Almost half a billion years ago, the largest animal on Earth was a 2-meter-long, helmet-headed sea creature that fed on some of the ocean’s tiniest prey. The newly described species is one of the largest arthropods yet discovered, a class of animals that includes spiders and crabs. The well-preserved remains of the multisegmented creature are providing clues about how subsequent arthropods’ legs may have evolved from the dozens of stubby flaps used to propel this beast through the water.

Fossils of the ancient leviathan were unearthed from 480-million-year-old rocks exposed on a hillside in southeastern Morocco. Besides a handful of relatively complete remains, researchers have recovered about 50 fragments that came from molted exoskeletons or decomposing carcasses before they were buried by sediment, says Peter Van Roy, a paleobiologist at Yale University. The largest nearly complete specimen measures about 1.3 meters long but likely would have stretched 1.6 meters if intact, he notes. Based on the sizes of isolated fragments, though, some of the creatures probably were about 2 meters long. It was likely the largest creature on Earth at the time, and only two other types of arthropods ever rivaled or exceeded it in size, the researchers report online today in Nature.

Van Roy and his colleagues dubbed the new species Aegirocassis benmoulae; Aegir is the god of the sea in Norse mythology, cassis is the Latin word for helmet, and benmoulae honors the Moroccan collector who first discovered fossils of the creature. Aegirocassis is a member of a group of creatures called anomalocaridids, a name derived from the Latin words for “strange shrimp.” Most known anomalocaridids were formidable predators, Van Roy says. But Aegirocassis, like a smaller and much older species of anomalocaridid, described last year, was a filter feeder that sifted millimeter-sized creatures—possibly including tiny crustaceans or the larvae of other marine organisms—from the water as it swam.

“Filter feeding and gigantism are associated, which is a pattern we see in different groups of animals across the tree of life,” says Gregory Edgecombe, a paleobiologist at the Natural History Museum in London, who wasn’t involved in the research. Just think of the filter-feeding leviathans swimming in today’s seas: The blue whale is the largest animal on Earth, tipping the scales at up to 190 metric tons despite feeding only on small free-swimming crustaceans, including the thumb-sized or smaller creatures called krill. Likewise, Van Roy and his colleagues note, in past eras several species of fish and sharks have evolved to huge proportions by directly exploiting the vast bounty at the base of the ocean’s food chain.

The new fossils also fill a critical gap in arthropod evolution, Van Roy says. In particular, they provide key insights into the evolution of arthropod limbs. Aquatic arthropods—today’s crustaceans such as shrimp, crabs, and lobsters as well as ancient creatures such as trilobites and sea scorpions—have limbs with two branches, one of which bears weight and another that is typically adorned with gills. Large gaps in the fossil record have previously obscured the origins of this configuration, but Aegirocassis provides the first look at how water-dwelling arthropods’ two-branched legs may have evolved. “This species is a very important intermediate, a transitional form,” says Javier Ortega-Hernández, a paleobiologist at the University of Cambridge in the United Kingdom, who wasn’t involved in the research. Although anomalocaridids died out (and thus aren’t the direct ancestors of any living arthropod lineage), they likely shared many anatomical features with close relatives living at the time that ended up on the surviving branches of life’s family tree, he explains.

Many Aegirocassis fossils that the team analyzed were preserved in three dimensions rather than being squished flat by accumulating sediments, as many previously described anomalocaridids had been. That enabled Van Roy and his team to see that each of the creature’s 11 segments had two flaps on each side—both of which helped propel the beast through the water, but the upper flap also seems to have been associated with the creature’s gills. Rather than undulating its entire body, Aegirocassis probably moved its lower flaps up and down in sequence, similar to the way a cuttlefish undulates its fleshy mantle (or sports fans repeatedly stand and sit as they “do the wave” in a stadium).

Through time and over generations, Van Roy and his colleagues suggest, arthropod species evolved such that the upper and lower flaps fused into one structure, with a stubby base and two branches growing from it. One branch came to bear the weight of walking whereas the other retained the gills. Many of today’s land-dwelling arthropods have legs with only one branch, having lost the need for gills as they developed other methods of breathing, he notes.

The 3D nature of the Aegirocassis fossils “made it easier to visualize what structures are on the upper surface versus the lower surface” of the creature, Edgecombe says. With previous teams having only strongly flattened fossils to analyze, he notes, “some really basic questions about how the body of anomalocaridids is arranged have had us chasing our tails for years.”