Scientists Discover Fossils Of Giant Sea Lizards That Ruled The Oceans 66 Million Years Ago

Researchers find fossils of giant sea lizards that ruled the oceans 66 million years ago

Artist’s rendering of Thalassotitan atrox. Credit: Andrey Atuchin

Researchers discovered a huge new mosasaur from Morocco, named Thalassotitan atrox, that filled the apex predator niche. With massive jaws and teeth like those of killer whales, Thalassotitan hunted other marine reptiles – plesiosaurs, sea turtles and other mosasaurs.

At the end of the Cretaceous, 66 million years ago, sea monsters really existed. While dinosaurs thrived on land, the seas were ruled by mosasaurs, giant marine reptiles.

Mosasaurs were not dinosaurs, but huge sea lizards that could reach 12 meters in length. They were distant relatives of modern iguanas and monitor lizards.

The mosasaurs looked like a Komodo dragon with fins instead of legs and a tail fin shaped like a shark. Mosasaurs grew larger and more specialized during the last 25 million years of the Cretaceous, taking up niches once occupied by marine reptiles like plesiosaurs and ichthyosaurs. Some have evolved to eat small prey like fish and squid. Others crushed ammonites and clams. The new mosasaur, named Thalassotitan atrox, evolved to prey on all other marine reptiles.

The remains of the new species were unearthed in Morocco, about an hour from Casablanca. Here, towards the end of the Cretaceous, the Atlantic flooded North Africa. Nutrient-rich waters rising from the depths fed the plankton blooms. These fed small fish, fed larger fish, which fed mosasaurs and plesiosaurs, and so on, these marine reptiles becoming food for the carnivorous giant Thalassotitan.

Researchers find fossils of giant sea lizards that ruled the oceans 66 million years ago

Nick Longrich with the mosasaur fossil. Credit: Nick Longrich

Thalassotitan, had an enormous skull measuring 1.4 meters (5 feet long) and grew to nearly 30 feet (9 meters) long, the size of a killer whale. While most mosasaurs had long jaws and fine teeth for catching fish, Thalassotitan had a short, broad snout and massive conical teeth like those of an orca. These allow it to grab and tear apart huge prey. These adaptations suggest that Thalassotitan was an apex predator, sitting at the top of the food chain. The giant mosasaur occupied the same ecological niche as modern-day killer whales and great white sharks.

Thalassotitan’s teeth are often broken and worn, but eating fish would not have produced this kind of tooth wear. Instead, it suggests the giant mosasaur attacked other marine reptiles, chipping, breaking and gnashing its teeth as it bit into their bones and tore them apart. Some teeth are so badly damaged that they have been almost crushed to the root.

Fossilized remains of prey

Remarkably, possible remains of Thalassotitan’s victims have been discovered. Fossils from the same beds show acid damage, teeth and bones being eaten away. Fossils with this particular damage include large predatory fish, a sea turtle, a half-meter-long plesiosaur head, and the jaws and skulls of at least three different mosasaur species. They would have been digested in Thalassotitan’s stomach before he coughed up their bones.

Researchers find fossils of giant sea lizards that ruled the oceans 66 million years ago

Size comparison of Thalassotitan atrox. Credit: Nick Longrich

“It’s circumstantial evidence,” said Dr Nick Longrich, a senior lecturer at the University of Bath’s Milner Center for Evolution and lead author of the study, published in Cretaceous research.

“We can’t say for sure what species of animal ate all those other mosasaurs. But we have the bones of marine reptiles killed and eaten by a large predator.

“And in the same place we find Thalassotitan, a species that fits the profile of the killer – it’s a mosasaur that specializes in preying on other marine reptiles. That’s probably no coincidence.”

Thalassotitan was a threat to everything in the oceans, including other Thalassotitan. The huge mosasaurs bear wounds sustained in heavy combat with other mosasaurs, with facial and jaw wounds sustained in combat. Other mosasaurs exhibit similar injuries, but in Thalassotitan these injuries were unusually common, suggesting frequent and intense fighting for feeding grounds or mates.

“Thalassotitan was an amazing and terrifying animal,” said Dr. Nick Longrich, who led the study. “Imagine a Komodo dragon crossed with a great white shark crossed with a T. rex crossed with a killer whale.”

The new mosasaur lived in the last million years of the age of the dinosaurs, a contemporary of animals like T. rex and Triceratops. Along with recent mosasaur finds from Morocco, this suggests that mosasaurs were not in decline before the asteroid impact that led to the Cretaceous Mass Extinction. Instead, they thrived.

Researchers find fossils of giant sea lizards that ruled the oceans 66 million years ago

Thalassotitan distribution map. Credit: Nick Longrich

Professor Nour-Eddine Jalil, co-author of the article from the Paris Natural History Museum, said: “Phosphate fossils from Morocco provide an unprecedented window into paleobiodiversity at the end of the Cretaceous.

“They tell us how rich and diverse life was just before the end of the ‘age of the dinosaurs’, when animals had to specialize to have a place in their ecosystems. Thalassotitan completes the picture by taking on the role of megapredator at the top of the food chain.”

“There’s so much more to do,” Longrich said. “Morocco has one of the richest and most diverse marine faunas known from the Cretaceous. We are just beginning to understand the diversity and biology of mosasaurs.”

Giant sea lizard fossil shows diversity of life before asteroid impact

More information:
Nicholas R. Longrich et al, Thalassotitan atrox, a giant predatory mosasaurid (Squamata) from the Late Maastrichtian phosphate rocks of Morocco, Cretaceous research (2022). DOI: 10.1016/j.cretres.2022.105315

Dr. Longrich has blogged about the research here:

Provided by the University of Bath

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