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Fossil Discovery May Link Life on Land and Water

April 6, 2006 at 12:00 AM EST
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JEFFREY BROWN: For fun, scientists are calling it a fishopod — part fish, part petropod, or four-legged animal. In all seriousness, it’s being seen as a crucial missing link in the evolutionary chain between life in the water and on land.

The fossils date from some 375 million years ago, and show a creature given the name Tiktaalik. As shown in this model, it ranged from four to nine feet, had fins and scales, but also a neck, ribs and rudimentary limbs.

The fossils were found on Ellesmere Island in the Canadian Arctic, and described in the journal “Nature” today.

Ted Daeschler, a paleontologist at the Academy of Natural Sciences in Philadelphia, was one of the two lead researches, and he joins us now. Welcome to you.

TED DAESCHLER, ACADEMY OF NATURAL SCIENCES: Thank you, Jeff.

JEFFREY BROWN: I saw this described as a transitional creature. Help us understand, would it have been both a land and sea animal?

TED DAESCHLER: This is still a fish. And so many of its features tie it back to the way we expect fish to behave. It probably lived primarily in the water, probably in very shallow water. We find it in rocks which were deposited in stream systems.

And so we’re making the hypothesis that this animal was specialized for living in shallow stream systems, perhaps swampy habitats, perhaps even to some of the ponds. And maybe occasionally, using its very specialized fins, for moving up overland. And that’s what is particularly important here. The animal is developing features which will eventually allow animals to exploit land.

JEFFREY BROWN: All right. You brought a skull along, I gather. You can show us some of those features that part fish, and that perhaps suggest that it could have moved on to land.

TED DAESCHLER: Absolutely. This is the skull of Tiktaalik. Now, we have several individuals — this is one of the more well preserved specimens. You see its broadly triangular shape. And it’s rather flattened, with eye sockets high on the head.

When we look closely at the features of this skull, especially at the lower jaw and the snout, we see that they are very fish-like. In fact, we actually originally found pieces of lower jaw and really couldn’t recognize them as anything other than some of the lobe-fin fish that are related to this thing.

But it’s when we saw some of the other features of the skull, particularly the short area behind the orbits here, as well as these large notches in the back of the skull, that we were startled by the similarity to the first limbed animals, which had skull design just like this.

Now, the entire specimen also includes the body, which occurs behind the skull. And in the body, there is also a number of features which suggest tetropod. The fin itself, although it may have looked like a fish fin on the outside, if you could look at the internal skeleton, you would see bony structures reminiscent of a tetropod limb, including a wrist joint and bones beyond the wrist joint which were giving that fin power and dexterity.

And again, we think that the animal used its fin for contacting the substrate and moving around.

One of the other surprising features is, this animal has broad ribs that overlap one another. And that’s a feature we see in animals that may live in very shallow water, where their body would be compressed under the force of gravity, or even an animal that comes on land.

Yet all those features are in an animal that still has scales, which is a very fish-like feature. So we’re seeing some features which show its ancestry in fish, but other features which are reminiscent of animals like the earliest tetropods

JEFFREY BROWN: All right. So I guess the key question here is what’s the significance in terms of understanding more about evolution?

TED DAESCHLER: Well, we know the broad structure of the tree of life, and many of the branches on that tree, including the tree of all limbed animals — so that’s everything from frogs, to dinosaurs, to us. We can see that they converge back in time to one point.

And in the case of the limbed animals, we see that the earliest limbed animals existed in the late part of the Devonian period. That’s about 365 million years ago. And we knew that those features of those earliest limbed animals, we also saw some of them in fishes and assumed that there was an evolutionary transition from those fishes to early tetropods, but we didn’t have a really good transitional form, an intermediate, to show us how those features were acquired.

And Tiktaalik fills that gap. It teaches us a lot about how the features which the earliest tetropods would use to build a limb and eventually come on land were first established in fish living in shallow water.

JEFFREY BROWN: Now tell us how you found this. Where did you find it and how did you know where to look?

TED DAESCHLER: I work closely with Dr. Neil Shubin at the University of Chicago. And many years ago, he and I were trying to expand our field area. We were working in Devonian-age rocks here in Pennsylvania, but we wondered where else in North America we might be able to find rocks of that age, and rocks that were formed in the right kinds of environments, in this case shallow stream systems.

And we learned that up on the — on Ellesmere Island in the high Canadian Arctic, there were rocks that fit those criteria. So we did the geologic homework, we talked to geologists who had been in the area, and then we just went. And we looked.

And there’s nothing to substitute for getting your feet on the ground and your nose up to the rock and looking for these fossils.

And it wasn’t easy. We spent four years there — excuse me, we spent four summers there so far. And it wasn’t until the last summer that we really hit it big with some of these very well preserved specimens. Although we have found numerous other kinds of fossil fish.

JEFFREY BROWN: And does the eureka moment happen in the field, or back in the lab, or is there even a moment when you say, wow, we’ve really come up with something?

TED DAESCHLER: Well, there is actually a lot of eureka moments. And that’s what is so exciting about exploration and discovery. Indeed, when we are in the field, we begin to see fragments, especially on the surface. We explore and try to find where those fragments were coming from — in other words, what layer were producing those fragments.

And then we dig in. We begin a small excavation. And if we find things that are more well preserved while we’re digging on that layer, then we can do a full-scale excavation.

But we wrap up what we find in a plaster jacket. It’s like a cocoon, with plaster on the outside and rock and fossil bone on the inside. So we know we have something good, but we haven’t seen all the details yet.

And it’s when we get that back to the preparation lab here in Philadelphia or in Chicago that very fine-scale work is done to expose all the features, to see just what the skeleton is like after the rock is removed. And that can take a lot of time, but it takes that much to get a good look and to carefully study what’s there.

JEFFREY BROWN: All right, Ted Daeschler, thanks for describing it to us.

TED DAESCHLER: My pleasure, thank you.