Ediacaran Life on Land

Australia: The Land Where Time Began

Ediacaran Life on Land - a critique

Xiao¹ has examined the evidence presented by Gregory Retallack in his paper in Nature, Ediacaran Life on Land, and disagrees with the conclusions (Retallack, 1994; Retallack, 2012) reached by Retallack. The fossils in question that were recovered from the Ediacaran Member, South Australia, have previously been interpreted as representing ancestral forms of marine organisms (Xiao & Laflamme, 2009). Retallack reinterprets the fossils as terrestrial organisms, suggesting the fossiliferous beds are actually palaeosols, and that some of the Ediacaran fossils are actually lichens or colonies of soil microbes. Xiao¹ finds the evidence presented by Retallack to be unconvincing.

As this unit predates the evolution of land plants the definitive identification of palaeosols in the Ediacaran Member is problematic because it lacks features that are considered to be diagnostic of palaeosols such as traces of plant roots. The evidence for the Ediacaran Member being the result of pedogenesis presented by Retallack included many observations, such as its reddish colour, elemental and stable isotope geochemistry, surface disruption patterns, and gypsum sand crystals and carbonate nodules.

Xiaor¹ considers the evidence to be ambiguous, citing the reddish colour and the depletion of certain elements that he suggests could have resulted from Cenozoic weathering, and not the result of chemical weathering through pedogenesis during the Ediacaran (Gehling, 2000). The response to this criticism is that reddish colour of the strata would have been continuous, and not the observed grey and red beds that alternate, Xiao¹ claiming that the colours produced by weathering can vary according to the lithological characteristics of the rocks, such as mineralogical composition and permeability. In marine sediments carbonate nodules and gypsum sand crystals are commonly found and post-depositional alterations not involving pedogenesis in the Ediacaran Member can account for the isotope signals of carbonate nodules.

According to Xiao¹ Retallack uses large-scale disruption structures as examples of structures that are characteristic of soils, Xiao¹ suggesting that such structures are very similar to slumps or load structures that are produced by subaqueous and sediment movements that occur after deposition. Retallack (Retallack, 2013) interprets small-scale disruption structures as being produced by tubules of millimetre size that could possibly be the remains of bacterial filaments, lichen rhizomes, that are filaments with a root-like appearance, or the hyphae of fungi. The Xiao¹ suggests this interpretation is dubious, as the tubules are too regular for an interpretation that compares them to structures of microbial origin.

Xiao¹ suggests there is compelling evidence for a subaqueous, if not marine origin for the Ediacaran Member, describing the evidence provided by Retallack as ambiguous, suggesting the evidence for a marine origin outweighs the evidence for pedogenesis. As an example Xiao¹ uses for a subaqueous origin of the benthic organisms such as Cyclomedusa davidi and Dickinsonia costata, have been found in situ on rippled bedding surfaces (Gehling, 2000). Some fossils from the Ediacaran also possess holdfasts, root-like structures that show signs of having been dragged in the same direction as the alignment of their stalks (Tarhan, Droser & Gehling, 2010). It is required for these structures to been formed by the action of waves or currents. A suite of features that are characteristic of deposition in a subaqueous environment (Gehling, 2000), such as ripple marks and lineations produced by currents that have been revealed by sedimentological analysis.

Retallack reinterprets fossils found in the Ediacaran Member as soil lichens, microbial colonies, fungi, slime-mould trails, or possibly casts of needle ice (formed in soils that are frozen). At other sites around the world many species that have been found in Australia have been found in formations that are unambiguously of marine origin, in rocks such as black shales and limestones. Other evidence cited by Xiao¹ includes Dickinsonia fossils showing evidence of movement, whereas lichen are incapable of movement. An interpretation of needle ice casts by Retallack for Radulichnus, a fossil from the Ediacaran, have been interpreted by other researchers as parallel scratches in fanning sets in an arrangement that is not explainable by a needle ice interpretation. Retallack interprets trace fossils from the Ediacaran as trails formed by land-cruising slugs or slime moulds in their aggregation stage, Xiao¹ suggesting that the organisms responsible for these and other trace fossils from this time were not capable of digging the burrows that are clearly visible in the Ediacaran Member.

According to Xiao¹ he disagrees with Retallack's interpretation of the fossils in the Ediacaran Member, but believes Retallack's persistence in pursuing his proposal that at least some of the Ediacaran fossils are of terrestrial origin, the rocks they were found in actually being derived from palaeosols, forces other researchers to rethink that possibility that lichens, either terrestrial or marine, might have existed as early as the Ediacaran before the vascular plants arose, though clearer, less ambiguous evidence is required before the idea if fully accepted.

Sources & Further reading