How to Recycle an Ichthyosaur

Whales have very active afterlives. Once they settle on the ocean bottom, their bodies become both food and shelter for a host of different organisms – an oasis of bone and rotting flesh called a whalefall. But whales aren’t the only animals to have enriched the seafloor. During the Late Jurassic, over 90 million years before whales even existed, the bodies of aquatic reptiles called ichthyosaurs hosted a vibrant succession of marine life.

Whale Fall (After Life of a Whale) from Sweet Fern Productions on Vimeo.

This week, Plymouth University paleontologist Silvia Danise and colleagues have described the ichthyosaur fall in Nature Communications. This isn’t the first time paleontologists have reported such a community. In 2008, Andrzej Kaim and coauthors described a pair of roughly 89 million year old plesiosaurs associated with snails that make their living in ephemeral undersea habitats. But the unfortunate ichthyosaur adds something new. The geologically older marine reptile underwent a slightly different trajectory during its breakdown.

The ichthyosaur in question is an Opthalmosaurus – a streamlined predator with eyes so big that paleontologists suspect it regularly delved into the dim deep to chase squid and small fish. Rather than dying in the dark, though, this marine reptile perished in the shallows of a 145 million year old sea that would eventually become part of southern England. And on its bones are the signs of several phases of deconstruction.

Large scavengers were the first to the party. Small grooves on some of the ichthyosaur’s ribs show that nibbling fish were some of the earliest to arrive, Danise and colleagues report, and repeating sets of star-shaped grooves were likely left by echinoderms that were not intent on eating the ichthyosaur itself, but the bacteria and worms covering the corpse. This part of the process corresponds to the “mobile scavenger phase”, which modern whale carcasses also go through. Then a smaller cast of characters took over.

The fossilized Opthalmosaurus bones are crusty. Many are wrapped in a kind of mineralized rind, and this, Danise and coauthors propose, is a sign of bacterial clumps or the feces of the worms and snails that came to feed on the bacteria. Danise and colleagues call this the “microbial mat and grazer stage.”

Both of the early decomposition phases can be seen on modern whale carcasses, and are often followed by what’s called the “sulphophilic stage.” This is a time when the microbial breakdown of a whale’s bone lipids produces sulphide and methane, which in turn support a community of small organisms that rely  on the energy created by this process. But the ichthyosaur didn’t go through this stage.

Perhaps the ichthyosaur wasn’t big enough to support such a stage, the researchers suspects, or the site was too shallow to be reached by the chemosynthetic organisms associated with it. Whatever the cause, the reptile’s body transitioned into a different final stage before burial.

Denuded of flesh and thoroughly picked-over by bacteria, the remains of the Opthalmosaurus were encrusted by filter-feeding invertebrates. For a few years, at the very least, the ichthyosaur became a reef. The absence of a modern whalefall regular might explain this.

Osedax – snotworms – are common whalefall critters. They burrow into the bones themselves, breaking them down as they feed on the lipids and proteins within. There’s no evidence of Osedax on the ichthyosaur, and the current hypothesis is that the worms didn’t evolve until sometime in the Cretaceous.

Safe from the big boring worms, the ichthyosaur skeleton held itself together for long enough to support an array of encrusters before finally becoming one with the fossil record. Preserved for over 145 million years, the ichthyosaur is a strikingly ancient example of how the sea reduces, reuses, and recycles.

Reference:

Danise, S., Twitchett, R., Matts, K. 2014. Ecological succession of a Jurassic shallow-water ichthyosaur fall. Nature Communications. doi: 0.1038/ncomms5789