The ancient marine reptile that turned into a reef

Beneath the sea, nothing goes to waste. Even around the bodies of dead animals, sea creatures establish thriving communities. This is an ancient tradition going back to the time when dinosaurs ruled the land and prehistoric giants swam the seas.

On the sea floor, a sunken whale carcass can be a big deal. As we saw just the other day, its body can provide nutrients, food and shelter for hordes of creatures, creating a temporary ecosystem that can endure for decades. But these "whale fall-style" communities existed well before the whales themselves.

During the Mesozoic Era, long before whales evolved, the oceans were home to diverse and sometimes enormous marine reptiles. One such creature, a dolphin-like reptile called Ophthalmosaurus, lived its life in what is today the southern UK. When it died, its body settled in shallow ocean water, and its fossilised remains tell the story of the ecosystems that grew around it.

A modern-day whale fall community goes through three major stages. Scavengers like sharks and crabs appear first to snap up the soft flesh. Microbes, worms and snails move in on the bones next, feeding on the nutritious scraps both inside and out. Finally, decomposers finish the job, breaking down the bones to produce gases that fuel yet another thriving community.

The star-shaped "footprints" on a rib fragment from the Ophthalmosaurus were left by ancient urchins. Image: Danise et al., 2014.

At the carcass of the ancient Ophthalmosaurus, the first two stages played out just as they do today. Small grooves on its ribs are evidence of bites from scavenging fish, while star-shaped "footprints" were left by ancient urchins. Then came the opportunists: the bones are covered in the crusty mineralised remains of microbe mats, as well as the fossilised faeces of the snails and worms alongside them. Tiny holes show where microbes were feeding upon the bone.

But stage three (the "sulphophilic" stage) doesn't seem to have happened. Why? Among the major bone-breakers of the modern oceans are the freaky and fascinating bone worms – but 145 million years ago, these creatures had not yet evolved. Without the worms, the Ophthalmosaurus skeleton remained intact long enough for other critters to attach themselves to the bones while they strained food from the water. This new community was no longer feeding from the carcass; it was building on top of it.

The ancient reptile's body was becoming a reef.

The idea of a marine giant's bones forming the basis of a reef is hauntingly beautiful, but it isn't something we typically see today, possibly because bones break down too quickly. Elsewhere in the UK, researchers have found 100 million-year-old fossils from a sea turtle and a long-necked marine reptile called a plesiosaur, which both show the tell-tale signs of bone-worm feeding. The evolution of these worms may have marked the beginning of the end for big carcasses turning into reefs. Whales, which wouldn't evolve for another 50 million years, never had a chance.

The Ophthalmosaurus tale ends with its burial. Along with many of the reef creatures living on and around it, its remains were finally covered up by sediment. This, too, was lucky. If the bones had been broken down more quickly, they may not have been fossilised, and we may never have learned the story of its incredible afterlife.