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The Missing Link  

Jenny Clack Jenny Clack in her laboratory at the University Museum of Zoology, Cambridge.
Diva of the Devonian

It's a classic chicken-and-egg conundrum: Did the distant ancestors of land animals come ashore and then evolve legs, or did they evolve legs and then come ashore? This is a daunting question considering the fish-to-four-legs transition took place some 370 million years ago. For decades, the former proposition held sway: land first, then legs. But Jenny Clack changed all that.

On an expedition to Greenland in 1987, this University of Cambridge paleontologist unearthed remains of a creature from the Devonian Period (408-360 million years ago) that skulked around swamps on four legs. Through careful study of the anatomy, Clack determined that this creature, known as Acanthostega, nevertheless didn't have a leg to stand on—that is, these rudimentary limbs could not support the animal's weight. But they do support the notion that legs came first. In this interview, Clack gives particulars of her field-shaking discovery and its impact.
Expedition to Greenland | From fish to four legs | Enter Acanthostega | A career paleontologist | The big picture
Greenland The world's largest island holds secrets about the world's first tetrapods—secrets that Clack and her crew began uncovering in 1987.

Expedition to Greenland
NOVA: How did you first become interested in going to Greenland?

Clack: It was my husband who really wanted to go. He thought it would be a really neat thing for me to do, because he wanted to. He hoped to find some more material of a thing called Ichthyostega, which is the Devonian tetrapod that most people have heard of if they've heard of Devonian tetrapods. It's the one often featured in children's books. I was very dubious about the possibility, but he kept nagging me.

Eventually, through a long series of coincidences, we did get information about putting in a grant proposal for some expedition money. Along the way, I spoke to various experts who were familiar with getting to Greenland, some of them across the road in our own Department of Earth Sciences. One of them, the geologist Peter Friend, has had a series of students over the years who have been to Greenland, looking at the sedimentology. He gave me their notes and said, "Have a look at this. It might help you in your decision as to where to go and how to get there."

I was reading through these notes, and I found that back in 1970, one of his students had actually found material of Ichthyostega—or so it said in his notes. So I went back to Peter Friend, and I said, "You know those specimens that were found in 1970, have you still got them?" "Yes," he said, "I think I've got them in a drawer somewhere." In fact, he came back with several drawers full of Greenland material, some of which was tetrapod, some of which was Ichthyostega, but some of it was this other animal, Acanthostega, which had only been known from two specimens collected back in 1952.

What was on the table in front of me was a block of material containing three different skulls and some other bits and pieces of Acanthostega. This suggested that, wherever the fossils came from, there was a big, rich vein of this animal waiting to be quarried out. To cut a long story short, we eventually did mount an expedition to go and find the spot.

"You begin thinking, Wow, this is going to be the start of something big."

NOVA: At the moment you saw that material, what were you thinking?

Clack: Well, very often you don't realize immediately what it is you're looking at. It will take several days, or even a week or two, for it to actually hit you that this is Acanthostega, this is a rich vein, go and get it! But at that point, a whole vista of things opens out. Because it's a crucial animal, it's a crucial period, and it's an area of study that nobody's looked at before. You begin thinking, Wow, this is going to be the start of something big.

NOVA: When you finally got to Greenland, how long was it before you found something?

Clack: We were there for a total of six weeks, but it must have been 10 days after we arrived before we hit the right spot. The landscape is vast. You have no sense of scale, because there are no trees. Something will look as though it will take you half an hour to reach, and it actually takes you several hours. Eventually we found the spot, 800 meters up on a hillside.

Greenland notes In a student's field notebook, Clack found one notation that helped inspire the Greenland expedition ("skull roof bones common") and another that temporarily hindered the same ("820m").

It took a little while to find, because the student's notes said 820 meters. We began thinking, Are we on the right mountain? We checked, and we were, so we decided we'd start from lower down and walk up until we hit the right level. Then we began to find things out on the scree: a piece of skull, a piece of limb bone. We followed that up to where it was all coming from, and we found a little exposure, a little cliff. Then it was just a matter of quarrying it.

NOVA: And was it hard to dig the stuff out?

Clack: We simply had to hack bits off of the cliff and split it. That's not too difficult, except that you're on a 45-degree slope, and you have to be careful not to drop stuff down. Getting it up and down the mountain was quite hard, however.

NOVA: How long were you working on it?

Clack: We used to go every other day. We would go up in the morning, spend all day up there, have lunch, and then come down. Initially it was a four-hour climb, but by the time we finished, we got it down to two-and-a-half hours. We did that for four weeks.

NOVA: How much material did you come back with?

Clack: Well, all together the expedition brought back, I think, a metric ton of material. About two-thirds of that was tetrapod material.

Continue: From fish to four legs

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