The bizarre animal with spiny, grasping head appendages, body blades and jointed legs thrived in the oceans 542 to 472 million years ago.
The world's first big predators grew larger, and existed for much longer, than previously thought.
These predators, known as anomalocaridids, possessed spiny head limbs, blade-like back filaments and plates around the mouth.
The world's earliest known large predators shoved prey into their mouths using long spiny limbs protruding from their heads and grew larger than previously thought, suggests a new fossil recently discovered in Morocco.
The more than 3-foot-long animal, described in the latest issue of Nature, was part of an unusual group of marine predators called anomalocaridids. They were the largest animals of the Cambrian period, known for its "Cambrian Explosion" that 540-500 million years ago resulted in the sudden appearance of all major animal groups.
It was previously thought that anomalocaridids died out at the end of the Cambrian, but the new fossil proves these sturdy hunters lived millions of years more, well into the Ordovician Era.
"This is important because, until now, it was believed that these animals had disappeared 30 million years earlier," co-author Peter Van Roy told Discovery News. "This new evidence shows that these giant predators continued to dominate the food chain as apex predators for a much longer time than had been thought."
Co-author Derek Briggs, director of the Yale Peabody Museum of Natural History, added, "These giant invertebrate predators and scavengers have come to symbolize the unfamiliar morphologies displayed by organisms that branched off early from lineages leading to modern marine animals, and then went extinct. Now we know that they died out much more recently than we thought."
Van Roy, formerly of Yale and now at Ghent University in Belgium, and Briggs analyzed the new Moroccan fossil, which is part of a treasure trove of animal remains dating to the Ordovician period from 488 to 472 million years ago.
The researchers admit that, to modern eyes, anomalocaridids are "very strange-looking animals."
"They have a head with stalked, probably compound eyes, and a pair of segmented, flexible spinose limbs at the front of the head," Van Roy explained. "These limbs most likely functioned for the capture and manipulation of prey, and to transfer the food to the mouth. The mouth itself is surrounded by a set of moveable toothed plates set in a circle."
These toothed plates likely crushed and tore apart prey, which consisted of soft-bodied smaller animals and possibly some small harder organisms, like trilobites.
The Moroccan specimen's body was elongated and segmented, with blade-like filaments across the back. The researchers think these might have functioned as gills.
All anomalocaridids were fast and agile, but probably hunted using different strategies. Some might have laid in wait while partially buried in mud at the sea floor, similar to modern cuttlefish, while others either patrolled the seafloor bottom or were more adapted for long distance swimming.
Since it's now known that these marine predators persisted into the Ordovician, their actual extinction may have been due to the rise of other hunter groups that evolved better survival adaptations.
Scientists can make that educated guess because the anomalocaridids' spot at the top of the food chain was later taken over by predators like eurypterids, which looked like scorpions and are related to today's horseshoe crabs, and nautiloids. The latter is a group of mainly extinct marine mollusks that includes the spiral-shelled nautilus.
Although no anomalocaridids are alive today, the world is now full of their distant relatives. These include animals like crabs, scorpions, spiders, centipedes millipedes, lobsters, insects and other members of what is now the planet's largest animal group: the arthropods.