Jago et al. (2012) regard problematic fossils described by Retallack (2011) as dubiofossils or pseudofossils from modern weathering, yet fail to propose a specific plausible model for formation of the disputed fossils in a weathering profile. Nevertheless, their comments are helpful for many reasons. First, they emphasize the problematic nature of the fossils, which were formally named to attract additional attention. Second, they have prepared colour illustrations of these green and red fossils to supplement the original black and white versions (although only a tiny corner of the originally arrowed fossil is seen in their panel C, which is centred on an unnamed mottle). Third, two authorities on trilobites (Jago and Paterson), after close inspection of the type material, find no similarity between Erytholus and enrolled trilobites, a conclusion shared by Retallack (2011). Fourth, an expert on Ediacaran fossils (Gehling) finds little similarity between Erytholus and Ernietta or Pambikalbae, another conclusion of Retallack (2011). Fifth, they allow that the Cambrian Lake Frome Group includes palaeosols (Retallack 2008), as also proposed by Stock (1974) and Moore (1990). Sixth, they highlight comparisons between Ediacaran and the Cambrian fossils and palaeosols, one of the aims of Retallack (2011). Seventh, they allow me to affirm the orientations of the fossils which were either crack-out specimens from outcrop, or slabs and thin sections oriented (and illustrated) vertical to bedding (Retallack 2008, 2011). Jago et al. (2012) were evidently unfamiliar with orientation markings used on these irregular and massive samples of palaeosols, which are marked by a circle drawn in felt pen on the upper side to define a plane parallel to regional bedding (Retallack 2001). The main claim of Jago et al. (2012) is that fossils named by Retallack (2011) lack regularity of form as a criterion of biogenicity. A more comprehensive list of criteria for assessing biogenicity of Precambrian problematica (from Hoffman 2004) is as follows: (1) known provenance, (2) plausible environment for life, (3) same age as the rock, (4) plausible composition, (5) taphonomic series, (6) repeated complexity. All six criteria are addressed in the following paragraphs, with specific reference to the disputed Cambrian fossils of South Australia. Few of these criteria can be evaluated from a handful of holotypes deposited in a museum collection as a standard of comparison for future collections. Retallack (2011) provided stratigraphic sections and locality photographs to specify the exact layers from which the described fossils were found within a long sequence of palaeosols, palaeochannels, tidal deposits, and marine shales and limestones (documented by Retallack 2008). The rocks with disputed fossils were not picked up from the surface as implied by Jago et al. (2012). All the fossils reported were quarried from palaeosol outcrops. The small size of some specimens is not owing to slaking or weathering in outcrop, but to fractures from closely spaced cutans (mainly clay skins), which split on quarrying (Retallack 2008, 2011). The fossils all came from hackly palaeosol beds and not from intervening shale, siltstone or limestone. Palaeosols are a likely environment for life, and undisputed for the Lake Frome Group by Jago et al. (2012). Abundant life, at least at the microbial level, is the most likely explanation for many documented palaeosol features: surficial enrichment in clay, segregation of calcite nodules and gypsum crystals, and depletion of alkali and alkaline earth elements and phosphorus within the palaeosol profiles (Retallack 2008, 2012a). The most convincing evidence that the disputed fossils are the same age as enclosing sediments is Prasinema in a redeposited clast of red palaeosol within a palaeochannel (Retallack 2008, fig. 9C). If this Prasinema were owing to post-Cambrian weathering, the entire 2 m [Palaeontology, Vol. 55, Part 4, 2012, pp. 919–921]
Read full abstract
Jun 1, 2020
Jasna Hrenović + 5
Open Access
