Small threadlike worms that consume the bones of dead whales on the ocean floor once made a living off the carcasses of ancient marine reptiles, a new study suggests. The find means that the worms may have been around at least 60 million years longer than researchers suspected.
“This [study] is very exciting,” says Anthony Martin, a paleontologist at Emory University in Atlanta who wasn’t involved in the work. “It shows how marine bones were being recycled” by organisms on the sea floor millions of years ago, he notes. “All those bones, they weren’t going to waste.”
Scientists first described worms in the genus Osedax (Latin for “bone-eating”) about a decade ago, after stumbling across the carcass of a dead whale on the sea floor off the California coast at a depth of thousands of meters. The sunken skeleton was covered with the tiny mouthless worms, which bore into the bones to extract fats and other nutrients for the symbiotic bacteria they host within their bodies. The circular cross section of the holes on the surface of the bones, as well as the cauliflowerlike caverns at the ends of those tunnels, have a very distinct shape, says Nicholas Higgs, a marine biologist at Plymouth University in the United Kingdom.
Previous genetic studies of Osedax worms, which estimated the past rates of mutations that had led to modern species, weren’t definitive about when the group first evolved. The earliest species might have appeared about 50 million years ago, just as the ancestors of whales returned to the sea from land, or they could have appeared even earlier: 125 million years ago or more. If the latter is the case, some researchers have proposed, the carcass-colonizing worms might have once feasted on the sunken remains of large marine reptiles such as long-necked plesiosaurs or dolphin-shaped ichthyosaurs, which ruled the seas when dinosaurs reigned on land during the Mesozoic era (which ended about 66 million years ago).
So Higgs and Silvia Danise, a paleobiologist also at Plymouth University, scrutinized fossils for traces left by Osedax worms. They used CT scanners—the same equipment that images a patient’s internal organs at hospitals—to make high-resolution, 3D images of the preserved remains of a plesiosaur and an ancient sea turtle. Bones from both carcasses were entombed in seafloor sediments laid down about 100 million years ago. And both sets of fossils included the distinct signs of Osedax colonization, the researchers report online today in Biology Letters. The two holes on the surface of the plesiosaur fossils measured 0.36 millimeters and 0.56 millimeters across; those on the sea turtle fossils were typically smaller. Yet all of these sizes fall within the range seen in modern-day and fossil whale bones, the team notes.
Although the holes and chambers made by those ancient organisms are very similar to those created by today’s Osedax worms, “it’s very hard to say they were made by the same genus of worms,” Martin says. Maybe the ancient borers were just behaving similarly, he suggests.
But Greg Rouse, a marine biologist now at the Scripps Institution of Oceanography in San Diego, California, and a member of the team that discovered Osedax worms in 2002, is more convinced than Martin that these newly described borings are truly traces of the modern group’s ancestors. “It’s great to finally find Mesozoic evidence of Osedax,” he says. The tunnels and chambers described by Higgs and Danise “look just like those made by Osedax,” he says. “This is pretty convincing. It’s solid, clean work.”
So if the ancestors of Osedax made their living by colonizing the carcasses of ancient marine reptiles that largely died out at the same time as the dinosaurs, how did they survive until the progenitors of whales first appeared millions of years later? The answer, Higgs says, is that the worms continued to feed on the sunken remains of other large creatures, including sea turtles. These reptiles not only survived the dino-killing apocalypse, but their carcasses were also large enough to avoid being covered by accumulating sediment before the worms could find them.