Fossils of The Burgess Shale - Fossil Preservation

Fossil Preservation

The processes responsible for the exceptional preservational quality of the Burgess Shale fossils are far from clear. The interpretation of what is preserved depends partly on two issues that are interlinked: whether the animals were buried where they lived, or washed long distances by sediment flows; and whether the water at the burial sites was anoxic or provided enough oxygen to sustain animals. The traditional view is that soft bodies and organs could only be preserved in anoxic conditions, otherwise oxygen-breathing bacteria would have made decomposition too rapid for fossilization. This would imply that the sea-floor organisms could not have lived there. However, in 2006 Caron and Jackson concluded that the sea-floor animals were buried where they lived. One of their main reasons was that many fossils represented partially decayed soft-bodied animals such as polychaetes, which had already died shortly before the burial event, and would have been fragmented if they had been transported any significant distance by a storm of swirling sediment. Other evidence for burial where the animals had lived includes the presence of tubes and burrows, and of assemblies of animals preserved while they fed – such as a group of carnivorous priapulids clustered round a freshly moulted arthropod whose new cuticle would not yet have hardened. Fossilized swimming organisms were also buried immediately below where they lived.

Fossil tracks are rare and no burrows under the sea-floor have so far been found in the Burgess Shale. These absences have been used to support the idea that the water near the sea-floor was anoxic. However it is possible that the water just above the sea-floor was oxygenated while the water in the sediment below it was anoxic, and also possible that there simply were no deep-burrowing animals in the Burgess Shale. Some fossils, such as Marrella, are almost always the right way up, which suggests they were not transported far if at all. Others, such as Wiwaxia, are often at odd angles, and some fossils of animals with shelly or toughened components very rarely contain remains of soft tissues. This suggests that the distances over which corpses were transported may have varied between genera, although most were buried where they had lived.

Fossils known as Girvanella and Morania may represent members of microbial mat communities. Morania appears on about a third of the slabs Caron and Jackson studied, and in some cases presents the wrinkled "elephant skin" texture typical of fossilized microbial mats. If such mats were present, they may have provided food for grazing animals and possibly helped to preserve soft bodies and organs, by creating oxygen-free zones under the mats and thus inhibiting the bacteria that cause decomposition.

The Burgess Shale animals were probably killed by changes in their environment either immediately preceding or during the mud-slides that buried them. Proposed killing mechanisms include: changes in salinity; poisoning by chemicals such as hydrogen sulfide or methane; changes in the availability of oxygen; and changing consistency of the sea floor. The death event was not necessarily related to the burial, and there may have been multiple death events between burial events; but only organisms killed immediately before a burial event would stand any chance of being fossilised, instead of rotting or being eaten.

Burgess shale type preservation is defined as the fossilization of non-biomineralized organisms as flattened carbonaceous films in marine shales. When the animals started to decompose, their tissues collapsed under the weight of the sediment that buried them. The typical flattened fossils are outlines of tougher parts such as cuticles and jaws, which resisted decomposition for long enough to be fossilized. Soft elements, such as muscles and gut contents, were sometimes squeezed out of the decomposing organism to produce dark stains on the fossils. Organisms that lack tougher structures, such as flatworms, nemerteans and shell-less molluscs, were not preserved by this process. Very soft but chemically active tissues may be preserved by different processes. For instance during decomposition, bacteria modify the chemically unusual mid-gut glands of some organisms into the durable mineral phosphate. This change happens extremely quickly, before the corpse is compressed, and leaves a three-dimensional mold of the tissues. Gills can also be preserved in something close to their original three-dimensional shape by this process. Both preservation mechanisms can appear in the same fossil. In Burgess-like shales, organisms and parts that are only quite soft, such as muscles, are generally lost, while those that are extremely soft and those that are fairly tough are preserved. The preservation of different body parts in different ways may sometimes help palaeontologists, by suggesting whether a body part was fairly tough like an arthropod limb (preserved as flat film) or very soft and chemically active, like a part of the gut (preserved as a solid piece of mineral). These differences may also help to identify fossils, by excluding from consideration organisms whose body parts do not match the combination of types of preservation found in a particular fossil bed.

It has often been suggested that this type of preservation was possible only when sediments were not disturbed by burrowing animals or the anchors of plants. However a similar type of preservation has been found in fossils from the Late Riphean period, about 850 to 750 million years ago, but in no known fossils between the end of that epoch and the start of the Cambrian. This suggests that such bioturbation has little to do with the appearance and disappearance of Burgess Shale type preservation. Such preservation may depend on the presence of clay-like minerals that inhibit decomposition, and ocean chemistry may only have favoured the production of such minerals for limited periods of time. If so, it is impossible to be sure when the animals known as "Burgess Shale fauna" first appeared or when they became extinct. A few fossils of animals similar to those found in the Burgess Shale have been found in rocks from the Silurian, Ordovician and Early Devonian periods, in other words up to 100 million years after the Burgess Shale.