What Is the Definition of Trace Fossils?

Video Transcript

"Hi, I'm Steve Jones and I'm going to tell you a little bit about trace fossils. Now, you all are interested in dinosaurs and all of these animals and we know that you find big dinosaur bones and you find shells of animals and you can find snails this big and so forth. And we all wonder at these wonderful things, but they don't actually tell us a great deal about the conditions at the time these animals were living. And sometimes we find out not by finding the animals themselves, but by finding the traces. And the trace would not be a mammoth bone, but a mammoth foot print for example. So if I find a huge foot print about this across, and I find next to it a mammoth, it's probably a mammoth foot print. I can tell a lot from that footprint, how far it sank into the mud. That tells me how soft the mud was and how heavy the mammoth was. The other kinds of things we might find... so we know it's not a mammoth bone, and we don't find a worm, but we might find the tunnel of a worm and the reason for that is that the worm will burrow under the mud in this way and as it does so it produces a little thing called a worm cast. This can tell us a great deal. We know that the worm burrows down. So if we find in a rock the remains of a worm tunnel... and what it will look like is probably dark material inside of light material and this material at the end. If we find something like that it tells us which way up this rock is and that is very important. That is a worm cast. This is a trace fossil. Sometimes on a glacier you get ice and the ice flows down. Let's say you get a huge chunk of ice, and in the bottom of it are embedded rocks. What happens if this is rock here... as the ice moves down, these rocks create grooves and looking at those grooves, we can tell which direction the ice was moving. So we know that this rock was covered with ice and the ice was moving in a certain direction. This tells us quite a lot about what the conditions were like. So this isn't a physical thing. This is a trace. So this is why trace fossils are really useful."