‘Sea monsters’ were real millions of years ago: New fossils tell about their rise and fall

Sixty-six million years ago, sea monsters were real. They were mosasaurs, huge sea lizards that lived at the same time as the last dinosaurs. Mosasaurs grew up to 12 meters in length and looked like a Komodo dragon with fins and a shark-like tail. They were also wildly diverse, developing dozens of species that filled different niches. Some ate fish and squid, some ate crustaceans or ammonites.

utilities we found a new mosasaur prey on large sea creatures, including: other mosasaurs.

The new species, Thalassotitan atroxwas excavated in the Oulad Abdoun Basin of Khouribga Province, an hour outside Casablanca in Morocco.

At the end of the Cretaceous, the sea level was highflooding much of Africa. Ocean currents propelled by the trade winds pulled nutrient-rich bottom water to the surface, creating a thriving marine ecosystem. The seas were full of fish and attracted predators – the mosasaurs. They brought their own predators, the giant Thalassotitan. Nine meters long and with a huge head measuring 1.3 meters long, it was the deadliest animal in the sea.

Most mosasaurs had long jaws and small teeth for catching fish. But Thalassotitan was built very differently. It had a short, broad snout and strong jaws, shaped like those of an orca. The back of the skull was wide to accommodate large jaw muscles, giving it a powerful bite. The anatomy tells us that this mosasaur was adapted to attack and maul large animals.

The massive, conical teeth look like the teeth of killer whales. And the tops of those teeth are chipped, broken and ground down. This heavy wear — not found in fish-eating mosasaurs — suggests Thalassotitan damaged its teeth by biting into the bones of marine reptiles such as plesiosaurs, sea turtles and other mosasaurs.

In the same spot, we found what appears to be the fossilized remains of his victims. The rocks that produce Thalassotitan skulls and skeletons are full of partially digested bones from mosasaurs and plesiosaurs. The teeth of these animals, including those of a half-meter skull of a long-necked plesiosaur, have been partially eaten away by acid. That suggests they were killed, eaten, and consumed by a large predator, which then… spit up the bones. We can’t prove Thalassotitan ate them, but it fits the killer’s profile, and nothing else, making it the prime suspect.

Thalassotitan, which is at the top of the food chain, also tells a lot about ancient marine food chains and how they developed in the Cretaceous.

Evolution of a killer

The discovery of Thalassotitan tells us about marine ecosystems just before the asteroid hit 66 million years ago, ending the age of the dinosaurs.

Thalassotitan was just one of dozens of mosasaur species that lived in the waters off Morocco. Mosasaurs were a fraction of all the thousands of species living in the oceans, but the fact that predators were so diverse implies that the lower levels of the food chain were also diverse, so the oceans could feed them all. This means that the marine ecosystem was not in decline before the asteroid struck.

Instead, mosasaurs and other animals – plesiosaurs, giant sea turtles, ammonites, countless species of fish, mollusks, sea urchins, crustaceans – suddenly died out when the 6 miles wide Chicxulub asteroid hit Earth, launching dust and soot into the air and blocking the sun. The extinction of the mosasaur was not the predictable result of gradual environmental changes. It was the unpredictable result of a sudden catastrophe. Like lightning from a clear blue sky, their end was swift, final and unpredictable.

But the evolution of the mosasaur may also have started with a catastrophe. Curiously, the evolution of the giant carnivorous mosasaurs resembles that of another family of predators – the Tyrannosauridae. The giant T. rex evolved on land around the same time that mosasaurs became apex predators in the seas. Is that a coincidence? Maybe not.

Both mosasaurs and tyrannosaurs begin to diversify and at the same time getting bigger, about 90 million years ago, in the Turonic stage of the Cretaceous. This followed major extinctions on the land and in the sea about 94 million years ago, on the Cenomanian-Turonian border.

These extinctions are associated with extreme global warming – a “super greenhouse climate” – driven by volcanoes that release C02 into the atmosphere. In the aftermath, giant predatory plesiosaurs disappeared from the seas and giant allosaurid predators were wiped out on land. With the predator niches empty, mosasaurs and tyrannosaurs moved to the upper predator niche. Although wiped out by a mass extinction, Thalassotitan and T. rex evolved only because of a mass extinction in the first place.

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 the higher up the food chain you go. It takes many small fish to feed a large fish, many large fish to feed a small mosasaur, and many small mosasaurs to feed one giant mosasaur. That means apex predators are rare. And apex predators need a lot of food, so they get in trouble if the food supply is disrupted.

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

It is this sensitivity to environmental changes that makes predators like Thalassotitan so interesting for studying extinctions. They suggest that being an apex predator is a risky evolutionary strategy. Over short timescales, evolution drives the evolution of larger and larger predators. Their size allows them to compete for and take down prey. But over long timescales, specialization for the apex predator niche increases vulnerability to disaster. Ultimately, a mass extinction wipes out the apex predators and the cycle begins again.

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