A recent fossil discovery off the northeast coast of Taiwan suggests that today's sand striker sea worm has an ancient ancestor lurking in its prehistoric family tree.

In a Nature paper, researchers examining samples from the Miocene strata of the Taiwanese seafloor described finding smooth-walled, L-shaped fossil burrows with distinctive feathering around the upper portion. Comparison with sand strikers today suggests that its ancestor also hunted by ambush, lurking before exploding upward from its burrow to snatch unsuspecting fish swimming overhead. Once captured, the prey was dragged deep into the burrow to its fate.

Fossil Evidence of an Ancient Sea Worm

Sea worms are soft-bodied, and they often don't leave much fossil evidence behind. Instead of skeletal remnants, sometimes only their teeth remain. In a 2017 paper also published in Nature, scientists reported finding the jaws of giant polychaete worms similar to the Taiwan fossils in Ontario, Canada. Extrapolating from the dimensions of the jaws they found, the research team estimated the worms' body size to be around one meter (just over three feet) long.

In the most recent Nature publication, there were no worms themselves to examine. The scientists instead relied on evidence from 319 fossil burrows left by the ancient sea worms to draw their conclusions. The team described these trace fossils as around two meters (over six feet) long and two to three centimeters (approximately an inch) in diameter. The burrows were L-shaped, with the upper portion descending vertically from the ocean floor then turning to run horizontally.

The team noted a distinct feathering pattern around the upper shaft, but otherwise, the traces had smooth walls. Further examination of the feathering by X-ray fluorescence (XRF) core scanning showed a sudden increase in iron deposition around these areas.

Comparing these with modern-day predatory polychaete worms helped researchers build a picture of what ancient life was like for the proposed new ichnogenus, Pennichnus formosae.

Predatory Polychaetes Hunted by Ambush

Drawing from their fossil evidence, the researchers think that, like their modern-day counterparts, P. formosae hunted by ambush. Today's sand striker worms, Eunice aphroditois hunt using powerful trap-jaw mouth parts that can split a fish in two. The soft-bodied worms burrow into the ocean floor sediment for protection and camouflage. They lurk at the entrance to their burrows with their mouth parts only just visible.

Antennae poke out of the burrow to both sense incoming prey and also to act as a lure. Once a fish swims within distance, the sand striker launches itself explosively from its burrow, just far enough to clamp its powerful jaws around its prey. The worm then drags its dinner back into its burrow. The prey's wriggling causes further collapse of the burrow walls, entombing it to its fate. According to the Smithsonian, an injection of toxins subdues the prey as the final blow.

Lifestyle Secrets of Ancient Worms

The distinct feathering was a major clue to the identity of the ancient burrow-dwellers, as the researchers reported. The lines showed repeated collapse of sediment walls around the entrance while the iron accumulation suggested that the damage was repaired over time.

The explosive ambush tactics of E. aphroditois sand strikers followed by their retreat with struggling prey causes sediment collapse at the burrow entrance. The sea worms then repair the damage, secreting mucus to strengthen the area. As National Geographic explains, since the mucus is rich in organic matter, it attracts bacteria that colonize the area. As part of their metabolism, certain bacteria often create conditions that lead to local iron deposition.

A Long History

In addition to discovering a new proposed species, the findings also show that gigantic predatory polychaetes have been exploiting their ecosystem niche for quite some time. Today's trap-jaw worms, E. aphroditois, can reach up to three meters (almost 10 feet) in length — and as Wired notes, they can also turn up in aquaria.

The fossil record from the Taiwan burrows and the Canadian mouth parts shows that giant worms living on the seabed isn't a recent phenomenon. These creatures have been around for millennia, and further exploration of the imprints they've left behind may help us further piece together the biological history of our planet.

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Eunice aphroditois

Ancient Sea Worm Leaves Fossil Burrows

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