Swarms of up to a thousand giant trilobites—extinct marine arthropods such as this 35-inch-long (90-centimeter-long) fossil specimen—roamed shallow prehistoric seas, new fossils show.

The 465-million-year-old fossils, found recently in northern Portugal, are of the largest trilobites ever discovered.

The trilobites may have clustered to mate and molt—shedding old exoskeletons as new ones grew in—as well as avoid predators, scientists say.

The benefits of swarming may explain why these distant relatives of horseshoe crabs were among the most widespread arthropods of the Paleozoic era (542 to 251 million years ago).

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Giant fossil sea monster found in the Arctic and known as “Predator X” had a bite that would make T-Rex look feeble, scientists said Monday.

The 50 ft (15 meter) long Jurassic era marine reptile had a crushing 33,000 lbs (15 tonnes) per square inch bite force, the Natural History Museum of Oslo University said of the new find on the Norwegian Arctic archipelago of Svalbard.

“With a skull that’s more than 10 feet long you’d expect the bite to be powerful but this is off the scale,” said Joern Hurum, an associate professor of vertebrate paleontology at the museum who led the international excavation in 2008.

“It’s much more powerful than T-Rex,” he said of the pliosaur reptile that would have been a top marine predator. Tyrannosaurus Rex was a top land carnivore among dinosaurs.

The scientists reconstructed the predator’s head and estimated the force by comparing it with the similarly-shaped jaws of alligators in a park in Florida.

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