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, they sink down to the seabed. Once their flesh has been stripped away by scavengers and microbes, their corpses are colonized by a variety of specialized invertebrates that feast on the fats and proteins locked inside their skeletons.

These bone-eating communities are ancient, and have been on Earth for at least 100 million years. Before whales existed, bone-eating worms were eating into the skeletons of mosasaurs, ichthyosaurs and plesiosaurs—leaving their distinct burrows behind in the process.

New research, in the journal PLOS One, has named seven of these burrow types. Sarah Jamison-Todd, a Ph.D. student who led the research, says that although the researchers can't be precisely sure which species made the holes preserved in fossil bones, some are very similar to the burrows of living bone-eating Osedax worms.

"We haven't found anything else that makes a similar burrow to these animals," Jamison-Todd explains. "As the ancient bores are so similar to modern Osedax species, and we don't have body fossils to contradict us, we assume that they were made by the same or a similar organism."

"It shows that the bone-eating worms are part of a lineage that stretches back at least to the , and perhaps further. We can see how the diversity of bone-eating worms changes across millions of years."

What are Osedax worms?

Bone-eating worms play an important role in recycling the ocean's nutrients. But despite this vital function , with dozens more species described since.

The worms can be found in oceans from the Arctic all the way down to the Antarctic. They live at a range of depths, from just 20 meters beneath the surface to more than four kilometers deep.

What makes their lives even more extraordinary is that they don't even have a mouth or anus. Instead, Osedax worms rely on bacteria to break down the skeletons for them before absorbing the released nutrients through a root-like system.

It appears that this between the worms and bacteria is truly ancient. Fossilized bones and teeth from plesiosaurs, ancient whales and turtles from the last 100 million years preserve the boreholes of these worms.

like the bone burrows can't be named as full species but can be described as an "ichnospecies."

Different species of modern bone-worm create different-shaped burrows that can then be used to identify the worms down to species levels. The researchers looking at these fossil traces suspected that the same thing could be done with the ancient burrows.

Burrow hunting

To start their search, the team turned to the vast array of marine reptile fossils looked after by the Natural History Museum. Dr. Marc Jones, who curates this collection, says that the team needed to start by getting an accurate date for the bones.

"Some of these specimens were collected in the 1800s, and the information on them can be very brief so it's difficult to know exactly where they're from," Jones explains. "We focused on specimens from the Chalk of the U.K., as using museum collections."

"They're also working to date fossils of an unknown age. By using the remains of small organisms that make up the Chalk itself, we were able to date the fossils to more precise time slices of the Cretaceous Period."

The team then searched for signs that the fossils had been bored into. In addition to the more obvious collapsed burrows, the team closely examined the bones and teeth for borehole entrances the size of pinholes.

"We examined over 130 different fossils for signs of bone-eating worm burrows," Jamison-Todd says. "However, not all fossils which look like they've been bored into by these worms have been—it could have been caused by different animals or environmental weathering."

"The only way to really confirm this is to put the fossils under a CT scanner. This allows us to create 3D reconstructions of the inside of the bone and see the different patterns that have been left behind."

New boring species

In total, the researchers identified a set of six burrows in bones and teeth that represent new bone-worm ichnospecies. They also included a previously studied fossil from the Royal Belgian Institute of Natural Sciences, bringing the total number of new ichnospecies to seven.

The team drew on a variety of different sources to inspire their new names. Some refer to the shape that the worms leave behind in the bone, with Osspecus arboreum's looking like a tree. Osspecus morsus, meanwhile, was named after the Latin word for "bite" because it's mostly found in fossil teeth.

Another species, Osspecus eunicefootia, was named in honor of Eunice Newton Foote, a scientist whose pioneering research into the greenhouse effect was overlooked for more than a century. She was the first person on experimental grounds to suggest that .

The team hope that naming the different types of burrows will make it easier for other scientists to find these traces in other fossils and document their true diversity.

"There are many more examples of boring that haven't yet been named from both ancient and modern bone-eating worms," Jamison-Todd adds. "In fact, some bores from the Cretaceous appear to be similar to ones that are still made today."

"Finding out whether these burrows are made by the same species, or are an example of convergent evolution, will give us a much better idea of how these animals have evolved, and how they have shaped marine ecosystems over millions of years."

This story is republished courtesy of Natural History Museum. Read the original story here.