Fossils of soft-bodied organisms were first discovered in the Ediacaran Hills, part of the Flinders Ranges of South Australia in 1946. At the time, these fossils were presumed to be Medusae (Jellyfish) of Early Cambrian age, since it was believed that there were no Precambrian fossils. In the 1950s, fossils were found in reliably-dated Precambrian rocks in Charnwood Forest, England, showing that this presumed Cambrian appearance of life was incorrect. Furthermore, the Charnwood fossils closely resembled fossils previously described from Namibia (then Southwest Africa) in the 1930s, suggesting that these were of similar age.
Eventually, all of these fossils were grouped together as the 'Ediacaran Fauna', found at numerous locations in the world, and eventually split into three separate assemblages, each with its own distinct fossils, laid down in different environments at different times; the Avalon Assemblage, which appeared about 578 million years ago, and persisted to about 555 million years ago, the White Sea Assemblage, which included the Ediacaran Hills fossils, and which appeared about 560 million years ago, and persisted to about 551 million years ago, and the Nama Assemblage, which appeared about 555 million years ago, and disapeared 539 million years ago (around the onset of the Cambrian).
The earliest of these Ediacaran fossil assemblages seemed to have appeared slightly after the Gaskiers Glaciation, between 580.9 and 579.2 million years ago, taken as a middle point for the Ediacaran Period, leading to the assumption that the appearance of large Metazoans post-dated this event. However, a number of locations have subsequently produced Ediacaran-type fossils that apparently predate the Gaskiers Glaciation. Notably, the Charnwood Forest fossils can be dated to 603 million years before the present (Early Ediacaran), while the Lantian Biota of Anhui Province, China, has been dated to 605 million years ago.
Since the discovery of the original Ediacaran Hills fossils, sporadic attempts have been made to find older fossils within the Flinders Ranges, although an absence of obvious fossils, combined with a perception that they were unlikely to exist, has tended to limit such searches. In 2021, Philip Plumber of the Department of Earth Sciences at the University of Adelaide, reported finding macro-fossils in the approximately 700 million years old (Cryogenian) Areyonga Formation, and 970–950 million years old (Tonian) Heavitree Formation of Central Australia, opening the possibility that pre-Middle Ediacaran fossils might be more widespread in Australia.
In a paper published in the journal Transactions of the Royal Society of South Australia on 2 September 2024, Philip Plumber reports macrofossils from the Early Ediacaran Brachina Sequence of the Flinders Ranges.
The Brachina Sequence spans the interval between the end of the end of the Marinoan Glaciation, 635 million years before the present, which marks the boundary point between the Cryogenian and the Ediacaran periods, and the Acraman Asteroid Impact, 580 million years ago, which is coeval with the onset of the Gaskiers Glaciation. The Brachina Sequence begins with the Nuccaleena Dolostones, which overlay the Marinoan glacial deposits, above which lies the purple Moolooloo Siltstone, made up of clastic deposits brought into a shallow marine basin by turbid bottom currents. Around 620 million years ago, the tectonic situation changed, causing a delta to spread across the basin from the southwest. These delta deposits form the ABC Range Quartzite, formed in a shallow, wave-dominated environment, while other parts of the basin were covered by a tidal flat environment, recorded as the Moorillah Siltstone.
The first fossil described by Plumber was first recorded in 1969 by palaeontologist Martin Glaessner, who identified it as a trace fossil, Bunyerichnus dalgarnoi, apparently made by a 'bilaterally symmetrical animal which used rhythmic muscular contractions rather than discrete appendages for propulsion'. The exact stratigraphic position where this fossil originated is unclear, but it was found on a surface bedding plane on a partly cross-laminated dark purplish micaceous siltstone, at the entrance to Bunyeroo Gorge in the central Flinders Ranges, which would imply it came from either the upper Moolooloo Siltstone or the lower Moorillah Siltstone.
Subsequent to this discovery, other intepretations of Bunyerichnus have been put forward. The curving shape of the fossil led to the suggestion that it might be a portion of a Medusa, but this did not explain why the specimen appeared to taper to one end. An alternative suggestion is that the specimen might be a trace left by a Rangeomorph (frond-like) Ediacaran sweeping over the sediment in a shallow setting. Plumber notes that Rangeomorph fronds were described from the base of the ABC Range Quartzite in 1985 (when the age of these deposits was unknown, although they were recognised as being stratigraphically significantly lower) by Ian Dyson of Flinders University, and that these would have been of approximately the same age as Bunyerichnus.
Plumber also notes a number of circular features 0.5 to 1.0 cm in diameter from the base of the Moorillah Siltstone about 22 km southeast of Bunyeroo Gorge. These were first described by Plumber in 1980, when he interpreted them as inorganic fluid escape structures. However, subsequent examination of the specimens by Jim Gehling of the South Australian Museum led them being re-interpreted as examples of Aspidella, a Rangeomorph holdfast impression, with a fallen frond-lying next to the largest example. Plumber dates the horizon from which these fossils were recovered to about 620 million years before the present, firmly within the Early Ediacaran, older than the Charnwood Forest fossils, and about 60 million years older than the Ediacaran Hills biota.
The Moorillah Siltstone of the Brachina Sequence has been dated to between 620 and 605 million years before the present. Philip Plumber reports the presence of frond-like Rangeomorph fossils near the base of the Moorillah Siltstone, suggesting that these must therefore be close to 620 million years in age. Such fossils are roughly coeval with the Lantian Biota of South China, and at least 40 million years older than the global Avalon Assemblage. These fossils therefore contribute to growing body of evidence for the emergence of Metazoan life before the Gaskiers Glaciation in the Middle Ediacaran Period.
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