Monthly Archives: October 2015

The Cambrian Explosion – An Enigma of Deep Time

Over half a billion years ago the biological world was a more peaceful place, or so the story goes. Simple multicellular organisms abounded, doing little more than filtering out food particles from the ocean waters. Then, in a geological blink of an eye, animals began to diversify, irreversibly changing the prevailing ecosystems, paving the way for modern life. They began eating each other, causing some to burrow into the sediment deeper and deeper, disturbing the slimy microbial matgrounds which had been key in the fading Ediacaran environs. Some developed armour, sporting hard parts which protected their soft flesh, leaving behind more distinct traces in the fossil record. Yet others became more mobile, learning to swim around to evade predators, or to seek out their own prey.

This event is often referred to as the Cambrian Explosion and has earned the nickname “biology’s Big Bang”. It is one of the most important events in the history of life on Earth and, due to its singular nature, is one of the most misunderstood. Misinformation proliferates on the internet, not least due to anti-evolution websites considering it to be a weapon in their arsenal, but also because it is a very mysterious time, where datasets often support multiple interpretations and cause and effect are difficult to untangle.

Image credit: Brest van Kempen

When Charles Darwin first pondered the Cambrian mystery, he was facing a fossil record where no Precambrian fossils were known – the lowest rocks contained the surprisingly complicated trilobites, not at all what he had hoped to see. Darwin invoked the incompleteness and imperfection of the fossil record as his explanation, in the hope that someday new fossils would elucidate this remarkable absence. It wasn’t until 1958 when Precambrian fossils were first recognised, but even they did little to diminish the seeming leap in complexity in the Cambrian. The Burgess Shale fossils, found by Charles Walcott in 1909, revealed a menagerie of bizarre creatures, many of which clearly had modern body plans, but many others seemed to have unique body plans, as though this Cambrian diversification was even bigger than expected.

The Burgess Shale was properly studied from the 1970s onwards and by the 1990s the diversification had been established as one of the key mysteries of palaeontology. By this point, a picture of the Cambrian Explosion had emerged which provided quite the conundrum. Those Precambrian fossils had originally been interpreted as belonging to modern animal groups but had since been questioned and even proposed as belonging to their own kingdom. Then, in perhaps as little as four million years, every modern phylum arrived on the scene, along with potentially three times as many more. It was an unprecedented diversification, expounded beautifully by Stephen Jay Gould in his book Wonderful Life. This is a picture which crops up repeatedly, despite twenty years of advances which show it to be wrong. In future posts, I will explain what has changed in our understanding of the Cambrian Explosion, which questions have been answered, and the mysteries that lie ahead.

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Filed under Cambrian, Cambrian Explosion, Ediacarans, Evolution, Palaeontology

What’s That Swimming Towards Me? – Reblog

I recently took the decision to share some of my older blog posts in order to pad out this iteration of the blog a bit and get me back into the swing of things. It beats ranting about Jurassic World and provides some hopefully readable content. Here is a piece I wrote in August 2010 as a bit of pop. science writing. 

With the vast size of the Earth’s oceans, it is not unlikely that many of us will swim in them from time to time. Whether you are going for an innocent paddle, catching waves on your surfboard, or sailing the seas on a fishing trawler or pirate ship, you will be sharing the waters with myriad different animals, some of which are dangerous. If you were to find yourself swimming under water with a big, moving animal coming ever closer, how do you know whether you have a friendly dolphin swimming towards you or whether it might be a hungry shark? Well, the easiest way can be done at a distance and is a simple observation with an interesting explanation – evolution.

So, with that large, looming shadow swimming towards you, what do you look out for? If you can see its tail, simply look at the orientation of the tail fluke, is it horizontal or vertical? Failing that, look at how it moves, does it undulate its body up and down, or does it move its body from side to side? Dolphins and whales have a horizontal tail fluke, which means they have to undulate their body up and down in order to propel themselves through water, so if you see either the horizontal tail fluke or the undulating movement, you have a dolphin or whale coming towards you.

Sharks, on the other hand, have a vertical tail fluke and so must flex their body from side to side for propulsion. If you see the vertical tail fluke or side to side motion, then what is coming towards you is a fish and so might be a shark.

As our fishy ancestors used a side by side motion to propel themselves through water, so did our earliest terrestrial ancestors and so do reptiles today. Snakes are an extreme example of this sort of movement, but the side to side motion is still there. During the Mesozoic era things began to change, as our ancestors (and convergently in dinosaurs too) developed a more upright posture, instead of the sprawling gait of reptiles. With an erect posture the more effective way to rapidly move is to flex the spine up and down whilst running, rather than side by side.

Many vertebrates have some of their vertebrae fused to facilitate particular movements, so future evolution can often be restricted to working within the confines of that movement. As dolphins evolved from terrestrial mammals, their semi-aquatic ancestors also used this up and down movement and so adapted this to movement in the water. Side to side motion, like that of a shark, would require a larger number of changes when there was the simpler solution of up and down movement (though note that evolution does not have the foresight, it simply uses what is available – quick fix solutions often work in evolution). The motion of whales and dolphins is testament to their ancestry, having descended from active land mammals.

During the Mesozoic, another group secondarily took to the waters and adopted the torpedo shape of dolphins and sharks. These were the ichthyosaurs, descending directly from terrestrial reptiles. As their ancestors used the side to side motion, so did the ichthyosaurs when they swam, also possessing a vertical tail fluke. They had some unusual traits for reptiles, giving live birth and being warm blooded, but their swimming motion gives away their reptilian status. This also quite ably demonstrates that they are not dinosaurs as many laymen mistakenly think, for dinosaurs did not have a sprawling posture which uses the side to side motion. So, if you are somehow in Mesozoic waters with a shape swimming towards you, it may be too late before you can discern whether or not it is an ichthyosaur or a shark.
Ichthyosaurus anningae

Artist: James McKay

 

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Update on updates

This blog never took off, which is a shame. For a number of reasons I haven’t been writing as much as I used to, but I clearly want to get back into it. In order to give this blog a little boost, I am going to share some posts from the old blog, which was hosted on Blogspot and had the same name.

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