Early offshoots of an evolutionary line that led to modern crustaceans, the so-called anomalocaridids looked sort of like modern shrimp or cuttlefish. But the fossil creatures had spiny limbs sprouting from their heads and circular, plated mouths, which opened and closed like the diaphragm of a camera.

Previous anomalocaridid fossils had shown the animals grew to perhaps 2 feet (0.6 meter) long, which already would have made them the largest animals of the Cambrian period (542 to 501 million years ago)—an evolutionarily explosive time, when invertebrate life evolved into many new varieties, such as sea lilies and worms.

(See "Earliest Animals Were Sea Sponges, Fossils Hint.")

But at a foot longer than previous specimens, the largest of the new anomalocaridids