Gigantic predatory marine worms that lived about 20 million years ago ambushed their prey by leaping at them from underground tunnels in the sea bottom, new fossils from Taiwan reveal. .

These monster worms may have been ancestors of trap-jawed modern Bobbit worms (Eunice aphroditois), which also hide in burrows under the ocean floor and can grow to be 10 feet (3 meters) long.

Based on fossil evidence from Taiwan, the ancient worms' burrows were L-shaped and measured about 7 feet (2 m) long and 0.8 to 1.2 inches (2 to 3 centimeters) in diameter, researchers recently reported in a new study.

Modern Bobbit worms build long tunnels to accommodate their bodies; they hide inside and then lunge out to snap prey between their jaws, hauling the struggling creature into the subterranean lair for eating.

"We hypothesize that about 20 million years ago, at the southeastern border of the Eurasian continent, ancient Bobbit worms colonized the seafloor waiting in ambush for a passing meal," the study authors reported.

Worms "exploded" from their burrows when prey came close, "grabbing and dragging the prey down into the sediment.

As the ancient worms retreated deeper into their tunnel with the thrashing prey, the struggle agitated the sediment, forming "distinct feather-like collapse structures" that were preserved in the trace fossils.

Though no fossilized remains of the worms were found, the scientists identified a new genus and species, Pennichnus formosae, to describe the ancient animals, based on their burrows' distinctive forms.