an autodidact meets a dilettante…

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Posts Tagged ‘fossils

a deeper dive into the shallow waters of Emu Bay

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Jacinta: So Emu Bay, shale, Cambrian, trilobites, early complex life, Kangaroo Island, why Kangaroo Island, where do we begin?

Canto: Well, let’s just begin. Apparently the first fossil finds, the first signs that there was something significant, date from the fifties, but it wasn’t until the seventies that real excitement grew.

Jacinta: And these finds were from the Cambrian. Can you give us some background on the so-called Cambrian explosion, and the geological epochs as they pertain to life forms?

Canto: The Cambrian explosion dates to around 530 million years ago. The most celebrated evidence of this comes from the Burgess shale in British Columbia, Canada, though the finds there date to about 510 million years ago – the middle Cambrian. Emu Bay’s fossils have been dated further back in time, though as always there’s some uncertainty as to precise dating. Another famous deposit, the oldest, is in southern China, the Chengjiang fauna.

Now, briefly, the planet’s life-span has been divided into six eons, which you can take as seriously as you like: first, the Hadean, from Earth’s formation 4.6 billion years ago to the end of the late heavy bombardment around 4 billion years ago; second, the Archaean, when life began and then photosynthesis evolved; third, the Proterozoic, from 2.5 billion years ago to 540 million years ago; fourth, the Paleozoic, to 250 million years ago, then the Mesozoic, to 65 million years ago, and finally the sixth, Cenozoic eon, up to the present day. Though the last three are sometimes called ‘eras’ under the title of the Phanerozoic eon.

Jacinta: So how does the Cambrian and other epochs or whatever, map onto this – the Cretaceous, the Jurassic and so forth?

Canto: Well, these are called periods, but let’s not get too caught up in all that, and let’s start with the Cambrian period, as we’re concerned with more or less recognisably modern complex life. It’s generally agreed to date from 540 million years ago, following on from the Ediacaran period, and has been divided into Early, Middle and Late, at least by some, and was followed by the Ordovician some 485 million years ago…

Jacinta: Okay, enough, let’s get back to Emu Bay and the Cambrian explosion.

Canto: Just look online and you’ll find a ton of info on this and the other Cambrian deposits, so I’ll provide links to those sites that have helped me.

Jacinta: And a glossary, maybe.

Canto: Arthropods – from which modern spiders, insects and crabs evolved – and molluscs came into being in the Cambrian, and they’re well represented in the Emu Bay shale, dating from around 520 million years ago. Trilobites (three-lobed critters), a type of arthropod, are particularly well represented. They’re the earliest known creatures to have developed ‘full’ eyesight, I think, which would make them pretty devastating predators at the time.

Jacinta: Eyesight’s an interesting one, and it seems complex sight requires brains as well as good lenses….

Canto: Yes it is complex, and sight is obviously going to be one development among many in the fight for advantage, and developed and used differently in different environments. Anyway, one of the most interesting and important things about Emu Bay is the preservation of soft tissue – crab muscle, trilobite antennae for example. The types of antennae are very revealing apparently. And they’ve even found the turds of these creatures…

Jacinta: So how is it possible for muscle tissue etc to be preserved for over half a billion years?

Canto: That’s a very good question. It’s obviously a rarity – unless there’s an explosion of such finds in the future. A Catalyst program on Emu Bay from 10 years ago puts it this way:

Why this rare occurrence happens is not entirely clear. But it appears that, 520 million years ago, the bottom layer of the sea was depleted of oxygen; no scavengers could disturb the dead and no bacteria could survive to decay the soft tissue.

Jacinta: But one of the big differences between this site and the Burgess shale is that these were shallow water creatures, and the Burgess shale preserved deep water creatures, is that right? So these might have been more exposed to air?

Canto: Well the issue we’re looking at here comes under the heading of ‘taphonomy’ – the branch of palaeontology that deals with the processes of fossilisation. And taphonomy seems very much a work in progress – progressed further by analysis of this site. But it does get very technical. Let me give you an example, from a paper published in the Journal of the Geological Society in 2016:

The EBS [Emu Bay Shale] seems to have been rapidly deposited in a relatively nearshore setting adjacent to an active tectonic margin that generated continual syndepositional faulting and slumping. The Konservat-Lagerstätte interval appears to form part of a localized, deeper-water micro-basin succession on the inner shelf that was subject to fluctuating oxygen levels, at least in the bottom waters (Gehling et al. 2011). This depositional setting is in stark contrast to the majority of other Cambrian Konservat-Lagerstätten, specifically Burgess Shale-type deposits that formed in outer shelf environments, either near or immediately adjacent to the seaward margins of expansive carbonate platforms (e.g. Burgess Shale), or offshore of broad clastic shelves (e.g. Chengjiang) (Gaines 2014).

Jacinta: Hmmm, I think I get the continental drift.

Canto: As to the oxygen question, that’s still being worked on. And as to the deposit being ‘adjacent to an active tectonic margin’, I don’t get that. The whole of Australia sits on a large tectonic plate, the Australian plate, which stretches way south of Kangaroo Island. Perhaps plates can be sub-divided into micro-plates, I don’t know.

Jacinta: Perhaps an active tectonic margin just means a fault-line. But enough of the geology, tell us about the creatures themselves – some of the first predators – and their well-developed eyes.

Canto: More than 50 separate species have been found there, though in terms of specimens, the trilobite Estaingia bilobata dominates. Trilobites are incredibly common in the fossil record, with some 17,000 species known. The earliest ones found seem already highly diversified but their origin in the pre-Cambrian is very much a mystery.

a heap of Estraingia bilobata trilobites found at Emu Bay. I’m guessing from the rule on the right that each one is 2-3 cms long

Jacinta: And do these trilobites have amazing eyes?

Canto: They’re among the first animals we know of to have complex eyes and their lenses were made of calcite, which fossilises well. It’s also hypothesised that the early success of trilobites with their weaponised, prey-catching eyes helped to trigger or speed up the Cambrian explosion of diversity. But the big story about eyes fossilised at Emu Bay isn’t trilobite eyes. An abstract from Nature describes a creature with eyes more complex, and better preserved than any others for the following 85 million years:

The arrangement and size of the lenses indicate that these eyes belonged to an active predator that was capable of seeing in low light. The eyes are more complex than those known from contemporaneous trilobites and are as advanced as those of many living forms. They provide further evidence that the Cambrian explosion involved rapid innovation in fine-scale anatomy as well as gross morphology, and are consistent with the concept that the development of advanced vision helped to drive this great evolutionary event.

Jacinta: I seem to remember reading about this a few years back.

Canto: Yes, Ed Yong of not exactly rocket science did a great post about it. The animal is called Anomalocaris, meaning strange shrimp, and it has been discovered, or uncovered, bit by bit over more than a century, in the Burgess shale and elsewhere. Its 3cm-wide eyes, stuck out on stalks, were about 30 times more powerful than those of trilobites, with at least 17,000 lenses in each eye – all of which was discovered from a single specimen at Emu Bay, the only place where soft tissue was preserved, though specimens of Anomalocaris have been discovered around the world. Including many specimens found at Emu Bay itself.

Jacinta: So, this discovery, of the eyes, really put Emu Bay and Kangaroo Island on the map for a time.

Canto: Well, sort of, among the cognoscenti. But yes, it was exciting to think of such a marvellous find so nearby. And there may well be a lot more to discover.

fossilised eyes of Anomalocaris

References

https://en.wikipedia.org/wiki/Emu_Bay_Shale

https://en.wikipedia.org/wiki/Anomalocaris

https://www.nature.com/articles/nature10689

http://blogs.discovermagazine.com/notrocketscience/2011/12/07/anomalocaris-sharp-eyes-predator/#.WtMuHS_L0go

http://jgs.lyellcollection.org/content/173/1/1

Written by stewart henderson

April 15, 2018 at 9:42 pm

Kangaroo Island – return to Emu Bay

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Canto: I wanted to find out more about Emu Bay’s famous fossils so I decided to return and take the trek round the bay to the eastern extremity, photographing any rocky scenario I could find. The good thing was that, again, the weather was perfect for a long walk, and my thoroughly salubrious saunter helped me to break the record for most daily steps recorded on my iphone since I bought one eighteen months ago. The bad thing was that I really had no idea what I was looking for – Emu Bay shale, Burgess shale, WTF is shale? Is it a kind of rock? What colour and texture does it have and how is it formed? I should have researched the matter before proceeding, perhaps.

No matter, I took plenty of photos and now it’s a matter of mapping what I’ve found onto the descriptions in the literature.

shale – typically exhibits varying degrees of fissility, breaking into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding plane because of the parallel orientation of clay mineral flakes.[1] Non-fissile rocks of similar composition but made of particles smaller than 0.06 mm are described as mudstones (1/3 to 2/3 silt particles) or claystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones.[1] Shale is the most common sedimentary rock. (Wikipedia)

This description doesn’t really help me. Fissility means the tendency of rocks to splinter along lines of weakness, which doesn’t help me either. I tried google images, but the variety of shale presented, and the near-complete lack of any connecting factors, didn’t help me either.

However, when I tried images for Emu Bay shale in particular, I felt some definite progress. Some of the shale was copper-brown, some was slate-grey, some dull yellow, some penicillin green. But I recognised some of the colours and textures in the rocks I photographed.

A trilobite in Emu Bay shale. Trilobites are the most long-lived class of complex creatures – in this case arthropods – of all time, I think, having inhabited the planet for about 270 million years: more than a thousand times longer than Homo sapiens (so far). Some 50 species of trilobite have been found at Emu Bay

Like many, of my generation at least, I learned at school that there were three kinds of rock – igneous, metamorphic and sedimentary. As Wikipedia informs us, shale is sedimentary, and that makes sense as it’s in sediment that fossils are found. Volcanic rock is extruded igneous rock, and there appears to be quite a bit of it at Emu Bay.

volcanic (extruded igneous) rock – I think – at Emu Bay. Lots of it about. Spongy and low density

A lot of this rock sits in gigantic chunks on the sand, but elsewhere they form craggy, small cliff-like structures.

One small area had what I suppose were sedimentary rocks with a coloration completely different from the rest – and of course the more you pay attention to rocks (and everything else) the more variety you find. They were a sulphurous yellow…

an anomalous bunch of yellowish rocks at the eastern end of Emu Bay

Other rocks looked like granite – intrusive igneous rocks – but another prevalent type I saw, forming ridges high above me, was a rock type I can’t easily identify, though no doubt it’s common enough.

Slate-like rock forming small cliffs, visible along much of the eastern side of Emu Bay

But it’s only through reading that I’ve found the type of rocks I’m after, and the more precise location of the fossil-rich shale. The colour is a dark coffee brown, as shown in the photo of the trilobite above, and these rocks only crop up (or crop out, to write technically) at the easternmost tip of the bay. In fact the onshore location of the fossil-rich shale is further still, a few hundred metres east of the bay proper, with a further site a few hundred metres inland. I would’ve had to clamber over the rocks photographed below, and get a bit wet, to find myself on-site.

the end of the road for me – but these are precisely the kind of rocks I was looking for, without knowing it. They’re only at the easternmost tip of the bay, and beyond

No matter. Hopefully the sites themselves are well-protected. I’m mindful of the concern about looters and trophy hunters, and the preciousness of such places as minefields of info about the extraordinary variety of the first highly successful complex life forms after the initial experiments of the Ediacaran biota a few million years before.

More about that in my next post.

Written by stewart henderson

April 5, 2018 at 12:53 pm

Kangaroo Island – Emu Bay’s Burgess Shale-type fossils

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Emu Bay, a lovely beach with hidden secrets. The 500 million-year-old fossils were found on the far, eastern side of the bay

Canto: We have a slightly disappointing tale to tell about Emu Bay, famous site for fossils from the so-called Cambrian explosion of some 500 million years ago.

Jacinta: Yes we went there in the naive expectation of ‘seeing something’ – not so much Cambrian-era fossils sticking out of the rocks, but a housed display, perhaps of a touristy nature, of at least photos of the many species of trilobite as well as an endemic species to Emu Bay, Anomalocaris briggsi.

Anomalocaris briggsi, named for famed Burgess shale paleontologist, Derek Briggs. Note the length, indicated by the scale

Canto: ‘Anomalocaris’ means ‘abnormal shrimp’. I recall using the term shrimp to indicate and abuse a small person, so when is a shrimp not a shrimp? I presume abnormal means rather large, as shrimps grow.

Jacinta: Well before we go into all that, let’s say that our trip was a disappointment because there was nothing on site to indicate, or celebrate, the place as one of the most important shale fossil deposits on the planet. Still, it was a nice beach. And I must say the weather here has been more than kind, so far.

Canto: Yes, after driving around for a bit in the tiny township of Emu Bay, we gave up and returned to Kingscote. On visiting the museum there, I asked the caretaker if he knew anything about Emu Bay’s fossils. Were any to be found on the island? He very much doubted it, and said that the exact location of the fossil deposits is not let out to the public – as fossil-fossickers were liable to desecrate the site, so to speak. In fact they’d already done so, it was said. Adelaide would be the most likely destination of the precious fossils, he said, or other parts unknown.

Jacinta: So, yes, in some respects a waste of time, but it was exciting to be so near the site of so many, and such old, fossil finds.

Canto: I’ve decided to return. Just to get a little closer, and fossick about.

Jacinta: Fossick – is that related to fossil?

Canto: Haha, good question – actually fossicking is probably related to fussing, but etymology, as I’ve learned from John Simpson – is often a fruitless endeavour. At least if you want to find definitive answers. But the word fossil has a much clearer etymology, ultimately from Latin fossilis, ‘something which has been dug up’.

Jacinta: But arguably the most important finding with respect to Anomalocaris was made in 2011, in Emu Bay. Six fossil finds of compound eyes belonging to Anomalocaris, which proved that it was an arthropod (as are shrimps), and that these eyes, which were 30 times more  powerful than those of trilobites, had developed very early in the evolutionary process. They dated back 515 million years! Trilobites were previously though to have the best eyes of the period!

Canto: I can see you’re impressed. In fact the Anomalocaris eye contained 16,000 lenses, which makes for pretty impressive resolution. But then, the modern  dragonfly has 28,000.

Jacinta: Hmmm, highly evolved eyes don’t seem to go with highly evolved brains. I’m sure there’s a lesson there…

artist’s impression of the sharp-eyed predator Anomalocaris, found fossilised at Emu Bay

Written by stewart henderson

April 2, 2018 at 8:05 pm