Australia: The Land Where Time Began

A biography of the Australian continent 

Ediacaran-Type Fossils in Sediments from the Cambrian

To understand the composition and diversity of past life it is essential that soft-bodied organisms are preserved in fossil assemblages. Ediacaran-type fossils have now been found in deposits around the world dating to the terminal Proterozoic, about 600-544 Ma, and the assemblages in such deposits are unique in that they are comprised entirely of soft-bodied organisms, their preservation typically being as impressions in coarse-grained sediments (Gehling, 1991; Runnegar1995; Conway Morris, 1993; Fedonkin & Wagoner, 1997). These lagerstätten pre-date the burst of skeletonisation that occurred near the start of the Cambrian, which makes them special (Conway Morris, 1993). The authors1 suggest higher metazoans, such as annelids and molluscs, may have also been represented (Gehling, 1991; Runnegar1995; Conway Morris, 1993; Fedonkin & Wagoner, 1997), though most Ediacaran-type fossils are interpreted as being cnidarians. Some specialists have preferred to interpret Ediacaran-type animals as non-metazoans, such as Vendobionta (Seilacher, 1992), because of the unusual preservation style and the difficulties involved in finding convincing morphological homologies with definite animals. The rarity of Ediacaran-type fossils in younger sediments has suggested to some that there may have been a mass extinction event in the terminal Proterozoic (Brasier, 1995). In this paper the authors1 report typical Ediacaran-type frond-shaped fossils occurring in an assemblage of Cambrian-type trace fossils in sediments that are unequivocally of Cambrian age in the Uratanna Formation of South Australia. The authors1 suggest this occurrence bridges the divide that apparently existed between fossil assemblages of the terminal Proterozoic and the Cambrian, also suggesting that closure of a taphonomic window, a time interval during which unique preservational conditions existed, was as important as extinction in the disappearance of Ediacaran-type organisms.

 

The authors1 suggest the attaining of a proper understanding will probably depend on taphonomic analysis. Shallow water environments where there was only limited bioturbation of the sediment appears to have been the typical habitat of most organisms of Ediacaran type, but more significantly, indirect evidence has been found that they occurred in association with extensive microbial mats (Gehling & Rigby, 1996; Gehling, 1988; Gehling, 1986). Therefore the unusual preservation in the terminal Proterozoic could possibly be a product of unique taphonomic conditions that involved sediment-binding and sediment-sealing microbial mats combined with a low level of bioturbation and scavenging (Gehling & Rigby, 1996; Gehling, 1988; Gehling, 1986). Many of the Ediacaran organisms were apparently benthic forms that had at best only limited mobility, or placed shallowly in the sediment or attached to the mat-bound sediment surface (Seilacher & Pflüger, 1994). The biology and preservation of these organisms would have been affected by the advent of extensive scavenging and disturbance (Droser & Li,  eds Zhuravlev & Riding) of the sediment. The extent and depth of bioturbation was still comparatively low in the early stages of the Cambrian (Droser & Li,  eds Zhuravlev & Riding). The authors1 suggest a glimpse through a taphonomic window that was rapidly closing is provided by the fossiliferous horizon in the Uratanna Formation, though not explaining the subsequent fate of the various components of the Ediacaran fauna, such as whether they continued on, evolved or went extinct. It does, however, demonstrate that at least some of the Ediacaran taxa continued on into the Cambrian.

 

Sources & Further reading

  1. Jensen, S., J. G. Gehling, and M. L. Droser. "Ediacara-Type Fossils in Cambrian Sediments." Nature 393 (// 1998): 567-69.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated  03/06/2013

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading