this sensitivity to environmental changes that makes predators like thalassotitan so interesting for studying species extinction. They suggest being a top predator is a risky evolutionary strategy. Over short periods of time, evolution drives the development of larger and larger predators. Their size means they can compete for prey and defeat it. But over long periods of time, specialization in the apex predator niche increases vulnerability to disaster. Eventually, a mass extinction wipes out the top predators and the cycle begins again.

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