A dead whale is more than a rotting mass of flesh and bone. In the deep sea, the descent of a leviathan is a fortuitous bonanza for the battalions of scavengers which gradually break down the cetacean’s body. And, for as long as the body lasts, the whale becomes a little island of diversity in the deep – a temporary deconstruction site where the remains of one life are distributed among the many.
But whales may only be the latest creatures to so enrich the sea floor. Long before whales, the carcasses of marine crocodiles, sea turtles, mosasaurs, ichthyosaurs, and plesiosaurs settled on the bottom. (Even the occasional dinosaur carcass floated out into deep water.) Unfortunately, we can’t study the decomposition of these Mesozoic creatures in real time, but the circumstances of preservation have created snapshots of marine communities which briefly sprung up around the bodies of these animals. For the most part, the associated fossils include bivalves and shark teeth, but at least two plesiosaur skeletons appear to have been colonized by a few archaic forerunners of today’s whalefall communities.
Paleontologist Andrzej Kaim and co-authors described the specimens in 2008. Each had been discovered in slightly different Late Cretaceous deposits near Hokkaido, Japan. One, dubbed the Turonian skeleton, was about 93 to 89 million years old, and the other – called the Conician skeleton – was about 89 to 85 million years old. And despite the fact that these skeletons were found at different sites and were not of the same species, both were found associated with numerous gastropod fossils.
By itself, the association between reptile and mollusk isn’t remarkable. Fossils of gastropods, as well as bivalves, are often found associated with skeletons of marine reptiles. But the identity of the invertebrates was a clue that ephemeral ecosystems had developed on the plesiosaur carcasses.
The small snails collected from the rock between and near the plesiosaur bones were provannids – a group of gastropods which are still around today, and are found in relatively short-lived habitats such as hydrothermal vents, cold seeps, and whalefalls. Indeed, Kaim and colleagues pointed out that similar snails also turned up at sites of Cretaceous cold seeps – places where methane and other chemicals oozed out into pools and nourished chemosynthetic communities of bacteria which formed the basis for islands of life on the seabottom. The snails on the skeletons were specialists at colonizing short-lived sites of localized resources.
Snails weren’t the only organisms to take up in the plesiosaur carcasses. Some of the plesiosaur bones were shot through with tiny boreholes similar to those found on decomposing whale skeletons. Exactly what creature or process creates this pattern of damage is unclear, but the tiny tubes may have been left behind by bacteria. This may also explain why the snails were present on the carcasses. As mats of bacteria spread over the picked-over plesiosaur skeletons, the snails grazed on the proliferation of bacteria – a situation similar to the final “sulfophilic stage” of modern deadfalls.
What remains unclear is whether the plesiosaur skeletons represent the true beginnings of dedicated deadfall communities, or whether the bacteria and snails were just opportunists. The plesiosaur carcasses settled in an area pocked by prehistoric methane seeps known to host such snails, and, hence, the bacteria on which the snails fed. Perhaps the creatures from a nearby seep simply took advantage of an easy meal, and distinct deadfall communities did not originate until the origin of entirely aquatic whales, around 45 million years ago. But, even if this is the case, the connection between the plesiosaur skeletons, colonizers, and methane seeps hints that bodies which fell in the vicinity of methane seeps or hydrothermal vents might have provided a stepping stone for deadfall communities to develop.
As Kaim in colleagues pointed out, we don’t have enough information to know whether modern whale falls are extensions of plesiosaur, ichthyosaur, and mosasaur falls of the past. No one even expected that such communities might exist until they were discovered in the modern seas. Now that paleontologists know what to look for, though, additional evidence of deep sea decay will hopefully be dredged from the rock.
Kaim, A., Kobayashi, Y., Echizenya, H., Jenkins, R., & Tanabe, K. (2008). Chemosynthesis-Based Associations on Cretaceous Plesiosaurid Carcasses Acta Palaeontologica Polonica, 53 (1), 97-104 DOI: 10.4202/app.2008.0106