Fossil may solve mystery of what one of the weirdest ever animals ate

One of the weirdest animals that ever lived may have been a scavenger. A re-examination of fossils first described in the 1970s seems to show a swarm of Hallucigenia feeding on the corpse of a comb jelly.

Hallucigenia was a small animal, up to 5 centimetres long. It had a worm-like body with multiple legs, as well as long, sharp spines on its back. Because of its peculiar appearance, palaeontologists at first reconstructed the animal upside-down, supposing the spines to be legs.

It lived in the deep seas during the Cambrian period (about 539 million to 487 million years ago), when many major animal groups emerged. Hallucigenia was first identified in rocks from the Burgess Shale deposits in British Columbia, Canada. It is related to velvet worms, tardigrades and arthropods (the group that includes insects and spiders).

Little is known about the ancient animal’s lifestyle. For instance, none of the Hallucigenia fossils found to date have preserved gut contents, so we don’t know what they ate.

Javier Ortega-Hernández at Harvard University re-examined a fossil from the Burgess Shale that was used in the original description of Hallucigenia in 1977, but which hadn’t been looked at since.

It is the remains of a soft-bodied, gelatinous organism, 3.5 cm by 1.9 cm, which has been severely damaged. Ortega-Hernández identified it as a comb jelly, or ctenophore.

Scattered over the comb jelly, Ortega-Hernández identified Hallucigenia spines, representing seven individuals. He suggests the comb jelly died and sank to the sea bed, where the Hallucigenia swarmed on it and fed, probably by suction feeding. While they were doing this, they were all buried in mud and eventually fossilised.

Ortega-Hernández declined to be interviewed by New Scientist because the paper hasn’t yet been peer-reviewed.

“I think it’s a convincing ecological interaction,” says palaeontologist Allison Daley at the University of Lausanne in Switzerland. She calls it a “snapshot”, representing “a moment in time that maybe only lasted some minutes or hours that happened to be caught in the fossil record”.

Hallucigenia is known to have lived in deep water, says Daley, and nutrients are scarce in the depths. It makes sense that the species survived by quickly finding and eating rich food sources like a dead comb jelly. “Suction feeding would be very effective on such a soft animal,” she says.

Jean-Bernard Caron at the Royal Ontario Museum in Toronto is less convinced. Just because the Hallucigenia and ctenophore fossils are found together, he says, doesn’t necessarily mean they were interacting in real life. It could be that undersea mudslides carried them to the same resting place.

Because most of the known Hallucigenia fossils are just spines, Caron suggests an alternative interpretation: the animals may have moulted, shedding their skins to grow.

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