These ancient giant worms stalked fish from their burrows

These ancient giant worms stalked fish from their burrows

Science

About twenty million years ago, the shores of northern Taiwan were riddled with burrows from which worms nearly two meters long sprang up to prey on fish. A team of researchers recently described 319 traces of these carnivorous marine worms, whose dens fossilized in Miocene mud.

The Bobbit worm (Eunice aphroditois) is a species of marine worms in the Eunicidae family that can grow up to three meters in length. His hunting technique is as follows: he conceals most of his slender body in long, narrow burrows dug in the seabed, then watches for any passers-by. If an unlucky fish gets too close, the Bobbit detects a disturbance outside its lair and springs up in a flash, grabs its prey and backs it up to its burrow to devour it. At the same time, the sand crumbles above the entrance, obscuring neither seen nor known from “the eyes of the world” the gruesome scene.

A recent study published in the journal Scientific Reports details a potential ancestor of the Bobbit with the same predatory techniques. It evolved in the seabed of the Eurasian continent about twenty million years ago.

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More than 300 traces preserved

Fossil tracks are remnants of the creatures that made them: footprints and other traces of movement, not the fossil remains of the creatures themselves. These worms, presumably composed of soft tissue, likely deteriorated shortly after death.

The study here examines a large burrow recreated from 319 specimens preserved in what was the seabed of northeast Taiwan during the Miocene.

Initially, the researchers thought it was a “very sophisticated shrimp burrow,” says Ludvig Löwemark, a sedimentologist at National Taiwan University. “And then, after talking to other experts, we looked at a bivalve hypothesis.” However, these two options did not add up.

Indeed, the shrimps would have left branching chambers “half-turn” in their dens and a suspect bivalve would have formed a “cul-de-sac” at the bottom of its den so as to be able to place its shell. A predatory ambush worm, which operates in one direction, therefore seemed to fit the profile much more. These traces of fossils were named Pennichnus formosae.

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Almost two meters deep

Through these scans, the researchers figured out that all of these burrows veered horizontally as they sank, forming a tapered boomerang-shaped den about 2.5 cm wide and about 1.8 m deep. This curvature suggests that it was becoming more difficult for these creatures to dig to a certain depth, or that the environment was becoming more and more anoxic (lack of oxygen, the worms “breathe” through their skin).

“To sum up, we hypothesize that around 20 million years ago, ancient worms colonized the seabed at the southeastern border of the Eurasian continent, waiting in ambush for a meal to present itself”, write the researchers. “When prey came close, these worms would surge out of their burrows, grabbing and dragging their victim into the sediment. Under the sea floor, the desperate prey wriggled to escape, causing further disturbance of the sediment around the burrow opening. ”

These findings fill a gap in our knowledge of how this type of creature has evolved and developed over time.