Not to toot my own horn, but I know a thing or two about bizarre animals. And I can tell you without a hint of doubt that the bobbit worm is by far the most bizarre. Growing to 10 feet long, the worm digs a burrow in the seafloor, leaving only its bear trap of a mouth sticking out. When a fish approaches, the bobbit worm shoots out of its burrow with astonishing speed, snapping its jaws around its prey. With violent tugs, the worm then drags the victim down into its lair, where it eats the fish alive. (Oh, there’s video.)
Now scientists say they’ve found evidence that an ancestor of the bobbit worm may have been menacing fish 20 million years ago. Writing today in the journal Scientific Reports, the researchers argue that hundreds of fossilized worm burrows, found in what is now Taiwan, show telltale signs of struggle. They haven’t found the worms themselves, mind you, as boneless critters like worms (known as invertebrates, because they lack spinal columns) very rarely fossilize. Instead, they discovered trace fossils, geological features that hint at the behavior of ancient animals, in sandstone that was once a seafloor.
“This is, we believe, the first time that we’ve actually found a trace fossil that shows how invertebrates like worms were feeding on vertebrates,” says National Taiwan University sedimentologist Ludvig Löwemark, coauthor of the new paper. “Because, typically, what we find in the sedimentary record is animals that are moving through the sediment.” Invertebrates, for instance, might dig tunnels through the sea bottom and pump water through their burrows, filtering out particles. “But this is a record of a much more active behavior,” he continues. “The worms were actually hiding in the sediment, jumping out, catching their prey, and then dragging this prey down into the sediment.”
The fossilized burrows are around 6.5 feet long. From their openings on the surface of the seafloor, they would have run more or less straight down into the muck. Then, halfway down, they’d bend at about 45 degrees, creating the shape of an L, or a boomerang. Near the entrances of the tunnels, Löwemark and his colleagues noticed “collapse funnels,” or piles of sediment that had built up inside the burrow. The researchers argue that this is a sign of struggle, preserved for millions of years in the fossil record: As a worm dragged a wriggling fish down into its lair, sediment would spill in to fill the void.
A cross-section of a burrow is shaped a bit like a feather, with the main channel being the shaft, and the collapse funnels branching off in the sediment on each side. The researchers argue that this is a hallmark of the worms’ feeding habits. “When the worm has digested its prey, it reemerges at the surface,” says Löwemark. “It reestablishes a tunnel system in the middle of these collapse structures, and that’s how these feather-like structures around the tube are formed.”
Now, bobbit worms and their ancestors aren’t the only critters out there digging burrows in the seafloor. Some kinds of shrimps and bivalves (shelled mollusks like clams) do the same. So how could the researchers be sure they were in fact excavating a worm’s home?
Shrimp are, well, shrimpy, yet nevertheless capable of digging lengthy tunnels. But because they’re working with sand grains that don’t typically stick together, they have to reinforce the walls of their burrows with mud to keep them from collapsing. Shrimp tunnels also tend to be mazelike, and the animals will build chambers to use as roundabouts so they can reverse direction. As for bivalves, their burrows are usually oval-shaped, since their body plan is two shells stuck together.
These 20-million-year-old burrows, on the other hand, are neatly circular. This tells the scientists that their residents were probably round, as modern bobbit worms are. The researchers can also tell that these burrows lacked reinforcement, which suggests that whatever lived in them instead supported the structures with their bodies to keep them from collapsing. Given that the burrows are more than 6 feet long, that means we could be dealing with one massive worm.
Or, more accurately, the unfortunate fish of the distant past could have been dealing with one massive worm. “The fact that we have a very nice, round shape all the way down, but without any strong lining, suggests that the animal was actually sitting in its burrow system most of the time, and then it rushed out,” says Löwemark. “And the collapse funnels—these feathers at the top—they then represent the struggle of the prey as it is being drawn down into the sediment.”
Such trace fossils thus preserve evidence of an ancient struggle between uber-predator and prey, the researchers argue. This gives scientists a better idea of how long the bobbit worm and its ancestors have been ruining the lives of fish, and provides clues that a fossilized body alone could never reveal. Even if the soft tissues of a worm fossilized instead of quickly rotting away, a preserved bobbit worm would provide information about morphology, but probably not behavior. “By understanding how the interaction between prey and predators worked in the past, we can get a better understanding of the paleoecosystem,” says Löwemark.
But the missing morphology is in fact a problem, says Terrence Gosliner, senior curator of invertebrate zoology at the California Academy of Sciences, who wasn’t involved in the work. Bobbit worms belong to a class of worms known as polychaetes, some of which are vegetarians, and some of which grow as large as the predator in question. So it’s possible that the feathering of a burrow isn’t a sign that the resident had been hunting fish, but rather just poking its head out to feed on other things. “I think anytime a worm retracts, it’s going to leave similar kinds of feathering and the collapse structures that they talk about, from my standpoint,” says Gosliner. “They could well be absolutely right, but there are lots of other explanations, too.”
Really, even the modern bobbit worm remains a largely mysterious creature. “Nobody has ever really examined what their burrow is, and whether it’s L-shaped,” says Gosliner. “And so I think it’s a really interesting paleontological find. But in my mind, it raises just as many questions as it does answers.”
Löwemark notes, however, that the feather-like structures of the burrow extend quite far vertically, indicating more of a wild struggle than peaceful grazing. The environment at the time, too, lends clues. “It is possible that other worms make similar burrows,” Löwemark says, “but we think that the fact that the burrows in our paper were found in a shallow marine paleoenvironment, where any plant material would have arrived as small fragments, speaks against a herbivorous worm.”
It’s food for thought, to be sure.
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