a deeper dive into the shallow waters of Emu Bay
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
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