Sidneyia fossil with Sidneyia model
Sidneyia fossil with Sidneyia model
Picture of Hugh

Hugh

The Cambrian Explosion and the Burgess Shale

Cambrian Explosion

The early Cambrian period marks one of the most spectacular evolutionary events in the history of life – The Cambrian Explosion. This was an explosion of life. Within ten million years, a very short period geologically, a host of hard-body and soft-body animals appeared in the fossil record. Trilobites, molluscs, chordates, cnidarians, brachiopods and many more phyla all appeared abruptly. Since, hard-body animals are more readily preserved compared to soft-bodied organisms, the fossil record has a bias. For example, there may be no fossils of trilobites in the time period before the Cambrian, known as the Ediacara. However, paleontologists know that soft-bodied trilobite ancestors roamed the sea floor at that time because these primitive animals left diagnostic tracks in the mud.

Factors driving evolutionary change in the Cambrian

The Cambrian Explosion is when animals first became capable of developing biomineralized shells and skeletons. Shells and skeletons provided animals with a framework to get larger. Hard body parts and increased size are two factors that improve an organisms chance to be fossilized.
 
Debate still rages as to the cause of the sudden development of skeletons. Skeleton formation may have been due to changes in ocean and atmosphere chemistry. Notably an increase in available ocean calcium during the Cambrian may have allowed organisms to form shells and skeletons. Additionally, an increase in the atmospheric oxygen level may have allowed for organisms to get bigger. The development of metabolic pathways allowing biomineralization was also necessary to form skeletons. It is possible that this metabolic pathway developed just before the Cambrian Explosion. In addition, new species interactions such as predation may have exerted selection pressure which favored survival of those with protective, hard exteriors.
 
The Burgess Shale is a record of the end of the Cambrian Explosion and is unique in its preservation of soft-bodied fossils that are under-represented in other parts of the geologic record (e.g., carbonates). This more complete preservation allows for paleontologists to more completely understand the entire ecosystem and not just hard-bodied organisms. Fossils like those preserved in the Burgess Shale allow for a more complete understanding of ecosystems and the complexity of individual organisms at this time.

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