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First Fossil Animals — Ediacara Biota of South Australia

Jim Gehling South Australian Museum

Wilpena Pound The Cradle of Animal Life

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The Ediacaran Period — a New Rung in the Ladder of Time

Five hundred and fifty million years ago, and more than 400 million years before dinosaurs roamed the Earth, most of eastern Australia was ocean lapping onto what is now South Australia and Northern Territory. This ancient ocean was inhabited by the first large, complex marine organisms — the Ediacara biota. They were smothered by sand on shallow seafloors, leaving only their impressions in sandstone. As sea levels changed and continents collided, these seafloor layers were buckled up and eroded to become part of what we today know as the Flinders Ranges in South Australia. In 1946, while exploring for minerals, geologist Reginald Sprigg discovered fossil imprints in sandstone around the low hills of the western Flinders Ranges at the old Ediacara minefield. The fossils preserved at Ediacara record the first known multi-cellular animal life on Earth. This diverse and exquisitely preserved community of ancient organisms represents a snapshot of the earliest evidence of large, complex life from which we all descended — and one that warranted its own geologic time division; the Ediacaran Period. Before the discovery of Ediacara fossils, the oldest known animal fossils were Cambrian in age. It was thought that only animals with hard parts, such as shells, spines and scales, could be preserved in the fossil record. The Ediacara fossils gave a new understanding of the early evolution of complex life on Earth, and of the preservation of soft-bodied organisms as casts and moulds.

The Ediacara fossils, ranging in size from a few centimetres up to more than a metre long, resemble soft corals, sponges, flat-segmented worms, molluscs, crustaceans and the possible ancestors of trilobites and vertebrates. Impressions of the largest known animal of its time, Dickinsonia rex, have also been found near Ediacara. Some of the smallest Ediacara fossils, like Spriggina, that appear to have heads and segmented bodies, may represent the earliest members of living animal groups.

Spriggina There are many questions surrounding these ancient organisms, like how they lived, fed, reproduced, dispersed around the globe, and if they had any living descendents. Some scientists think that the Ediacara biota may represent multi-cellular ‘experiments’ that bear little resemblance to living groups of organisms. However, it is more likely that amongst the host of weird creatures in the Ediacara biota we can identify the forebears of all living animal groups. The Ediacaran Period was the first geological period to be declared in 120 years and the first to be based in rocks of the Southern Hemisphere. Covering an interval from 635 to 542 million years ago, the start of the Ediacaran Period corresponds to the end of a world-wide glaciation, often referred to as ‘Snowball Earth’, as well as significant changes in ocean chemistry. Fossils of the Ediacara biota represent a biodiversity revolution: the first explosion in abundance, size, complexity and diversity of life forms since the origin of life some 3000 million years earlier. Around 30 other Ediacara localities are now known globally, including sites in Namibia, Russia, Newfoundland, China, NW Canada, England and Siberia.

Arkarua

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National Heritage Ediacara Fossil Site at Nilpena.

Unlike the Ediacara Conservation Park, where collecting went on unmanaged for 50 years, the fossil site at Nilpena has been conserved as an outdoor museum, where we have exposed, excavated, flipped and reassembled large samples of fossil covered seafloors, so that they can be studied for their fossil ecology. We collect only the specimens required to describe new species. Everything else is retained on site for current and future research. Unlike most fossils, the sandstone impressions do not suffer when exposed to a desert climate. The Nilpena site allows us to study not only the fossils but also their ancient life environments. This means that Nilpena is uniquely important as a field laboratory for researchers to use to try to understand this critical time in Earth history, when large life bloomed for the first time. No other site on Earth offers such a fine record of early life and environments. The last 10 years of study at Nilpena have revealed many organisms new to science as well as insights to how they lived, fed and reproduced. We have discovered that: 1) Almost every fossil-bearing surface was coated with microbial mats and organic debris, since

very few Ediacarans were efficient bottom feeders. 2) Flat, segmented animals like Dickinsonia fed by resting on seafloor mats and digesting them,

before moving to another place. 3) Masses of finger-shaped tubes called Funisia reproduced sexually and colonized entire

seafloors where their larvae settled. 4) Eoandromeda, a clockwise spiralling, eight-armed animal, first found at Nilpena, was later

discovered in Ediacara sedimentary layers of south China. 5) Some animals were torn from the shallow seafloor and washed into deeper water, leaving their

anchoring discs behind. 6) Groove traces, like you would find on a beach at low tide, were made by shallow burrowing

animals too small to be preserved clearly. 7) Most Ediacarans were bottom living rather than swimmers or floaters. 8) Each new layer of sand appears to have preserved a different community of bottom living

animals. 9) The Ediacaran Period ended when animals began to burrow into the seafloors, either to escape

being eaten by newly evolved predators, or to exploit the buried organic matter trapped between layers of sand.

10) The overlying sediments of the Cambrian Period preserve very few animals without shells or hard parts.

11) Rare deposits of Cambrian fossils without hard parts, such as that being studied at Emu Bay on Kangaroo Island, show that the explosion of life in the Cambrian was a product of new ecosystems quite unlike those of the Ediacaran Period.

Nilpena will be managed to allow the next generation of scientists to ask new questions about the origins and evolution of early animals. All fossils are conserved on site. The only fossils that end up in the South Australian Museum are those used to describe new species or modes of preservation previously unknown.

Volunteers moulding fossils on an excavated

fossil bed

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Brachina Gorge Geological Trail — a Corridor Through Time The Brachina Gorge Geological Trail in the Flinders Ranges National Park can be entered from the Hawker Leigh Creek road, west of the ranges, or the Hawker Blinman road up the centre of the ranges. The trail through Brachina Gorge offers one of the most spectacular scenic views in the Flinders Ranges. At either end an interpretation booth explains the highlights of the trail. Entering from the west, the trail is marked by a series of signs indicating the named rock formations and the key geological sites, from youngest to oldest. The trail winds through layered rock tilting steeply toward the west. Thus, in 20 km one can pass through rock formations that represent 150 million years of the most important two chapters in Earth history. Key sites on the trail include:

1) The Cambrian rocks occurring at the western entrance preserve the fossil skeletons of trilobites. Trilobites were the most common arthropods of Cambrian seas, and a dominant group for almost 250 million years. These rocks are rich in all kinds of small shelly marine animals, like brachiopods and molluscs.

Atops

2) The first section of Brachina Gorge is lined by

massive limestone cliffs built from microbes and the skeletons of ancient sponges. In the creek bed, 2 km into the gorge, stop to look at a water-polished mass of limestone that is a reef of these sponges called archaeocyatha. They can best be studied by splashing water on the red or black limestone. The rock shows wagon-wheel like cross sections through these beautiful porous vase shaped sponges or V-shaped sections parallel to their body axis. They are index fossils for the Cambrian, world-wide, but went extinct half way through the Cambrian Period.

Archaeocyatha (c. 530 Ma)

3) As the road passes the Teamster

Campground, a sign points up a small gully where we see the most significant geological boundary in Earth history. Sloping sandstone layers on the east side have no fossils. Crumbling, rusty sandstone on the west side is entirely churned by burrowing crustaceans or worms. Vertical U burrows represent the first evidence of deep burrowing on Earth. The boundary between is the Ediacaran-Cambrian boundary marking the demise of the Ediacara biota and the onset of the so-called “Cambrian explosion of life”. The fact that animals began to burrow suggests that predators had evolved by this time.

Straddling the Ediacaran — Cambrian Boundary & U-

shaped burrows

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4) One kilometre east, though the steepest and narrowest part of Brachina Gorge, a sign indicates the level of the Ediacara fossils. They are found where red, thinly layered sandstone, in a side gully on the south side, gives way to thicker layers of cream-coloured sandstone. Fossils are common on what look like carpet rolls of sand. This is a cross section through a delta, where sand cascaded down over deeper water muddy silts and sands. The crumbling cliff makes this a very dangerous site. A replica of fossils collected here is fixed to a rock near the signs in the parking bay.

5) The track winds though the gorge, and 1km further on, it passes a talus slope where an active fault has shattered the sandstone. This is the habitat of the endangered Yellow Footed Rock Wallaby. The area is fenced off to allow these beautiful animals to be observed without scaring them.

6) The road trends east though older Ediacaran formations with sparse Ediacara fossils, until it opens into Aroona Gorge. One route goes south to the spectacular Bunyeroo Gorge. The other continues east to a lookout over the Aroona Valley. Here a sign explains the significance of a single layer of much older volcanic fragments — the Acraman ejecta layer — marking an event when a meteor struck northern Eyre Peninsula, 300 km west in what are now the Gawler Ranges, forming the Acraman crater and blasting red volcanic rock into the atmosphere, At this time, 680 million years ago, the Flinders Ranges was the site of a relatively deep ocean. The impact ejecta settled as one layer in the muddy maroon coloured sediment of the Bunyeroo Formation. After the ocean basin was filled and Proto-Australia collided with other continents, the layers of seafloor were buckled and domed. As erosion wore the upthrust rocks away, the Acraman layer was exposed. It can be followed thoughout the ranges — marking a time line that is used to correlate these rocks with its trace in bore holes 600 km away in the NW of South Australia!

Acraman discoverers: Peter Haines and Victor Gostin.

7) The base of the Ediacaran Period is marked in Enorama Creek. 500 m north of the Brachina

Trial, some 5 km further east. Take the track toward Trezona Camp Ground, off the Brachina road. A sign where the track turns left before Enorama Creek, directs visitors to walk 300 m up stream to the Ediacaran “Golden Spike”. On the south bank, under a gum tree that shades tent-like structures in the yellow dolomite formation, you can find a brass disc making the global position of the designated point marking the base of the Ediacaran Period. This is the global reference point that allows geologists to correlate this level into south China, Namibia and Oman — wherever rocks of this time are preserved. This “Golden Spike” marks the end of the

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greatest ice age ever recorded. The red muddy sandstone in the creek to the east consists of debris transported and deposited by massive ice sheets. Chunks of exotic rock lie within a matrix of gravel, sand, silt and clay. Only ice is capable of carrying such a mixture. What makes this site unique is that the palaeomagnetism (magnetic orientation of mineral grains in the rock) shows that the ice deposited sediment in this ancient ocean, 635 million years ago, when this part of Australia was within 5-10° of the equator. This evidence led to the concept of “Snowball Earth” — that the Earth was all but covered with ice, on at least two occasions, during the appropriately named Cryogenian Period, spanning 635 to about 800 million years ago.

Ediacaran GSSP (“Golden Spike”) Enorama Creek, Flinders Ranges National Park.

8) The only visible fossils in the Cryogenian Period were stromatolites — limestone domes

built by microbes. Driving 1.5 km east of the Trezona Camp Ground turn off, along the Brachina Road, an interpretive sign in a parking bay on the north side of the road marks a layer of stromatolites in cross-section. However a well-exposed stromatolite reef occurs in Enorama Creek 500 m north of the road. To get there, turn left (north) at the Youngoona Track and park at the turn-about near the ruin of an old shepherd hut. Follow the walking track 200 m west into the creek. Staying on the south side, walk 100 m until you see a sloping rock bench with domed stromatolites of all shapes and sizes.

Trezona Formation: stromatolite reefs.

Common Fossils of the Ediacara Biota

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Gehling Flinders Ranges Treasures 8

Background Reading 1. Gehling, J.G. and Droser, M.L., 2013. How well do fossil assemblages of the Ediacara Biota time?

Geology, 41, 447-450.

2. Gehling, J.G. and Droser, M.L. 2012. Ediacaran stratigraphy and the biota of the Adelaide Geosyncline, South Australia. Episodes 35, 236-246.

3. Gehling, J. G., and Droser, M. L. 2009. Textured organic surfaces associated with the Ediacara biota in South Australia. Earth Science Reviews 96, 196-206.

4. Droser. M.L. and Gehling, J.G. 2008. Reproduction and synchronous aggregate growth in a new Ediacaran tubular organism. Science 319, 1660-1662.

5. Gehling, J.G. & Vickers-Rich, P. 2008. The Ediacara Hills. In Fedonkin, M.A., Gehling, J.G., Grey, K., Narbonne, G.M. and Vickers-Rich, P., The Rise of Animals. Evolution and Diversification of the kingdom Animalia. Johns Hopkins Press, Washington, Chapter 5, 88-113.

6. Peterson, K.J., Cotton, J.A., Gehling, J.G. and Pisani, D. 2008. The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records. Philos Trans R Soc Lond B Biol Sci.10.1098/rstb.2007.2233.

7. Droser, M. L., Gehling, J.G. and Jensen, S.R. 2006. Assemblage palaeoecology of the Ediacara biota: The unabridged edition? Palaeogeography, Palaeoclimatology, Palaeoecology 232, 131-147.

8. Gehling, J.G., 1999. Microbial mats in terminal Proterozoic siliciclastics: Ediacaran death masks: Palaios 14, 40-57.

9. Gehling, J.G., 1991.The case for Ediacaran fossil roots to the metazoan tree. Journal of Geological Society of India Memoir 20, 181-224.

10. Gostin, V.A., McKirdy, D.M., Webster, L.J., and Williams, G.E. 2010. Ediacaran ice-rafting and coeval asteroid impact, South Australia: insights into the terminal Proterozoic environment. Australian Journal of Earth Sciences 57, 859-869.

11. Zhu, M., Gehling, J.G., Xiao, S., Zhao, Y. and Droser, M.L. 2008. Eight-armed Ediacara fossil preserved in contrasting taphonomic windows from China and Australia. Geology 36, 867-870: doi: 10:1130/G25205A.1;2; 2 figures: Data Repository item 2008222.