A creature resembling a human-sized lobster that roamed the Earth's oceans millions of years ago is providing new insights into the evolution of modern arthropods such as shrimp, ants and beetles.

The fossilized remains of an anomalocaridid (Aegirocassis benmoulae), measuring about two metres long, was unearthed in Morocco several years ago. Paleontologists have since spent hundreds of hours chipping away at the rock it was found in, slowly uncovering more details about its anatomy.

"It's an amazing specimen," Dr. Allison Daley, a paleontologist at Oxford University and Canadian co-author of the report in Nature Magazine, told CBC Radio's Quirks and Quarks.

That's because it's three-dimensionally preserved, allowing researchers to get a better idea of the creature's shape. Most fossils as old as these have been flattened after being preserved and compressed in rock over tens (or hundreds) of millions of years, giving us only a two-dimensional view of their bodies.

"It was a monster for its time — pretty much the biggest thing around," Daley said.

Filter Feeder lateral view

A lateral view of the anomalocaridid fossil, showing a pair of flaps that bear some similarities to the limbs of modern arthropods such as shrimps. (Peter Van Roy/Yale University)

It lived during the Early Ordovician period, about 480 million years ago. That would make it one of the largest and oldest living arthropods ever discovered, dwarfing the modern, smaller crustaceans and insects that share its evolutionary lineage.

“It’s fair to say I was in shock at the discovery, and its implications,” said Peter Van Roy, co-author on the research and a research scientist at Yale University.

“It once and for all resolves the debate on where anomalocaridids belong in the arthropod tree, and clears up one of the most problematic aspects of their anatomy.”

Gentle giant ate sea plankton

The new look revealed two sets of swimming flaps on either side of the body, possibly filling in a gap in the evolutionary path to modern arthropods such as shrimp.

Unusually, the creature survived by feeding on tiny organisms known as plankton in the ocean, filtering them out of the water with appendages on its head, much as modern whales do.

There was a greater volume and diversity of plankton in the oceans 480 million years ago, allowing such a large creature to subsist.

Most anomalocaridids who existed around this era (or at least within a few million years of it) were apex predators. Their head appendages were more like spiny claws that could catch larger prey.

“I’d love to swim alongside this guy, because I'd know he wasn’t going to try to eat me,” joked Daley.

You can listen to the full interview with Dr. Allison Daley on CBC Radio’s Quirks and Quarks this Saturday at noon.