from Anne in Adelaide, Australia: Walking on the Cryogenian, thinking of an Urmetazoan

River red gum in Alligator Gorge

6 September. The second day of our travels north was spent in the Mt Remarkable National Park, near the town of Wilmington, 320kms north of Adelaide. On reflection, this felt like being on the edge of the known world. The countryside north of Wilmington suffers from a severe shortage of rain and is dotted with half-deserted towns and collapsing stone buildings built over a century ago.

My husband and I elected to visit Alligator Gorge within the park. The origin of the name is in dispute. Rest assured, there are no alligators, nor crocodiles in this gorge. You might find a large goanna which looks intimidating but will not rise from the murky depths to grab your leg and refuse to let go. A long time ago, a large goanna raided my Queensland campsite. I learnt that they have an efficient sense of smell, a liking for cheese and powerful jaws.

salt water crocodile in the Daintree River, Queensland. Far away.

This National park was relatively green and we saw may euros, or common wallaroos, on our drive to the gorge. Euros are marsupials – they are smaller than the larger common Western Grey kangaroos and have noticeably darker paws and tails. They did not seem afraid of us, enjoying the grass in open areas, a teenage joey doing circuits round its mum. Visitors to Australia struggle with the many names of our marsupials and tend to call all hopping creatures of this shape, ‘kangaroos’. But we have potoroos, quokkas, bilbies, bandicoots, euros and wallabies to name a few survivors.

a Euro ‘jill’ (female) checking the joey in her pouch

Over time, rivers have cut into the 700 myo quartzite of Mt Remarkable leaving a steep gorge. We made it down the 272 steps, counting all the way. At the end, a small river was running and we did not feel inclined to wade through the slippery Narrows. Instead, we marvelled at the rocks beneath us, where the ripples marks of ancient seas were frozen in time. This is a geologist’s paradise: a moment to ponder the question of the origin of animals.

Ripple Rocks on the descent into Alligator Gorge, Mt Remarkable

These were rocks of the Precambrian (older than 541 million years before present): a period which covers almost 90 percent of Earth’s history. The Precambrian is split into three time spans and the Alligator Gorge rocks belonged to the more recent of those eras: the Proterozoic (good news – this is when oxygen first entered the earth’s atmosphere).

Keep on splitting as geologists do: the Proterozoic is again divided into three periods. The most recent is the Neoproterozoic era from 1,000 to 541 million years ago. We are getting there. More splits: the Neoproterozoic is split into three as well – the middle period of this is the Cryogenian period (cold birth) and the rocks on which we walked on the 24th August, 2020 were formed during the Cryogenian – between 720-635 million years ago.

I am sorry I cannot be more specific on the date.

the rocky Narrows of Alligator Gorge

I can only say we walked on the shore of an estuary or beach formed into rock when nothing lived on the land – since it was a frozen waste without much oxygen. Some call this the time of ‘Snowball Earth’, others, ‘Slushball Earth’. A matter of the degree of freezing.

The big question for scientists is when did animals originate? They evolved before the Ediacaran Period (635-542 million years ago). It is currently agreed that animals originated during the Cryogenian period, either in the depths of the frozen ocean around hydrothermal vents OR close to the ocean surface around a slightly warmer equator. So, we were walking on the rocks formed during the time the first animals appeared.

Another point of interest. The Adelaide Geosyncline or the Adelaide Superbasin (includes the Flinders Ranges, Mt Lofty Ranges and Kangaroo Island) consists of sedimentary rocks (some volcanic) that were deposited during the time of the breakup of the super-continent Rodinia (motherland). Thus, where we were standing in Alligator Gorge was on the eastern shore of Rodinia, an ancient land where the first animals stirred in the superocean of Mirovia.

https://earthlyuniverse.com/cryogenian-glaciations-birth-animals/

What I like most of all was learning this new word, a very important word. Urmetazoan. The urmetazoan, is our common mysterious ancestor living in the Cryogenian Period. What did it look like? – probably like a sponge.

All this science and questions of millions of years, puts our current world’s woes into a little perspective. Such issues were later discussed by our Field Geology Club of SA members, but not resolved, over dinner at the local Wilmington pub.

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