The discovery of a fossilised ichthyosaur has revealed more about what happened to the ancient sea creatures after death than during life, but it appears both were important to the ecology of the Jurassic oceans.
The 3m long Ophthalmosaurus was discovered in Dorset, England when a road was built through the Late Jurassic mudstones in which it lay. The recently discovered genus gets its name from its huge eye sockets, suggesting it hunted at great depths. Ophthalmosaurus, like all Ichthyosaurs, had a basically dolphin-shaped body, and filled a similar niche in the oceans of the time.
Dr. Silvia Danise of Plymouth University found narrow grooves on the fossil’s rib bones and concluded these were from fish picking at its flesh after it sank to the bottom of the ocean. She attributed star-shaped marks to sea urchins, eating bacteria that had fed on remaining flesh.
As Danise explains in Nature Communications, the carcasses of cetaceans, known as whale falls, are home to rich food webs, and the record from this ancient version reveals both similarities and differences.
“The early ‘mobile-scavenger’ and ‘enrichment-opportunist’ stages were not succeeded by a ‘sulphophilic stage’ characterized by chemosynthetic molluscs, but instead the bones were colonized by microbial mats that attracted echinoids [sea urchins] and other mat-grazing invertebrates,” Danise writes.
The bones went through “a well developed ‘reef stage’" where oyster-like creatures colonized the bones. Today such a stage is rare, with bone-eating Osedax worms consuming the skeleton before this can occur. Osedax have not been identified before the late Cretaceous, so may not have been around to play a similar role when this Ophthalmosaurus died. However, having only been discovered in 2002, there is a lot we don’t know about the worms.
This is the first ichthyosaur to be studied in this way, but with some of the ancient reptiles growing to 21m long, it is likely their graveyards represented an important part of the ocean ecology at the time. The authors note that in many cases, it is hard to identify the creatures responsible for particular marks as a result of convergent evolution of reproductive behaviour among unrelated organisms that exploit similar environments.
In addition to studying marine responses to ichthyosaurs and whales, Danise is studying the little-researched area of how deep sea ecosystems responded to major extinction events.
Nature Communications. Narrow grooves suspected to be from scavenging fish.
Nature Communications, A close up of the marks left by sea urchins on the ichthyosaur bones.