NOVA Online | The Missing Link | Diva of the Devonian (3)

Not five but eight fingers on each "hand" emerged from the rock encasing the remains of Acanthostega.

Diva of the Devonian
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Enter Acanthostega

NOVA: How did this new idea come about?

Clack: The new idea stems from the material we collected in East Greenland in 1987. That proved to be a really rich haul of material, though it took several years to develop it out of the rock and expose what was there. [For the skinny on fossil preparation, see Confessions of a Preparator.]

One of the really surprising things was that there was a complete limb with its digits in place. That's rare enough among this age of fossil. But what was astonishing was that instead of the conventional five digits on the end of this limb, there were eight. The person who was preparing them was preparing them one by one, and when he got to five he thought he'd finished. But he hadn't! He went on to find three more. Of course we thought, Does this all belong to the same animal? Eventually we concluded that yes, it had to.

Then we began to look at the animal as a whole, and how it compared with other creatures of this sort of similar date, and how it related. There were several things about this specimen, this animal called Acanthostega, which suggested to us that it was a very primitive tetrapod, one of the very first with legs. If you look at the structure of the foot on the leg, it's clearly not designed for walking on. It had no proper ankle or wrist, so it wasn't a weight-supporting device. In fact, it looks more like a paddle. The animal also has a deep tailfin, and it has a number of other fish-like characters that are lost in all subsequent tetrapods.

"Basically, yes, it was a fish with fingers."

NOVA: Did it have gills?

Clack: We believe it also had gills, yes, because we found some gill skeleton material with several of these specimens.

NOVA: So this was basically an aquatic creature with hands?

Clack: This was a swimming creature. We don't know whether it could ever have come out on the land, but it certainly wouldn't have walked in the conventional sense. It had gills as well as lungs. It had very short ribs, so we don't think it used its ribs for breathing. It had a deep tail with bony supports all along it. It also had features of the internal workings of the skull that were also fish-like. Basically, yes, it was a fish with fingers.

NOVA: What did this animal do on a day-to-day basis?

Clack: This beastie probably spent most of its time hanging around in the shallows, in the swamps, waiting for something to go by, and perhaps using its hands to steady itself while it waited. Then as something went by, it would swish its tail, which would give it a good thrust, and it would lunge, catch the prey in its open mouth, and then wriggle back into the shallows.

NOVA: So this was a big deal, wasn't it, this find about Acanthostega?

Clack: This was pretty revolutionary, yes. Partly it was because nobody had really looked at this transition in any detail. And there was very little evidence prior to this as to exactly what had gone on, so any new evidence was going to be quite startling. But it did turn on their heads all the preconceptions that had gone before. With respect to the origin of limbs in particular, it rewrote the textbooks.

Humble to behold, Acanthostega shattered long-held preconceptions in the field of vertebrate paleontology.

Not only that, but because people are beginning to take an interest in the development of limbs from a developmental genetics point of view, and looking at the early embryology of limbs, it also fed into what they were doing. So paleontology and modern developmental genetics helped each other understand what exactly was happening very early on in the development of limbs.

NOVA: How so?

Clack: Well, up until about the mid-1980s, there was an idea or conception that the five-digited hand was the archetype, and that at the fish-tetrapod transition you went straight from the fin of something like Eusthenopteron to a five-digited limb. They tried to make parallels between the bone structure of one and the bone structure of the other. It was difficult if not impossible to do without having to make up a lot of intermediate forms.

When we found the limbs of Acanthostega, with eight digits, that completely knocked all the old arguments on the head. It was not something that had been conceived of under the old preconceptions. How do you get an eight-digited hand from a fin like Eusthenopteron's? Well, you can't.

Fortunately, it tied in with some work in embryology that suggested that all the old ideas of how you do it were wrong anyway. That work showed that the digits in modern animals do not arise in a regular pyramidal branching pattern but are formed around an arch that runs through the wrist or ankle, thumb or big toe last. In theory, all you'd need to do to produce eight digits is keep on going farther along the arch. The two discoveries very much came together, and one was able to explain the other.

It can take years of painstaking preparation for a single specimen from the Devonian to fully emerge from its sarcophagus of stone.

NOVA: So what was the best thing that you found in Greenland?

Clack: After three or four years of preparation, it was clear that the very first block that we looked at was the most complete and articulated individual skeleton. That's the one with the limb preserved on it. Then from the same level are another two or three individuals, one of which has got a beautifully preserved head. It's in three dimensions, and it's rather cute. Those are the two best specimens. But putting them all together, we were able to reconstruct practically the complete bony skeleton of Acanthostega.

NOVA: Preparing those fossils is a major operation, right?

Clack: Absolutely. When you bring a specimen home from the field, you may have a hint that there's something inside the rock by a cross-section here, a piece of worn bone there. But you won't know what's in it until you've removed all the rock that sits on top of it. This is a special skill, and it requires a great deal of patience.

The overburden, the rock that's lying on the top of the bones, is called matrix, and there's usually some kind of color or texture difference between the matrix and the bone. You can often see a separation there. But you've got to do this under a powerful binocular microscope, and you have to do it very, very carefully—in the last stages, grain by grain with a mounted needle.

It can take literally years to dig out a good specimen from a lump of rock. Some of the ones we brought back from Greenland have taken three or four years. [For a personal take on the process from Clack's own technician, see Confessions of a Preparator.]

Continue: A career paleontologist

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