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Ted Daeschler and Neil Shubin: Early Tetrapod Fossils

Transcript:

Q: Tell me about some of the important fossils you've found in Pennsylvania.

TD: One of the fascinating things we discovered early in the project was an animal that we named Hynerpeton, which means crawling animal from Hyner, which is the town near where we found it.

The first part of the body that we found -- actually, the original material and all we had to work with for awhile -- was its shoulder girdle. A shoulder girdle is a very interesting part to find if you're looking at some of the earliest limbed animals, because it shows you where that limb attached into the body. When you look at the shoulder girdle, especially the inside, you see a large, deep area for muscle attachments. That's telling us that Hynerpeton had a very powerful forelimb. It's not like the shoulder girdle of a fish at all. So it was easily separable from all the fish material that we found.

From that shoulder girdle we could make some presumptions that this animal used that limb to do a lot of different things. We don't know exactly what it used it for, but most importantly, it shows that in this animal, limbs were becoming an important part of its tool kit. It may also have used its tail for swimming through the water, as fish do, but most fish don't use their limbs for anything significant. This early limbed animal, although it was probably still a lot like a fish and probably used its tail for propulsion and all the rest, was using those limbs for some unique functions as well.

We also have a major piece of a lower jaw of Hynerpeton. It was found very close to the shoulder girdle, and it has many of the features that are distinctive of early tetrapod lower jaws. What's really neat about it is it's a very skinny jaw -- it's lightly built, and so it would've held very light teeth.

Q: How do you know that Hynerpeton was still aquatic?

TD: We're making that judgment based on other early tetrapods that are found in other places around the world and are clearly still aquatic at that same point in time. It would be a bit of a jump and a bit bold for us to say that from this small part of shoulder girdle we had enough evidence to say this was an animal that actually could walk around on land. That would be too bold.

Q: How did that discovery come about?

TD: We became aware of a fossil site in Central Clinton County from some work that had gone on about 20 years previously. They did not find a lot of fossils in this site, but they found a few. We came back to this site using the old field notes from the previous workers and found a few fossils as well, but there was enough there to make us want to come back and look for more. Very early in the project we were back at the site, which we now call Red Hill. As we were surveying it, getting our nose out to the rock, we noticed a bone sticking right out on the surface of the rock that did not fit what we would expect for a typical fish bone.

It was unique, and very simply we collected that bone by taking some of the rock around it as well as the bone itself. I wrapped it up really carefully, took it with me in the car back to Philadelphia, called Neil and said, "Neil, I found a fairly interesting bone. I don't know what it is exactly." Between the two of us and a colleague named Dr. Keith Thompson we looked at it and almost couldn't believe that we had found an early tetrapod fossil.

We used comparisons. There are some early tetrapods from Greenland that are known from very good skeletons, Ichthyostega and Icanthostega, and part of that skeleton is the shoulder girdle -- where the forelimb attaches into the animal. And we found that this oddly shaped piece of bone that I'd found up on the side of the road at Red Hill was very similar to what we see in those forms from Greenland. It was also a little different, and that's what's really nice. We could compare it to what had been known in the past and we could also make some new observations about it and learn some new information from it.

This early tetrapod from the side of the road in Clinton County, Pennsylvania, was the first evidence of these early tetrapods from all of North America. That made it very exciting. Think about the hundreds of years nobody had been able to find evidence of these late Devonian limbed animals on this continent. People hadn't really understood that early tetrapods might have been part of the group of animals that lived at this time in North America.

Personally, it was critical for our project here in Pennsylvania because it provided the stimulus to get further funding and to really explore the area with much more detail. And it's that continued work at the Red Hill site in particular that has given us so much more information about what the area was like at the time that these early limbed animals were evolving here.

NS: One of the great things about that discovery was that if you could choose one bone of an early tetrapod to have, it would be the shoulder. Why? The shoulder is a window into how the animal walked or moved around, and even how it breathed. It's central to where the gills and the limbs lie, so that by having that single bone, one can reconstruct quite a bit about how that animal made a living. What that discovery told us is that this animal, Hynerpeton, was a fairly advanced sort of creature despite its old age, that it had a very robust powerful limb with large muscle attachments that enabled the limb to move backward and forward, and that it was a creature that was clearly able to make some forays on to land. Hynerpeton was a big strong guy, and you could tell that by the size and shape of the muscle attachments on the shoulder.

TD: Perhaps most importantly is to see that these animals with limbs were still living in the stream systems, for all the evidence we have. They may have been very able to make forays out on to the land for a limited amount of time, but they were very much tied to water, and that's an important part of understanding that early limbed animals were actually pretty much fish with limbs.

NS: There's another important evolutionary point here. There's not just one type of early limbed animal here, but several. Red Hill -- and other places like it -- has great diversity of early limbed animals. Even in the earliest stages of their evolution, there's not just one type. Having diversity, having variation is very important for evolution to proceed. That's the raw material by which evolution happens, that's the fuel.

Q: Did you make other discoveries?

TD: Another discovery we made of an early limbed animal was something that we named Densignathus rowe. It was discovered by Doug Rowe, one of the local guys we work with, and we also found a lower jaw of that fossil. We know that it's very different from the lower jaw of Hynerpeton because we did find a piece of lower jaw along with some of the shoulder material.

Densignathus looks very different from Hynerpeton, although they both are early tetrapods. One's jaw is very narrow and thin, and the other's is very deep and robust. So they were doing different things. They were probably catching different kinds of prey. They found different ecological niches in these freshwater stream systems.

So at this point in time, experimentation was going on in being a tetrapod. What kinds of things could you do with these new limblike fins or limbs, if you want to call them limbs? What could you do with these new tools? Some of these early tetrapods may have specialized in catching invertebrate animals, picking worms off the bottom of these stream systems. Others, like Densignathus with a thick jaw, maybe specialized in crushing the armored fishes. We're not sure, but we can easily see that they were trying different lifestyles.

Q: And what about Sauripterus?

TD: Sauripterus is clearly still a fish. It has many of the rods that are very characteristic of lobe-finned fish fins. The fin is rather large, about the size of a tennis racket head. We can see the internal bony structure that showed it was a muscular fin; it probably had a lot of muscle attachments within the fin that gave it a lot of mobility and a lot of strength. So here's a fish doing something with a fin that we don't expect fish to do with their fin, something new.

It was probably a large predator as well. So maybe it sat on the mud and used its fins to help it get started to chase its prey. It's really a little bit hard to know exactly how it used it, but we can certainly say that it was developing some novel features with the fin. And in fact, many of those features it was developing are very much like the limbs of the earliest limbed animals.

Q: And what about Hyneria?

NS: Hyneria is one of the more common things we find in the Red Hill site. It is not unusual to find the sharp teeth of Hyneria. In fact, its prevalence is what makes me think how hostile these freshwater streams were likely to have been. Hyneria was an 8- to 12-foot-long predatory fish with a massive jaw and huge teeth that look like railroad spikes. It was clearly the top of the food chain, a voracious predator. There was nothing else as big, nothing else as ferocious. It was essentially consuming our distant relatives, our ancestors. It was eating all the other fish in the streams.

TD: Hyneria is a lobe-fin fish, but its fins were not specialized for the function of perhaps pushing on the bottom or such. Its fins may have been specialized for speed. It clearly was an animal with a very large tail fin. It may have been something like a pike in today's fish. It probably sat and waited and was able to make quick starts and shoot out after its intended prey. So there is a lobe-fin doing something with its fins which is not the same as the lobe-fins that may have led directly to the earliest limbed animals. But it's part of the experimentation with all the things that you can do with this lobe-fin structure.

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