[Palaeontology, Vol. 54, Part 6, 2011, pp. 1223–1242] PROBLEMATIC MEGAFOSSILS IN CAMBRIAN PALAEOSOLS OF SOUTH AUSTRALIA by GREGORY J. RETALLACK Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA; e-mail: [email protected] Typescript received 13 February 2009; accepted in revised form 20 August 2009 Abstract: Red calcareous Middle Cambrian palaeosols from Other axial structures (Prasinema nodosum and P. adunatum the upper Moodlatana Formation in the eastern Flinders gen. et spp. nov.) are larger and show distinctive surface Ranges of South Australia formed in well-drained subhumid irregularities (short protuberances and irregular striations, floodplains and include a variety of problematic fossils. The respectively). The size and form of these filaments are most fossils are preserved like trace fossil endichnia but do not like rhizines of soil-crust lichens. Other evidence of life on appear to be traces of burrows or other animal movement. land includes quilted spheroids (Erytholus globosus gen. et sp. They are here regarded as remains of sessile organisms, com- nov.) and thallose impressions (Farghera sp. indet.), which parable with fungi or plants living in place, and are formally may have been slime moulds and lichens, respectively. These named as palaeobotanical form genera under provisions of distinctive fossils in Cambrian palaeosols represent commu- the International Code of Botanical Nomenclature. Most nities comparable with modern biological soil crusts. common are slender (0.5–2 mm) branching filaments flanked by green-grey reduction haloes within the red matrix of pal- Key words: Cambrian, South Australia, palaeosol, fungus, aeosol surface horizons (Prasinema gracile gen. et sp. nov.). slime mould, lichen. B iological soil crusts in the distant geological past Ranges, South Australia (Text-figs 1–3). The Cambrian have long been suspected because of dispersed spores palaeosols described here predate the evolution of land (Gray 1981; Strother 2000), abundance of pedogenic clay plants but nevertheless contain three distinct kinds of (Kennedy et al. 2006), unusually deeply weathered com- enigmatic megafossils: (1) drab-haloed filament traces position of Cambrian sandstones (Dott 2003), carbon iso- (Prasinema gen. nov.), (2) quilted spheroids (Erytholus topic composition of palaeosols (Watanabe et al. 2000) gen. nov) and (3) thalloid impressions (Farghera sp. in- and microbially textured bedding planes (Prave 2002). det.). Although the biological affinities of these fossils Middle Cambrian microbial filaments (Southgate 1986) remain uncertain, they provide new guides to the appear- and lichen-like fossils (Fleming and Rigby 1972; Mu¨ller ance of Cambrian life on land. and Hinz 1992; Retallack 1994) from phosphorites of Formal naming of fossils aids future investigation and western Queensland are in sedimentary facies with evi- wide recognition, as demonstrated by other problematic dence of exposure within tidal flats and rock platforms fossils. For example, Vendobionta were informally noted (Southgate 1986), but such marine-influenced communi- by Mawson (1938, p. 259) as ‘fossil impressions resem- ties are not directly comparable with biological soil crusts bling brachiopod or bivalve form’, but formal description of modern deserts. Unlike grey marine cherts and phos- of five species by Sprigg (1947) was needed before their phorites with permineralized fossils or grey shales with global distribution and importance as Ediacaran fossils carbonaceous compressions, Silurian to Quaternary red could be appreciated (Fedonkin et al. 2007). The various oxidized palaeosols seldom preserve cellular detail of car- fossils in Cambrian palaeosols formally named here are bonaceous fossils, so efficient is recycling in well-drained preserved as bedding disruptions comparable with some soils (Retallack 1998). Nevertheless, post-Silurian oxidized kinds of trace fossil (endichnia of Martinsson 1970) but palaeosols commonly preserve impressions of leaves, lack backfills, sequential prints or shapes recording move- stems and roots, often with drab mottling from biochemi- ment or behaviour of motile organisms. Each of the dif- cal reduction of buried organic matter (Retallack 1997c). ferent fossil genera described here is preserved in a Problematic megafossil traces of life on dry land now different way, but all appear to have been remains of come from numerous green-red-mottled palaeosols in the sessile organisms such as fungi, lichens, algae or plants in Middle Cambrian, Moodlatana Formation of the Flinders place of growth. Thus, the nomenclatural system ª The Palaeontological Association doi: 10.1111/j.1475-4983.2011.01099.x 1223 1224 PALAEONTOLOGY, VOLUME 54 TEXT-FIG. 1. Geological map and fossil locality on Ten Mile Creek, South Australia. appropriate to these fossils is not that of ichnofossils in GEOLOGICAL SETTING the International Code of Zoological Nomenclature (Ha¨ntzschel 1975; Ride et al. 1999) but rather of palaeo- All fossils reported here were collected from a large expo- botanical form genera in the International Code of Botan- sure of the upper Moodlatana and lower Balcoracana For- ical Nomenclature (McNeill et al. 2006). Form genera mation, within a prominent anticline, north of the big such as Thallites (Walton 1923) and Algites (Seward bend in Ten Mile Creek, six miles west of the road to 1894), for example, are used for fossils with the distinctive Martins Wells, on Wirrealpa Station, South Australia dichotomizing form of algal, liverwort or lichen thalli, but (31°25¢N, 138°94¢E). These palaeosols are early Middle whose exact systematic affinities are uncertain, because Cambrian in age, immediately above upper Moodlatana histological and reproductive structures are not preserved. Formation grey shales with the trilobite Onaraspis rubra stromatolitic limestone Mindi palaeosol Natala palaeosol Viparri palaeosol A hammer B TEXT-FIG. 2. Selected fossiliferous palaeosols (A) and distinctive features (B) of the fossil locality in the uppermost Moodlatana Formation in cliffs flanking Ten Mile Creek, South Australia. The stromatolitic limestone marker here is the base of the Balcoracana Formation. For other palaeosols, see Retallack (2008). RETALLACK: CAMBRIAN PALAEOSOL FOSSILS 1225 TEXT-FIG. 3. Field sketch of three successive palaeosols, including those yielding fossils described here (upper two only), Middle Cambrian, uppermost Moodlatana Formation, Ten Mile Creek, South Australia. The palaeosols are at 3602 m in Ten Mile Creek section and measured palaeochannels from 3557 and 3561 m, in the next outcrop to the south and west. (Jago et al. 2006), equivalent to the Oryctocephalus indicus in contrast have extensively disrupted bedding and subsur- zone (Gradstein et al. 2004). At 3602 m in the composite face caliche nodules (Text-fig. 3) of floodplain soils. The section in Ten Mile Creek, these palaeosols are 508.8 Ma Mindi pedotype is intermediate between these extremes, old in the age model of Retallack (2008). with small subsurface gypsum crystals and some persistent Unlike thin marine dolomites and shales of the Moodla- bedding, and is interpreted as a high supratidal palaeosol tana and overlying Balcoracana Formations (Moore 1990), (by Retallack 2008). All the fossils described here are from the fossiliferous palaeosols represent dry land in terms of only three (Mindi, Natala and Viparri) of seven pedotypes both soil drainage and palaeoclimate (Table 1). Pervasive known in the upper Moodlatana Formation (Table 1). cracking, haematite, loess-like grain-size distribution and Both Natala and Viparri pedotypes have a calcic horizon low FeO content of red parts of the palaeosols (Table 2) deeper than usual for the Moodlatana Formation and rep- are evidence of well-drained soils of floodplains and supra- resent a time of subhumid, rather than semi-arid climate, tidal flats. Crack orientation orthogonal to fluvial palaeo- immediately before marine transgression of the basal Balc- channels (Text-fig. 3) is characteristic of gilgai microrelief oracana Formation (Retallack 2008). of Vertisols (Paton 1974). Nodules of gypsum and micrit- The fossils described here are surprisingly large and ic, low magnesium calcite at shallow depths within the pal- plentiful for what would be expected in Cambrian palaeo- aeosols are evidence of semiarid to subhumid Middle sols (Retallack 2008), and the question may be raised Cambrian palaeoclimate. Different kinds of palaeosols whether they represent biological activity after the Cam- (pedotypes) have been interpreted to represent different brian. They do not appear to be products of modern local conditions (Retallack 2008), ranging from intertidal weathering, Cenozoic lateritization or Permian glacial to fluvial (Table 1). The Irkili pedotype for example has landscapes because found in deep boreholes: Prasinema is much relict bedding, including flaser and linsen bedding, common at 1493–1504 feet, Farghera at 1498–1500 feet in and prominent calcite geodes after gypsum crystals, as in Lake Frome no. 2 core and Prasinema at 2089–2090 feet in soils of supratidal flats. The Natala and Viparri pedotypes Lake Frome no. 3 cores archived in the Primary Industries TABLE 1. Palaeosol pedotypes in the Middle Cambrian, Moodlatana Formation, Ten Mile Creek, South Australia. 1226 Pedotype Adnamatna Diagnosis Palaeoclimate Former biota Palaeotopography Time for formation PALAEONTOLOGY, VOLUME 54 name meaning Imba Ash Thin green mottles (A) Not relevant Fluvial microbial mat, Supratidal-alluvial 5–10 years