So what exactly is a fossil?

James Tytko has been speaking to Professor Paul Barrett who is a dinosaur expert at the Natural History Museum...

Paul - Fossils are the remains of past life, often animals that are completely extinct that have no living counterparts and they can be any part of an organism. So they can be shells, they can be bones, they can be fossil leads, they can be pollen, they can be the microscopic insides of single cell organisms. All of them are essentially the hard parts of those animals that, over time, have become replaced by minerals and turned to rock. There are a few other, rarer fossils that we get that are things like soft tissue fossils, things that preserve the insides of an animal like organs or things like skin or fur, but these are much, much rarer and get preserved in lots of very different ways to do with the chemistry of the rocks that they were deposited in. Most of the fossils we have are the hard parts of an organism. And generally all of the soft, squishy bits are long lost to the ravages of time.

James - Let's take those hard bits, the bones, the skeletons. What is the process by which they're usually preserved over millions of years?

Paul - So the main way in which any of these things get preserved is that the animal dies somewhere where there's active sedimentation going on. That is the deposition of things like muds and sand and gravel so that the remains get buried very quickly, they're not on the surface too long for scavengers and wind and rain and weather to affect their appearance. And as they get buried, they become encased in the rock as it forms. Over time, those sediments are compacted, lose water, change their minerals slightly and turn into rock. And the bones or shells preserved within them also change. On a molecule by molecule basis, those original organic components of bone and shell are replaced with rock forming minerals to form an exact rock replica down to the microscopic level of what the original shell or bone used to look like.

James - And where is it that we are most likely to find these rock replicas of the bones and shells?

Paul - Mainly in rocks we call sedimentary rocks. These are rocks that are deposited by rivers and lakes and in oceans and that formed by the accumulation of sediment. They're found in various other places as well, such as deserts, a couple of other places on land. So that's the first thing you want to find. And then, depending on the type of fossil you're interested in, you need the rocks to be at the right age as well. For me as a dinosaur worker, I'm particularly interested in rocks that date from the Triassic, Jurassic and Cretaceous periods between about 240 and 66 million years ago, which is when dinosaurs were alive.

James - When we talk about how they're found, this discipline that we know as palaeontology, when did this all begin? How long have we been looking at the fossil record for evidence of what life was like on earth all those years ago?

Paul - So people have been finding fossils as long as there have been people. And we have examples from very early human sites where fossils were obviously picked up because they were realised to be something unusual or something curious. But the scientific study of fossils really only goes back a couple of hundred years. So a few pioneers, people like Leonardo Da Vinci in fact, were already speculating about fossil remains, wondering why these shells turned to stone appeared at the top of mountains long ways from ocean. But it's only really in the 19th century where we also have the development of various other branches in science, in particular geology, that starts to give us an idea about the age of the earth, the length of time that there has been for animals to appear and evolve and also developing ideas about evolution. And as a result, we've really only started to think about them scientifically for a couple of hundred years and bring them into this bigger view of the evolution of the earth as a whole and also their influence on what we know about the evolution of life.

James - And then fast forward a couple of hundreds of years: dinosaurs, they're a staple of any natural history museum, let alone the Natural History Museum where you are speaking to us from. And I understand you've got a brand new exhibit at the moment. Can you tell me a bit about it?

Paul - We do. Dinosaurs always capture the imagination, partly through their bizarre appearance and size. They're a very good gateway for getting people interested in thinking about past lives and aspects of earth science and biology that stem from those. So at the moment we have a big new exhibition that's only been open a month or so so far, which is called Titanosaur. Titanosaur focuses on one particular dinosaur, an animal called Patagotitan, which is from Argentina, which is a contender for the crown of the largest animal that's ever walked the earth. And I'll be very specific when I say "walked the earth," this is in terms of an animal that lives on land. So the largest animal ever is a blue whale, which lives in the sea, but living on land poses a large number of extra challenges to becoming big. And Patagotitan is an animal that may well be pushing at the envelope of the very largest animals ever.

James - Incredible. What did it look like?

Paul - So Patagotitan was what we call a sauropod dinosaur. These are the dinosaurs with large barrel shaped bodies, very long flexible necks and tails, and four stout columnar legs. And there are a lot of famous examples that people might be familiar with, like diplodocus for example. So it's a member of that general family of dinosaurs. It's a very large example. We think it weighed about 57 tonnes and was about 37 metres in length. And to put that in some perspective, that means it weighed about the same as eight or nine fully grown African elephants. And actually it's about 10 metres longer than our beloved diplodocus replica that we have at the Natural History Museum. So these are really gigantic animals and you get a sense of the scale of them when you walk up to it and you realise your head just about comes up to its knee.