The Deep History of the Sea’s Bone-Eating Worms

Bones are symbols of death. That makes sense. Before X-rays and CT scans allowed us to look inside ourselves, we’d only see our skeletons after death and decay had stripped everything else away. Not to mention that poison labels, the Jolly Roger, and visages of the Grim Reaper have all reminded us of our eventual fate. But while these associations aren’t wrong, they suffer from being one dimensional. Bone isn’t just about death. For some creatures, bone is a giver of life.

On the surface, the phrase “bone-burrowing worm” sounds like something David Cronenberg thought up. Perhaps, had they been discovered sooner, the body horror director would have used them. The invertebrates weren’t found until the year after The Fly debuted, pocking the skeleton of a whale that had perished off the California coast. In time, these specialized worms – known by the scientific name Osedax – became known as central players in the succession of “deadfall” critters that make their living on bodies that sink to the bottom. The end of one life enriches countless others.

But when did Osedax start drilling their way into the deceased? Pinholes in fossil whale bones, bolstered by estimates of genetic divergence among living worm groups, showed that Osedax – or very similar annelids – have been sinking their roots into undersea skeletons for millions of years. For as long as there have been whales, it seems, there have been worms that could take up residence on their remains.

Whales were not the first creatures to trade life on land for one spent entirely at sea, though. Starting around 245 million years ago, about 190 million years before the earliest whales took their first dip in the water, multiple marine reptile groups slid into the seas. Those disparate forms – the fish-like ichthyosaurs, quad-paddled plesiosaurs, sea turtles, and more – flourished throughout the Mesozoic, the last of these “sea dragons” going extinct about 66 million years ago. Surely these marine reptiles died and sank to the bottom just as whales do today. Is it possible that Osedax evolved to inhabit their bones and only later continued the tradition with whales?

Osedax worms on the surface (a), the root and ovisac of Osedax (b), and Osedax-type burrows (in orange) on a plesiosaur bone (d). From Danise and Higgs, 2015.
Osedax worms on the surface (a), the root and ovisac of Osedax (b), and Osedax-type burrows (in orange) on a plesiosaur bone (d). From Danise and Higgs, 2015.

The recent discovery of modern deadfalls sent paleontologists searching for more ancient equivalents. And they found them. A pair of plesiosaurs excavated from the 86 million year old rock of Japan hosted late stage deadfall communities where snails grazed on mats of bacteria, and, last year, Plymouth University paleontologist Silvia Danise and colleagues reported on a 145 million year old ichthyosaur from southern England that documented how the marine reptile hosted a changing array of scavengers. Yet no one was able to find signs of Osedax. The oldest confirmed traces of the worm were 30 million years old, and it was unclear whether the worm hadn’t evolved in the time of the marine reptiles or whether its taphonomic calling cards were absent from the known finds. Now, in a new Biology Letters paper by Danise and Nicholas Higgs, paleontologists have their answer.

Osedax have been taking advantage of deadfalls since the Cretaceous. The evidence, Danise and Higgs report, can be seen on an upper arm bone of a plesiosaur and two sea turtle bone fragments found in the 100-93.9 million year old sediment of England. The Cretaceous bones bear burrows that correspond to those made by modern Osedax species, showing a narrow opening to the surface with a glob-like chamber beneath. Even though the worms themselves didn’t become preserved, the traces of their osteological feast give them away.

The occurrence of Osedax through time. From Danise and Higgs, 2015.
The occurrence of Osedax through time. From Danise and Higgs, 2015.

But here the fossil record would seem to hit a snag. The last whale-sized marine reptiles died out 66 million years ago, and the first fully-marine whales didn’t evolve until about 20 million years later. This bottleneck in the food supply could mean that worms with an Osedax-like lifestyle evolved more than once – just as multiple forms of bone-burrowing beetles have utilized dinosaur and fossil mammal bones – but, more likely, it speaks to how flexible these worms are.

While some marine reptile lines ended by the close of the Cretaceous, sea turtles survived. Osedax could have kept persisting on their skeletons, at the very least. And, from geologically younger finds as well as experimental studies, marine biologists have found that Osedax are not especially picky about whose bones they’re colonizing. A whole whale is great, but cow, bird, or fish bones will do in a pinch. Magnificent giants that paddle through the surface waters have come and gone, but, for over 93 million years, the bone-eating worms have been waiting for them.

Reference:

Danise, S., Higgs, N. 2015. Bone-eating Osedax worms lived on Mesozoic marine reptile deadfalls. Biology Letters. doi: 10.1098/rsbl.2015.0072

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