Burgess Shale type faunas

The Burgess organisms were predominantly benthic or burrowing creatures, swept out of their shallow marine habitat by turbulent sediment flows, transported for some distance and then deposited, in all likelihood already dead, in a layer of mud. The fossils are preserved as organic films and impressions in the sediment. Compaction took place, but fine mud particles infilled every space in and around the organism, preserving a degree of three-dimensionality and allowing the Cambridge team to disect down through the fossils to reveal hidden structures. A Burgess Shale fossil of the arthropod Sanctacaris. From The fossils of the Burgess Shale. Briggs D.E.G., et al, 1994.

Burgess Shale type faunas have now been discovered at over 30 other localities in Greenland, China, Poland, Australia, and from rocks ranging in age through Lower and Middle Cambrian. The significance of these faunas is twofold. Firstly they contain organisms exhibiting a large and seemingly bizarre range of morphologies, "Weird Wonders" as Stephen Gould puts it in his book Wonderful Life, some of which don't seem to fit into any taxonomic group. This is seen as evidence for decreasing disparity of body forms through time, with morphological range being at its greatest in the Cambrian. More recently however the Burgess organisms, in particular the arthropods, are being recognised as belonging to various known taxonomic groups.

Reconstruction of the Burgess Shale arthropod Sanctacaris, by Marianne Collins. From The fossils of the Burgess Shale. Briggs D.E.G., et al, 1994.

Secondly, the Burgess faunas are significant because over 80% of the organisms are soft-bodied or lightly skeletalized, and so would not normally preserve (see references Conway Morris. 1986). The hard bodied component, trilobites and the like, is no different from other Cambrian deposits. Therefore we can conclude that the majority of Cambrian deposits only reflect a fraction of the living fauna, and that by the Middle Cambrian the variety of forms seen in the Burgess were present all over the globe. Comparing this with the paucity of fossils from the Precambrian, it seems that an morphological explosion of immense proportions did indeed take place.