If you went diving between 541 million and 252 million years ago, the apex predators of today would pale in comparison to the creatures you’d find roaming the Paleozoic Era seas.

During this historic time period, arthropods, or animals with exoskeletons like crustaceans and spiders, were some of the largest animals on Earth. In the oceans around Australia, these creatures took the form of giant sea scorpions called Eurypterids.

A new study by the University of New England and the Australian Museum takes a closer look at these nightmarish creatures, which could grow more than eight feet long and are actually the cousins of modern scorpions.

Fossil records show these creatures preyed the way that great white sharks do today, earning them a top spot on the food chain. They were good swimmers and had powerful clawed front limbs to snatch food. Then, they’d crush their prey using teeth-like structures on their legs called gnathobasic spines.

With all the unique creatures endemic to Australia, giant sea scorpions may come at no surprise. But the origin stories of these predators have been a bit of a puzzle.

The first recorded specimen was a part of an exoskeleton that was found in Melbourne in 1899.

“Prior to our new research examining the completeness of the group in Australia, there were about 10 records — and only one other attempt to pool everything together,” say the lead authors of the study, Russell Dean Christopher Bicknell and Patrick Mark Smith, in an article for The Conversation. “As such, the diversity and spread of these fossils was fairly uncertain.”

Related Reading: What to Know About the Fourth Global Coral Bleaching Event

The scientists traveled around to different Australian museums and had fossils sent to their labs at the University of New England, discovering many uncovered sea scorpion fossils in the process.

“As a result, we now have evidence of a possible six different groups that existed in Australia,” Bicknell and Smith say.

However, there is still much unknown about these prehistoric predators. Future research, the scientists say, will involve visiting sites where the fossils were originally found and trying to locate more complete specimens.

“Not only will this help document Australian sea scorpion species better,” the researchers say, “it will also allow for a more complete understanding of the environments in which they lived.”