Burgess Shale Type Preservation - Closing The Taphonomic Window

Closing The Taphonomic Window

Burgess Shale type preservation is known from the "pre-snowball" earth, and from the early to middle Cambrian; reports during the interlying Ediacaran period are rare, although such deposits are now being found. Burgess-shale type Konzervat-lagerstätten are statistically overabundant during the Cambrian compared to later time periods, which represents a global megabias. The mode of preservation is more abundant before the Cambrian substrate revolution, a development in which burrowing organisms established a foothold, permanently changing the nature of the sediment in a fashion that made soft-part preservation almost impossible. Consequently, the quantity of post-Cambrian Burgess Shale-type assemblages is very low. Although burrowing reduced the number of environments that could support BST deposits, it alone cannot explain their demise, and changing ocean chemistry - in particular the oxygenation of ocean sediments - also contributed to the disappearance of BST preservation. The number of pre-Cambrian assemblages is limited primarily by the rarity of soft-bodied organisms large enough to be preserved; however as more and more Ediacaran sediments are examined, Burgess Shale type preservation is becoming increasingly well known in this time period.

While the post-revolution world was full of scavenging and predatory organisms, the contribution of direct consumption of carcasses to the rarity of post-Cambrian Burgess Shale type lagerstätten was relatively minor, compared to the changes brought about in sediments' chemistry, porosity, and microbiology, which made it difficult for the chemical gradients necessary for soft-tissue mineralisation to develop. Just like microbial mats, environments which could produce this mode of fossilisation became increasingly restricted to harsher and deeper areas, where burrowers could not establish a foothold; as time progressed, the extent of burrowing increased sufficiently to effectively make this mode of preservation impossible.

However, Burgess Shale type biotas do in fact exist after the Cambrian (albeit somewhat more rarely). Other factors may have contributed to the closure of the window at the end of the Amgan (middle Mid Cambrian), with many factors changing around this time. A transition from an icehouse to a greenhouse world has been associated with an increase in storm intensity, which may have hindered exceptional preservation. Other environmental factors change around this time: Phosphatic units disappear; and there s a stem change in organisms' shell thickness.