Burial is a critical factor in the preservation of most fossils, keeping scavengers away and preventing the disarticulation of hard elements and the decay of soft parts.
Although exceptional fossil preservation is often attributed to the rapid burial of live specimens, the type of deposition associated with this process is still poorly studied.
In a new study, the authors used brittlestars, or ophiuroids, to simulate burial under different fast-flowing sediment avalanches.
Turbidites (fast-moving sediment avalanches) and tempestites (sediment surges moved by storm waves) are considered the main burial agents of organisms, especially smaller invertebrates, in a marine environment. Brittlestars are common animals living on the seafloor and are known from the fossil record dating back at least 410 million years.
The authors simulated sudden burial by simply emptying a buckle of mud, clay and muddy sand along a ramp into an aquarium with living brittlestars. All specimens escaped under the deposition of thin (1 centimeter) sand layer. A brittlestar will use one of its five arms to drag the central disc and the rest of the arms along the same track to the surface. As the layer covering the animals gets thicker, they need more time to escape, eventually also losing one or two arm tips as they wiggle through the coarse sand grains. The burial of complete specimens was only recorded from the deposition of 10 centimeters of sand. Most specimens reached exhaustion during this phase, helped by a lack of oxygen in the lower sediment layers.
The study shows that bed thickness is a key factor in rapid burial and fossil preservation. Layers of mud or clay are often seen as a relevant factors for the exceptional preservation of an animal as a fossil, as they quickly cover and suffocate smaller organisms. But the experiment showed that this is not necessarily the case, as most brittlestar specimens remained active even with dropping oxygen levels long enough to escape. The authors suggest that not grain size, but other factors associated with the mud layer play a role in trapping the animals. Freshwater pulses will temporally “stun” a saltwater animal like a brittlestar, preventing a successful escape.
The full research paper “How does rapid burial work? New insights from experiments with echinoderms” was published in the journal Palaeontology and can be found online here.
Additional material provided by the Palaeontological Association.
