Fascinating Fossils

In front of a live audience at the Cambridge Science Centre, Chris Smith is joined by three paleontologists to discuss fascinating fossils!
06 May 2014


A life reconstruction of the early tetrapod Ichthyostega from the Late Devonian of East Greenland


In front of a live audience at the Cambridge Science Centre, Chris Smith is joined by three paleontologists to discuss fascinating fossils! Alex Liu explains where the first animals evolved from, Stephanie Pierce describes how animals first crawled out of the oceans and Jon Tennant digs into how the dinosaurs died out. The team also answer questions like how big are fossilied spiders? Plus, Dave Ansell and Kate Lamble break down bones and discover how we know how fast dinosaurs ran...

In this episode

Rotated version of Image:Charnia_Half_Crop.jpg. == Scale == Frond length 20cm.

03:43 - Earth's first animals

Alex Liu explains what the fossil record tells us about how Earth's first animals developed from bacteria...

Earth's first animals
with Alex Liu, University of Cambridge

So Alex, tell us a bit about your work and how life began. How come we study that?

Alex - Well, in terms of when life began, we're looking around 3.8 billion years ago. So, the Earth is around 4.6 billion years ago and for almost a billion years, there's no evidence in the fossil record of any life at all on the planet. But as around 3.8, we get the first evidence of fossilised cells. So, things looking like modern bacteria. It seems that for the next almost 3 billion years, we don't get any fossils of anything that's bigger than those bacteria-sized organisms. Slowly, they become a little bit more complex but generally, there's nothing at all. It's only in the period I particularly look at around 500 to 600 million years ago that we see the first large fossils - things that you could see without the use of a microscope. The earliest ones are called the Ediacaran biota because they come from the Ediacaran period which is very recently named the same as the Jurassic called the cretaceous. It's very similar.

Chris - What about when you say, 'life got started after a billion years or so' but there's not really any fossil record of it. How do we know life got started 3.9 or 4 billion years ago?

Alex - Well, it could've got started earlier and we're just not seeing the record of it. We say that it started around 3.8 because that is the time where we see the first fossils of these bacterial cells.

Chris - Well, you can see fossilised bacteria.

Alex - Yes.

Chris - What do they look like? I mean obviously, bacteria presumably, but what do they actually look like? Where do you find them?

Alex - The oldest ones are in Greenland and in western Australia. The two main forms are either small rod-shaped things like drain pipes, but on a very microscopic scale or ball-shaped spheres.

Chris - How do we know that they're microorganisms though and not just say rock formations?

Alex - The way we look at this is to look at the chemistry of what they're actually made of and the chemistry of the surrounding rock as well. And so, a real cell, say, will have been made of carbon or at least that have had some carbon within them and biological carbon has a very distinct signature that we can see that it is very different to any carbon that is formed non-biologically. And so, if we look at the chemistry of the carbon we see that makes up these fossils, we can determine whether they are really biological or whether they're not.

Chris - Any questions let's come to you. What's your name?

Liz - My name is Liz and I'm from Longstanton. My question is, you must go looking on purpose to find fossils that small because you wouldn't possibly find them by accident. So, do people look everywhere? Why would they look in Western Australia or in Greenland?

Alex - There's only certain places around the world that have rock types of particularly ages. Generally, the older the rocks get, the more they've been subjected to pressure and temperature. And therefore, the original fossils that you might see in them and the chemical signatures you might get from them, they become less reliable the further you go back. And so, when we're looking for rocks about 3.8 billion years old, most of the ones that we find on the planet are either buried under other rocks or igneous or metamorphic, so they're not suitable environments, they're not places where the organisms would've lived in the first place. So, we need very specific sedimentary rocks say, sands and mudstones, and things like that. And we also need them to have not being crushed or heated, to the extent that all the fossils are lost. There's very few places in the world that that happens which is why people focus on let's say, northern Greenland. And they will go and look specifically for evidence of fossils in those regions.

Chris - Any other questions?

Neil - Hi. I'm Neil from Cambridge. So, when you find these, how do you actually know how old they are?

Alex - So, in rock sequences, the rocks are all made up of different types of minerals and there's a particular mineral called zircon which isn't a very large component of rocks, but they're very, very useful because they form in volcanoes generally. When the volcanoes erupt, these zircons are deposited in the ash. But as they crystallise and go from a liquid state to a solid state, they trap within them various elements. And so, uranium is one that is regularly trapped. But uranium has various states or isotopes that decay. And so, we know the rate at which they decay and so, each of these zircons is like a tiny clock. If we can measure the amount of uranium in those zircons compared to the amount of lead which is what the uranium decays to and we know exactly how long it takes for that ratio to occur, we can measure lots of different zircons from around the fossils and compare all of those ages. We should get a very definite single one that dates the rocks.

Chris - So, how do you then turn those tiny microorganisms into the big life that then you have beautiful examples of in front of you? Tell us about these.


Alex - The microfossils as I said for about 3 billion years, we don't see very much else other than these bacterial forms. Suddenly, around 700 million years ago, you get the first evidence of things that were getting slightly larger. There's lots of different ideas for why this might have taken place - a rise in oxygen levels is one big idea. There's also various glaciation events that may have covered the whole planet in ice for tens of millions of years at that time at about this time and they may have played a role in preparing the planet for larger organisms, making it more habitable for them. And so, those organisms, very rapidly diversify into lots of different forms. But they're very unusual and although we can tell that they're probably complex organisms, we can't say for certain that they're animals because they don't look anything like any modern animal that we know of. I've got a few here. A lot of these fossils resemble leaves or fern fronds. So, this is one from Leicestershire in Charnwood Forest - if any of you are familiar with that - and it's called Charnia. It's one of the earliest macrofossils, large fossils the first to be found from rocks of this age and secondly, to have actually lived on Earth at any point.

Chris - How do you know it's an animal and not a plant?

Alex - We don't. That is, the main focus of my research is trying to find out exactly what these creatures were. They could've been fungal. They could've been animals. They probably weren't plants just because of the environments that we find them in. So, these are all found in marine rocks, marine sediments that were laid down under the sea at a depth of around a kilometre or so. The important thing about that is that light can't penetrate that deep into the oceans. So, there's no way these could've photosynthesised in the way that modern plants do.

Chris - Kate...

Kate - I've got a question that's come in Facebook that's related to that and Jo has asked, "Is it frustrating studying creatures that you'll never get to see?"

Alex - Slightly, it does mean that you'll never be able to know whether you are absolutely right about the ideas that you've come up with. But it also does mean that no one can prove you wrong very easily. That does help.

Chris - What's your name?

Meluka - Meluka and I'm from Cambridge. My question is, with the microorganisms that you find, roughly how big are they - are they tiny or can you see them with the naked eye?

Chris - Are they the same sort of sizes we see microorganisms today or are they much smaller in history? How do they relate?

Alex - They're comparable to microbes that we find today. If you think of a millimetre, typically, bacterial cells, you're looking at hundredth to a thousandth of a millimetre in length. And so, you can't see them with your naked eye. You'd need a microscope. Luckily, universities have lots of microscopes and say it's very, very easy for us to study them if we prepare them in the right way.

Chris - There's a big jump therefore between turning from something that is a thousandth of a millimetre across to these specimens you've got in front of you which are roughly the size of your hand. So, how do you think that happened?

Alex - That's one of the biggest questions in palaeontology and it's very difficult to work out. It's the switch from organisms being single-cells to becoming multicellular and grouping together, not only to just survive as a colony, but to actually start to differentiate all of the tissues to do certain roles. So some of the cells will start feeding, some cells will be just for respiring or breathing and then that's a very complex biological problem. It is really the biologists who are looking into that rather than the palaeontologists because we can't see changes on that scale from the fossils that we can find in the fossil record. The biologists are the only ones who can actually look at how those processes might be taking place.

Niko - My name is Nikko and I'm from Longstanton. My question is, do you know how long the creatures lived?

Alex:: We know that they lasted as a species for millions of years, but each individual one, we don't know at all. Some of Ediacaran fossils I look at, the way that they're preserved is underneath volcanic ashes. So, we know exactly what they look like when they died because it's almost like Pompeii. You had a big volcanic eruption that's completely smothered lots of living organisms all together on a seafloor. And so, we get this replica of the communities and we can see what the whole seafloor looks like and all the organisms on it. But we can't tell how old any of those individual organisms were.

Chris - Any other questions?

Arushon - Hello. My name is Arusha and I'm from Cambridge. My question is, what you research is fascinating. Is it useful?

Chris - Yeah. So a taxpayer is saying, "Look! I'm paying for this."

Alex - Yes. Firstly, it is a very curiosity-based project - I have to admit - in terms of, we all want to know as humans where we come from. Some people look to religious sites for guidance on how that might have happened. As a scientist, I look for how we might have evolved and how life might have evolved. The key questions I look at is some of the more fundamental ones in explaining the diversity of the planet we see around us. But you're right. In the end, it is just a curiosity-driven question. The thing that we do that does help society - one of the things - is the techniques we need to look at some of these fossils really are pushing the boundaries of science. The machinery and the equipment is often developed in scientific labs, looking at questions like these that then gets farmed out into companies and turned into beneficial equipment for the medical industry or even to end up in your own home. So, indirectly science as a whole actually, it is not just curiosity-driven. Often, that's what gets us started on some of the questions, but it can lead to a very useful societal impacts.

What new studies suggest a tetrapod looks like

19:14 - Climbing out of the oceans

Stephanie Pierce explains to a live audience at the Cambridge Science Centre how early life crawled out of the oceans....

Climbing out of the oceans
with Stephanie Pierce, Royal Vetinary College

Chris - Why is that a challenge for things coming out of the water?

Stephanie - The challenge is, is that in the water, we usually find things like fish and fish have fins and they can float around in a very buoyant way. But on land, that just doesn't work. You can't move around on land with fins and the air doesn't keep you buoyant. So, in order to come out of the water onto land, you have to reconstruct your whole skeleton, all your muscles, your physiology, your sensory systems, how you deal with dehydration, all sorts of things, in order to be able to survive out of water and under the forces of gravity.

Chris - So, it's not trivially just crawling out of the water one day and going onto land. There's a lot of changes that have to happen to an animal to make it capable of surviving on land for any period of time.

Stephanie - Yes, there's a whole suite of morphological and biological changes that need to occur in order for this major transition to happen.

Chris - Do we know what order they must've happened in? Did they all at once? Did they happen sort of slowly, everything at the same time? What do we know about this?

Stephanie - So, it happened in a period of time known as the Devonian and that's about 400 million years ago. It happened basically over about a 50-million year period. At the beginning of this period, we find fish and at the end of this period, we find land animals. And these land animals are known as tetrapods. Tetrapods are all animals that have four limbs with digits - so fingers and toes.

Chris - What is striking to me is that you're talking about a period which is not that long after the period that Alex was talking about. He's got some of the most primitive life or at least impressions of it in front of him, very, very basic things that look like leaves. Within a hundred million years, you've got animals crawling out onto land.

Stephanie - Yeah, well for a long time, life was fairly simple. We had very unique organisms as Alex was mentioning before where we don't really understand their morphology and biology. But when we get to about 530 million years ago, we came upon a Cambrian explosion. The Cambrian explosion is very important because this is where we see an explosion in the diversity of life, all sorts of different animals. Not just simple animals, but animals that are plant-like, all the way up into giant predators. It's after this period of time that organisms start to focus and start to evolve in different directions, into animals that we have around today. In the Cambrian explosion, we actually have the precursors to vertebrate animals. So, these are animals that have backbones and these are the animals that eventually turn into tetrapods.

Chris - What would the land that these animals were beginning to come up onto have look like? What was there already if anything?

Stephanie - Well, at the beginning of the Devonian and up until sort of the precursors before tetrapods were starting to evolve, there wasn't really anything on land. Almost everything lived in the water. So, if you actually got in a time machine and you went back to the Devonian, and you stood on the land, it would've been really, really quiet. There wouldn't have been any birds flying around and chirping, there wouldn't have been any bees humming around.

Chris - I bet, if you lived in Britain, it would've been raining.

Stephanie - There might have been a bit of rain. But it would've been a very, very quiet place. The waters were a different story. They were very, very active with marine life. From invertebrates, so things like shelled animals and corals and stuff like that, all the way through to a massive diversity of fish. And so, it's during this time that the fish start to change. Certain groups of fish start to change. These are known as the lobefin fishes. They're called lobefin fishes because their fins are actually much different than other fish fins. They start to develop bones in their fins that look much like our bones. So, they start to develop things like a humerus and a radius and ulna. And it's from within these lobefin fishes that we see the evolution of tetrapods.

Chris - So, if it was such a boring place on land, what was the lure for these animals to come out of the water and want to go onto the land? There were plants there presumably.

Stephanie - Yes. So, by the time that these animals were starting to want to come out onto the land, we have the beginning of plants actually forming communities on land. So, we start to see an environment which could protect animals. The waters were incredibly busy - big predators, huge fish, a few meters long. They were really fierce predators. So, if you could survive that of the water, just for a short period of time during the day, you probably had a very good shot of surviving that day. But there's also a food source because we now have some invertebrates living on the land. Another factor is oxygen. So, we're starting to see the development of plant communities on the land and plants actually burrow their roots into soils and this starts to erode the soils. That actually gets deposited into water. We also have leaf litter and stuff like that and when that goes into the water as well, we start to reduce the amount of oxygen in the water because things are decomposing. Now, these early fish tetrapod animals were actually living right at the water's edge. This water might have become depleted in oxygen. So, if you could find a way to get your head out of the water and breath oxygen from the air, you might have also had a better shot of living.

Chris - What's your name?

Meluka - Meluka and I'm from Cambridge. My question is, when you said that to go out onto the land, the animals had to change all their muscles and their bone structure. How much time would it take?

Stephanie - That's a very good question. I would have to say - I mean, as I said, the period of time where we go from animals that are very much fish into animals that are tetrapods - so animals with limbs, with digits - is about 50 million years. So, it took about 50 million years to get to the body plan that we see today in a modern tetrapod.

Niko - I am Niko and I am from Longstanton. My question is, what's the amphibian up there?



A life reconstruction of the early tetrapod Ichthyostega from the Late Devonian of East Greenland - ulia Molnar, Stephanie E Pierce, The Royal Veterinary College.


Chris - There's very large animal on the screen behind you. What is the animal?

Stephanie - The animal. Well, this is an animal that I worked on a lot. Its name is Ichthyostega and it is an early tetrapod. It's actually one of the first early tetrapods to evolve. So, this animal isn't a fish anymore. It actually has limbs with digits. So, it has fingers and toes, but it still mainly lived in the water.

Chris - How big was that?

Stephanie - That animal could've got up to about a meter and a half. So, it's probably the size of a giant dog.

Chris - It looks like a massive lizard. It's a sort of big lizard-like thing with very big teeth.

Stephanie - Yeah. It wouldn't have lizard-like skin. It would be more like salamander-like skin or fish-like skin, but it was a fearsome predator in the water. It had a huge head with massive teeth. It had huge big arms and it had a long tail. If you look at the very back, what you'll notice is that it has its hind limbs sticking out and they very much look like paddles. They actually look like seal flippers. And so, this animal is really, really good at swimming. But because it had this big four limbs, it actually could haul - what we think - that it could haul itself out of the water, using its four limbs and probably bask in the sun on mudflats and perhaps even feed on some of the animals that were on the shoreline.

Chris - How do you know it was pulling itself up because you've got sort of fossilised tracks left by these creatures in the mud?

Stephanie - Well, we actually performed a study on these animals. So, part of the research that I'm involved in is actually reconstructing the 3D skeletons of these very early tetrapods. To do these, we use x-rays and specifically, we use micro CT scans. What that allows us to do is force x-rays through the fossils and then we can resolve the fossils that are hidden inside the rock. We can use intricate 3D modelling software to go through, colour in each of the different bones. And eventually, you can come up with a whole 3-dimensional reconstruction of the skeleton. From this, we can start to move the limbs around, how did they move, how much mobility was there, and what can that tell us about how the animal was moving.

Chris - Kate...

Kate - This picture on the wall is green, like a lot of the dinosaurs that I've seen, but (Chris Barrow) has asked us on Facebook, how do we know what colour dinosaurs were?

Stephanie - If we talk about what colour early tetrapods were, that is a very hard question indeed because we actually don't have the material in order to test that. If we want to talk about the colour of dinosaurs, well, a lot of work has been done recently, looking at the chemical signatures of feathers using various new technologies. Some of the techniques that they've used in order to examine these feathers have shown that different dinosaurs, well, the non-avian dinosaurs might have been for instance, white or black or red in colour.

John - My name is John from Steeple Bumpstead. Is there any evidence that any animals went back into the sea?

Stephanie - Lots of animals went back into the sea. So, one question is, it's very hard to determine if a very early tetrapod had an ancestor that was land based and really quickly went back into the sea because the timeline isn't that great. Our fossils, we get one at a time, but whales are mammals that went back into the sea. We have in the Jurassic and Triassic a huge amount of reptiles that went back into the sea. These are called plesiosaurus and ichthyosaurus. Ichthyosaurs actually looked like dolphins, but they were indeed reptiles. So, there's many instances of tetrapods going back into the see and it's probably for a very similar reason for why tetrapods came out - to get away from predators, to find food sources that weren't being exploited.

Arushon - My name is Arusha from Cambridge and my question is, is there a particular place on the planet where most of this coming out to the sea onto the land happened?

Stephanie - Yes. One of the areas that produces a lot of early tetrapod fossils is in Greenland, just like we heard about the beginning of life. So, they actually have rocks which are of an ideal age - the Devonian period - in order to try and trace this evolutionary transition. Some really important fossils also come from arctic Canada. Recently, there are some stuff in the UK even. So, up in Scotland or some really important specimens that can really give us some insight into this important event.

Liz - Hello. It's Liz from Longstanton again. When you say that you find these fossils of the animals coming out of the water in these places, would it be like it is now cold or was the climate completely different when they did it?

Stephanie - The continents as you know them today weren't like that in the past. So for instance, Greenland wouldn't be way up north. It actually would've been much closer to the equator than it is today. So, they would've been more in temperate climates and there are some ideas that perhaps during this time, there might have even been some arid conditions some time and then even some really wet conditions. So, it depends on what specific formation that you're looking in, in terms of what the exact environment was like, but the animals where we find them, that's not the environment that they were living in back 400 million years ago.

Replicas of dinosaur footprints found in La Rioja (Science Museum in Logroño)

36:19 - How did the dinosaurs die out?

Jon Tennant explains how mass extinction events changed the face of Earth's biodiversity and answers questions from the audience...

How did the dinosaurs die out?
with Jon Tennant, Imperial College London

Chris - We're talking this week about the science of palaeontology. In other Dinosaur footprintswords, things that are hundreds of millions of years old, like most of the politicians in the House of Lords. Our guests this time are Alex Lu who actually studies some of the first life on Earth, Stephanie Pierce who looks at how things moved and came out of the water onto land, and sitting next to Stephanie from Imperial College is JT, Jon Tennant. Hello, Jon.

Jon - Hello.

Chris - So, you're interested in actually not where life came from, but where it went, how it got wiped out, why it disappeared.

Jon - Yeah. I look at very long and large patterns in the history of life on Earth. This is a field of study called macroevolution. So, 'macro' is Latin for big, evolution is obviously everyone's favourite subject. In the fossil record, over the last five hundred million years or so, we're punctuated by these very catastrophic periods called mass extinctions. I look at a period about 150 million years ago where depending on how you look at the fossil record and how you interpret what we've got at that particular period in time - whether or not it can achieve the status of a mass extinction or not.

So, a mass extinction is usually where something like 75% to 95% of all life on Earth just gets absolutely obliterated. Usually, many different reasons for this. Everyone loves the end of the dinosaur's meteor strike story, but I look in a lot more detail to see whether there are potentially biological factors such as what animals were eating or something which might have drove them to extinction.

Chris - How do you know that there's been a mass extinction? I mean, if I was to walk out into Cambridge now and there had been a mass extinction, it might be obvious. But how do you know from the fossil record that all to 75% of life disappeared?

Jon - So, there were periods of time where we have lots and lots of fossils and there were periods of time where we have very little. So, around 250 million years ago, there was an event which we think happened called the Permo-Triassic mass extinction where up until this point, we had quite a lot of fossils happening. We had a lot of fossils around, both on land and in the sea. But then all of a sudden, it's just gone and there's almost nothing. We think that's because the continents came together and there was this massive volcanism happening and it just made life very uncomfortable on Earth and they all just kind of died out. They were actually quite lucky to hang on. There were a few strugglers which survived on and then went on to radiate again. But then about 50 million years later, were hit again with another mass extinction, and we actually find the last 500 million years so punctuated in time by these large extinction periods.

Chris - Is the cause of the mass extinctions written into the geology? If you study rocks, can you generally find out what's happened?

Jon - Yeah, to a degree. There's a lot of debate at the moment whether there are more kind of biological factors which are recorded in the fossils themselves which drove of extinction or environmental and geological factors which are caught in the rock themselves which drove extinctions. So, if you go back to the end of the non-avian dinosaurs again, we find evidence for a meteor strike hidden in the geology at this time. There's a very thin layer of rock all around the globe and if you look at the elements and stuff which are preserved in it, we actually find an element called iridium. There's a little spike there every time and it tells us that at the same time as the dinosaurs went extinct or most of the dinosaurs went extinct. We find this spike in extra-terrestrial material, so extra-terrestrial, meaning not from Earth. If you combine that with evidence which we have done in Mexico for a huge meteor impact and perhaps even another one just off the coast of India, then we actually find very strong and compelling evidence for a meteor strike in the geological record. We also find the biological record being severely depleted in the fossils at that time.

Chris - When you say huge, how big is huge?

Jon - I think the one down in Mexico is 120 km wide. So, that's one big meteor.

Niko - My name is Niko. I'm from Longstanton. My question is, how do you know that the meteor was the only thing that led to the extinction of the dinosaurs?

Chris - Yeah, how do you know that?

Jon - We don't. This is one of the things I'm studying but at a different period in time. It's very nice to have the story of just one meteor come along and just obliterating life on Earth. But what might have happened is actually something like more of a perfect storm of events. So imagine life being pushed to the very brink. You have something like mass volcanism happening and churning up tons of gases and ash into the atmosphere. Imagine walking around Cambridge and a volcano just got off. It's going to be very difficult to breathe. We think this kind of put a lot of pressure on animals just to stay alive and it made the environment very unliveable. And then just as things were getting really, really bad, a meteor comes along and just smashes everything apart and life is just very unhappy at that time unfortunately.

So, it's not just one thing. It could be several different things contributing towards ecological breakdown, environmental breakdown. It's also very important to look at that because when you consider what humans are doing today to the planet, we're pumping carbon into the atmosphere. If you kind of think that's like almost an analogue for mass volcanism happening, we can look at these extinction events, see how animals responded, see that they suffered greatly. We can almost kind of predict exactly what's going to happen in the future.

Greyden - Hello. My name is Greyden and do you know how the meteor killed all of the dinosaurs even if they're really far away?

Jon - Even if they're really far away.

Chris - I mean, can we slightly adjust that also to add to the point that, why did some things not disappear because crocodiles for example, their ancestors have been around for 300 plus million years and they're still here, aren't they? So, that's an excellent question. Why did some things succumb and not others?

Jon - Some things are just better designed for surviving. So, if we take the typical dinosaur analogue again, dinosaurs at this time when the meteor hit were very big and they are very specialised. Generally, the bigger you are and the more pressure that's put on an ecosystem which you're living in, the more likely you are to die because you need more resources to survive. So, if you have things like small birds and crocodiles which are living in the oceans which might not have been as affected, then there's different probabilities that they might have survived or died.

Malcolm - My name is Malcolm and I'm from Longstanton. My question is, how did the meteorite affect flying animals, apart from the ones that were right under the meteor?

Jon - So, there were two different groups of flying animals at this time. There are the pterosaurs and there were the birds. They're actually in direct competition in the kind of duel for the skies. Imagine the battle of Britain. You have giant pterosaurs. Some of them were humongous. It's like 10 or 15 meters in wingspan.  Birds generally got a lot smaller. So, it might've been again that pterosaurs which is too big, there just wasn't enough food or just places for them to live. But perhaps the ability to fly as well meant that they can migrate away from areas which were being worst affected by the meteor so maybe if you could fly down to the poles. But again, this is very much an ongoing point of research. People are just beginning to be able to assemble the data sets which enable them to ask these massive questions about extinction.

Chris - Kate...

Kate - Bill Pope who's @underbundle on Twitter asks, "What might the world look like today if the Permian great extinction hadn't happened?"

Jon - Probably a lot better because there probably aren't be any humans around to destroy the planet. So, life would probably actually be doing quite well.

Chris - Can you just tell us what the Permian...?

Jon - Sorry, the Permian extinction. The end-Permian extinction was an event, 252 million years ago where all of the continents that we know today crashed together into a giant super continent called Pangea. There are combination of factors relating to this which led to about the wiping out of about 95% of life on Earth. So, if you have reduced coastlines because you've got all the continents coming together, there's less coastal environments for animals to live in the ocean. If you got less animals going on then there's, you know, ecosystem degradation, things begin to - like food chains and things breakdown and there's less suitable places for animals to live. If you like living in a shallow sea and then all of a sudden, your shallow sea is replaced by a really big deep sea, you're not going to be too happy there and you probably are going to go extinct or radiate, or evolve into something different.

Kate - Kevin Nagel asks, I think this is sort of a fight question - which were more vicious, pre-historic land animals like T. rex and raptors or crocodiles and sharks?

Jon - Well, if somebody wants to pit a T-Rex against a shark. I'm pretty sure a shark will win because the T-Rex will drown. It's a very interesting question. We don't actually know what and raptors were like. We have Jurassic Park to go on, but these are obviously exaggerated because directors aren't scientists and they just want what is going to draw in an audience. But we don't really know. I mean, if you look at a chicken, they're actually really friendly. Have you ever picked up a chicken? It's not going to come and try and bite your head off. They're generally quite friendly. Most birds are. Apart from occasionally pooping on your car, there's not that much which they do which is really life threatening. So, we really don't know that much about the actual behaviour of dinosaurs to guess. Let your imagination run riot.

Chris - So do you think they're actually quite friendly then, some of them? You don't think they were violent?

Jon - I probably wouldn't try and hug a T-Rex still, but maybe. You don't know.

Chris - What about these giant birds that lived in South America? The Terror birds, weren't they? I mean, these had a beak a meter long, some of them, even bigger. I don't think they were notoriously friendly. I think they were peckish.

Jon - Probably. I don't think humans are around really to record that much, but if you consider things like ostriches, again, today, ostriches are very primitive birds, probably some of the closest ancestors we have to dinosaurs. They're generally quite friendly. If you start poking them or trying to annoy them then they're going to get a little bit angry. But generally, they're quite placid.

Chris - Let's hear it for dinosaurs. What's your question?

Lisa - My name is Lisa, I'm from Cambridge. Are you able to detect disease?

Jon - Disease. Yes, we are actually. I think again, if I use T-Rex as an example, if you look at the jawbones in T-Rex, you can actually see evidence of like bacterial infection and things. I think there might be some evidence of cancerous growths in the bones of dinosaurs as well. You can actually maybe suggest this as a cause of death. They probably didn't have chemotherapy or anything, back then to give them really much of a fair shot. But yeah, you'd certainly do see traces of disease and often these are just left in the remains of bones. It can be quite interesting.  This is evidence of what was happening to an animal when it was alive. When you find a bone of a dinosaur, that's evidence of just it being dead, but if you find these marks and things on a fossil - disease, even things like bite wounds and things, you can actually see what was going on during life or what killed the actual animal it's really quite cool.

Chris - Kate, anything on the Twitter or...?

Kate - full circle man on Twitter is trying to give us all nightmares. He says, what fossils do we have on spiders? Were they larger and more venomous?

Jon - Mister invertebrate, would you like to take that one?

Chris - Alex, do spiders fossilise? They do, don't they? I mean, we've got amber spiders, haven't we?

Alex - Yes, so spiders do fossilise. I think there's one in the Cambridge Museum actually in the Sedgwick which is called Mega arachni and it's 300 million years old, and about 60 centimetres in diameter - very, very large spider fossil. I think the fossil record of spiders goes back to around 400 million years ago, but the interesting thing as well is you don't just get the spiders. There's also evidence that their webs have been preserved and particularly in amber. I know there's an example of cretaceous amber from Sussex actually where if you look with a microscope, actually, inside the amber, you can see these little coiled up bits of spider web. So, not only do we know that the spiders are present, but we also see that they're behaving in exactly the same way as modern spiders are.

Corrine - My name is Corrine and I'm from Cambridge. So, you're talking about amber and that takes us to that famous moment in the Jurassic Park movie. Is there ever a chance that we could bring dinosaurs back?

Jon - So, there are some very hopeful scientists out there. They're generally considered to be a little bit crazy in the paleontological community. There's research going on at the moment over in Japan where some guy is trying to almost reverse engineer a dinosaur from a chicken. So, what he want us to do is mess with a chicken embryo and mess with its genes a bit so that you superficially create something that looked a bit like a velociraptor.

Even in your wildest dreams, you won't ever get an actual dinosaur. You probably have something that you know lays eggs, clucks, and goes around making chicken sounds and looks like a dinosaur. But it won't ever actually be one. They're gone forever. Until we create some kind of zombification process, it's not going to happen.

But as well as that, I think they mentioned earlier that something about proteins and molecules had been found. This is by a team of scientists led by Mary Schweitzer over in the US and they believe that they've actually found like almost the structural residue of DNA and they've used various techniques borrowed from biotechnology and biochemistry to actually demonstrate that we have this extremely fine level preservation which even 5 or 10 years ago, we wouldn't have been able to dream of finding. At the end of one her papers, she said, "I think about 5 or 10 years. We'll be able to have about 20% of a genome of a T-Rex" and there's a reason why that paper never really made it, particularly much in the media and it's because she was dismissed as a bit of a wacko after that. But if you kind of open your imagination, we could be able to do things in the future. I mean, there are incredible developments in genomics and being able to sequence DNA, happening. It's just a case of what the fossil record lets us do.

Alex - I was just going to add to that. There's another team in America. We may not be able to get dinosaurs back, but there's a team in California looking to try and get mammoths and also the passenger pigeon. That's their first effort because it didn't go extinct too long ago, I think 60 years or so. But from genetic material for the passenger pigeon, that's from a museum specimen and the mammoth, they've found frozen mammoths about 10,000 years old in Siberia. They can actually extract DNA from there. So although amber is not involved, you can still get the DNA and it's not perfect, but they are trying to find host animals like elephants for the mammoth that they might be able to bring back those sort of animals.

Chris - We'll just interject a second because our third experiment needs a little bit of time. So Kate and Dave, what have you got in mind?

Kate - So, for some prehistoric creatures, they're really funny looking and this is my favourite. What's this?

Boy - Dimetrodon.

Kate - See, everybody knows it but me. So, the dimetrodon as we can see has a big sail on its back. Now, that doesn't look to me like it's for fighting. Dave, what's it for?

Dave - So again, it's a fossil record so noone knows for certain, but one of the theories is it's all to do with heat and how well it can take advantage of the temperature in the outside world. Now, to get an idea why having some kind of big sail on your back might be useful in the outside world, what I'm going to do is look at how water changes temperature in two different ways. I've got a pot of water here and I've got a thermometer. At the moment, it's sitting at about 56 degrees centigrade and I have two identical glasses and I'm going to fill each one of them with the same amount of water.

Okay, so you can imagine this is an animal which is producing lots of heat and it's nice and warm. Now, I'm going to consider two different shapes of animal. One of them is a nice, kind of compact animal like a glass of water for example. The other one is a really kind of spread-out animal. So, I'm going to pour the water into a tray. So, it's only about half centimetre deep.

Kate -  So, this water in the tray is like the blood going through the dimetrodon's sail I suppose.

Dave - So, if the body is hot in the environment outside and it's pumping around the sail then that would be very similar, yes. Or if you're thinking about other modern animals, it's very like the blood going through the ears of elephant.

Kate - So, we'll wait and see what happens to the water after some more questions.

Chris - Okay, any more questions while we wait for the water to do its thing. There is one over here.

Meluka - Hi. My name is Meluka and I'm from Cambridge. Going back to the subject about the fossils of the enormous spider, if you found one of those alive and it bit you and it was like venomous, what would happen to you? Would you like die or...?

Chris - They did have venom, did they, these spiders, do we think?

Alex - Yes, we're not entirely sure whether this particular spider had venom. I mean, there are spiders around today which don't and obviously, spiders that do and it's really the strength of the venom that determines what happens to you, whether you'll die from the bite or whether you'll just get a little bit ill, or whether actually, nothing will happen at all. But with something that big, I'd imagine it would leave some quite large puncture holes.

Arusha - Arusha from Cambridge. Just following on from the spider questions, a spider that's 60 centimetres in length, what might it have fed on?

Alex - Jon has just suggested humans, that's not the case because humans weren't around. But there's quite a lot of large spiders around today. So, in the tropics of Malaysia, there are bird-eating spiders that can reach 30 or 40 centimetres in diameter and they do catch birds. So, if you have something that big, presumably, it was feeding off something, or could feed off things that were a lot charger than any of the other spiders around at that time could.

But the interesting thing about the carboniferous, appeared that these giant spiders and the giant scorpions were living is that pretty much, every type of insect we see at the time could reach giant sizes. There were dragonflies with wingspans of 70 centimetres. Millipedes that were several meters, well. Not several meters. Maybe a couple of meters long. It is the one period in time in Earth history when things could get very, very large compared to modern organisms. The idea for why that might be is that there's evidence that oxygen levels on the planet were a lot higher at that particular point in time. So at current levels we're at 21% of our atmosphere is oxygen. The estimates are that in the carboniferous, 35% of the atmosphere was oxygen. If you're an insect, the way you breath is almost like diffusion. So, you're limited in your size by the amount of oxygen that's around you because the oxygen can only diffuse a certain distance before you've used it all up into your body. So, if there's higher oxygen levels, any organism that survives by diffusing in the oxygen can get to bigger sizes. We think that's what was driving this massive evolution, this gigantism at that time.

Chris - Kate...

Kate - Jeremy on Facebook wants to know what gap in our knowledge of pre-history are you guys most vexed by? What's missing from our knowledge?

Chris - Jon?

Jon - Well, one of the things which we always hear is how biased the fossil record is. People who study modern animals and DNA, they love throwing that one at us "Oh, you can't do anything with the fossil record. It's so biased." By this, they mean that only in certain periods of time and in certain places do we find certain types of fossils. But, palaeontologists, we don't see that as a kind of end to all things. We actually have now many different techniques where we can overcome these biases. Although our data may never be as good as going out into a forest and being able to sample every animal out there. We're certainly able to overcome some of these biases which other scientists actually think makes us almost like, not able to be useful in any way. But yeah, I'd say, just what the fossil record is able to offer can sometimes be incredibly frustrating. I mean, we can have periods of 5 or 10 million years where there's just nothing there.

Chris - Stephanie...

Stephanie - Gosh! Well I'd love to go out there and find an amazing early tetrapod fossil. That would be really great! Well for me, it would be amazing to see the point in time where we actually see the evolution of modern locomotion behaviours. So, as I said with some of the early tetrapods that I've worked on, they sort of weren't really using their limbs like modern animals do. They were sort of hauling themselves out of the water onto mudflats. I'm really interested in, at what point in time did animals put four limbs on the ground, lift their body weight off the ground and start moving one limb at a time. Because after that happened, we start to see an explosion in a way and diversification in the way that animals move. We not only have animals moving one leg at a time. We start seeing animals walking, running, animals that are moving their limbs under their body, animals that are going from four limbs to two limbs like we see in tyrannosaurus rex, and everything in between. They can gallop, they can run fast like a cheetah, and then eventually, of course, they can fly in the air. So, this point where animals start to move like a modern tetrapod is really, really important.

Chris - Kate and Dave, how's your water coming along?

Kate - How's water coming on Dave? You've been measuring it as we'be been going along.

Dave - So, if we measure the temperature of the one which has been sitting in a nice compact glass...

Kate - So, what kind of animal is this representing?

Dave - So, this is something kind of spherical. So, if you imagine a kind of really big fat animal. Polar bears are really good example of this kind of really big fat, and almost round. That water is now at about 51 degrees centigrade.

Kate - So, hasn't come down by that much.

Dave -  It's only come down by maybe 5 or 6 degrees centigrade whereas the ones which have been spread-out - so this is an animal with great big, long kind of spindly limbs or kind of big flat animal, the temperature has come all the way down to about 36 degrees centigrade. This is because basically, things can only lose heat at their surface. So, be it animals or lumps of water, if you've got a small surface, you're a really compact thing then you can't lose or gain heat very well.  If you're spread-out, you've got much more surface so you can gain or lose heat, and a lot more places at the same time.

Kate - So, our dimetrodon on the war here. Was it trying to cool itself down or warm itself up?

Dave - Well, the theories go that it was probably trying to warm itself up because it was a predator. It was trying to catch things. If it could hang around in the sun and have this great big sail to warm up its blood then all the chemical reactions go in faster which means it can run faster so it can catch the other animals that are kind of still a little bit cold and a bit kind of sleepy in the morning. You can kind of charge after them and grab them and get its breakfast very efficiently.

Kate - So, the early dinosaur catches the dinosaur I suppose.


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