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ImageMap - turn on images In order to study the radiation of these metazoans after this time, we went to the Aldan River in Siberia, 38copterrusS.jpg courtesy of Russian helicopters and 39shipS.jpg their research boat, with a group of people on a field trip in 1990, to look at rocks along the Aldan and Lena Rivers. This is the "Pinnacles of the Lena". These are Cambrian in age but we wanted to look at rocks a little bit lower so we had to fly from here down to the Aldan River. 40cliffS.jpg Here we are on the Aldan River looking at rocks 41beachS.jpg. You can see that white layer right there and up here it's darker in color. That's the boundary between the Cambrian the Precambrian rocks below. At that point, many shelled organisms appear in the fossil record. This particular part had some diagenetic problems because the rock below is dolomite and it is limestone above, and there's a problem with how the fossils may have gotten to the base of the limestone.

In any case, these people, 42gangS.jpg Alexei Rosanov from Moscow, Francois Debrenne from Paris, Stefan Bengtson from Sweden, our Chinese colleague from Nanjing, who's name I've forgotten, standing on the outcrop debating the validity of these kinds of fossils. Science is not a bunch of people coming up with the facts and saying this is the way it is. It's a debate. As Stan says, it's a continuing process, often self-correcting or eventually self-correcting. Sometimes these things last for hundreds of years before somebody gets around to setting them straight. But this kind of relationship amongst people standing on the outcrop debating it, is the way science works. It's a social process and the social studies of science is really something quite interesting that we don't need to get into. But you can't teach science unless you teach about scientists. The scientists are the people that do it and interpret it and they are just like everybody else. They have their hopes, their dreams, their loves and they get excited and it's very creative but they make mistakes, some of them even lie. I mean, they are people! So that, I think, has to be said, as well. Science isn't just objectivity.

43cliff14S.jpg Here's the boundary right there between the Precambrian the Cambrian right at that point. The reason these guys are photographing it, is that's where shelly fossils begin. That's sharp, at least in that part of Siberia. You can see it that way in China, you can see it that way in some other places, as well. This has been studied in various parts of the world. So what is it that we see at that point? 44hyolithidsS.jpg Here are some of the microscopic fossils--what we call small shelly fossils. These are just little spines and little cap-shaped elements that were probably spicules on the backs of larger organisms. One organism may have had several hundred of these on it, but when the organism died and deteriorated, these things fell apart and became incorporated into the sediment. For many years, just to show you how science works, people were trying to jam some kind of a snail inside of this or a slug or a worm. It wasn't until Stefan Bengtson, the guy I showed the slide of, sitting in his office in Uppsala wondering about these things, came up with the idea that these were sclerites or little pieces covering a much larger organism. Eventually other paleontologists found the fossil in place with all these neatly arranged on the back of a slug-like animal and Stefan was proved to be correct, and all the Russians and Americans and Chinese that tried to jam a snail or slug in those little shells were wrong. That may not be a big time thing for NBC, but it is, I would say, the sort of thing that could be made into a dramatic story if it were a little bit better in terms of impact. I have to admit, some of this does not have NBC-style impact.

I told you about the phytoplankton and we see a change in those as well, at this boundary where we found these small shelly fossils. 45skiagiaS.jpg This one looks quite different from the previous one that I showed you that was 850 million years old. This one is 540-545 million years old and it's got a lot more structure to it and many more spines on it. This is a major change in the phytoplankton. So phytoplankton changed at this time, as well. It's not just metazoans, the animals, 46starS.jpg that radiated with skeletons. These things are doing something, too. And they're radiating into many more species, into many more morphologies. We can see by studying their distribution over wider ranges of geography that they had biogeographic patterns that are analogous to the kinds of patterns we see in 47sporesS.jpg phytoplankton today. They radiate into a whole variety of different kinds, too, as you move from the Cambrian into the overlying Ordovician and Silurian. These phytoplankton are tracing evolutionary patterns; I should say trace, the evolutionary patterns of animals through this period of time starting with some simple-looking things sort of like that. The animals were relatively simple in many ways and then became more complex through the Cambrian and into the Ordovician and Silurian when everything gets going really well.

There were also forminifera, 48yanisS.jpg another kind of single-celled protozoan. These made tubes and they're worldwide in their distribution as all of the other things are. 49platysS.jpg Here are some of the flattened tubes from up in the Baltic region. They're very widespread in that area. In fact, they're very widespread around the world. 50barS.jpg Here's one from California still embedded in the rock. You see, these kinds of organisms made their shells of sand or silt in this case. At the base of the Cambrian or soon thereafter, we have the organic-walled phytoplankton, the siliceous radiolarians, the agglutinated (cemented sand or silt) skeletons of foraminifera, and the secreted calcium carbonate and phosphoric skeletons of the earliest metazoans. This will come to bear on this problem of radiation at this period of time, in a few more minutes. 51idahoS.jpg As the foraminifera radiate through time in the Cambrian, they develop other structures and other morphologies, and other species just like the metazoans did. In other words, we see as a same pattern of evolution in these quite diverse groups--animals, protozoans, algae, and phytoplankton--that live in different parts of the marine environment, on the bottom, in the water column, onshore and offshore. They are all radiating at the same time near the base of the Cambrian.

At the base of the Cambrian we have also the appearance of a different set of trace fossils. 52fingerS.jpg This is a trace fossil. In fact, it marks the base of the Cambrian in the reference section in Newfoundland. Phycodes pedum, it's called. That's my thumb, for scale. And you'll notice here, just a few stratigraphic meters above the first appearance, there are some really fancy and big trace fossils. The guys that made these were cruising across the surface of the mud in search patterns that are highly sophisticated and quite different from what we saw in the Precambrian or Vendian rocks, say at the White Sea or down in Australia. 54pickaxS.jpg You see them everywhere; here are some other ones. To a trained eye, even to your eye I hope, you can see the difference. So, with all kinds of fossils, we see a major time of radiation, even when there are no skeletons preserved.

In Nevada, near Fish Lake Valley and other places, are a series of rocks of this lower Cambrian age and in those rocks are some of these big limestones, these blue rocks there. Those rocks contain a group of organisms called archaeocyathids. 56vfossilS.jpg These are skeletonized probably sponge-like organisms. We've had them in different phyla. Maybe they have been in a different kingdom even, but they are probably sponge-like animals. 57revolverS.jpg These things were able, because--this is their internal structure--these things were able to build big reef-like structures just like you would have in the ocean today. And they did it by growing upward and outward. 58veinsS.jpg Here's one right there and if you look carefully you can see pieces and little bits of other ones--and they build a framework which became a reef. That's what you see in these situations. 59ridgeS.jpg These are big Cambrian reefs built by archaeocyathids and an association of algae and other organisms, as well. So very early in the history of metazoans, after they obtained skeletons at the base of the Cambrian, they radiate into all the modern kinds of habitats, all the modern kinds, major kinds of way of making a living in the marine environment. 60ridge2S.jpg Here you can see some more of these big reef structures. When you look at this really carefully and compare it, as I've done, to modern reefs, say at Tahiti or any other place we saw, you can see all the same kinds of structures. Just a different group of animals. But they made these reefs and they probably did it by actually using photosynthesizing algae in their tissues, but that's another story that we can get into later. 61veinsS.jpg More of them here.

Then of course, about the same time, we have 62triloS.jpg trilobites. And now we're into that part of the geological column that you know so well; trilobites and so on. And the action is essentially all over at this time, in the early Cambrian. And it happened in 10 or 20 million years. We have the sudden appearance of these animals evolving into the Burgess Shale kinds of organisms. 63lobsterS.jpgThey may well have been around because we actually have them earlier in the Cambrian in China and in a couple of other places in the world now. The Burgess Shale is turning out to be less unique and more of a depositional artifact, perhaps, than a special new kind of wonderful life. They really are not. They were probably all around and certainly there are arthropod specialists who think there's nothing special about them either, other than many of the things are extinct. That does make it sort of special in a certain sense but there's nothing special about these things in the way that some people have described them. Certainly they are wonderful but maybe not so significant.

So that's the story and it happened very quickly. We came through a very long period of bacteria, a very long period later on with eukaryotic algae, then some weird things appear, and then the Vendian animals, those three classes or groups of animals, and finally animals evolve skeletons and radiate like crazy. But they were not alone. And this is one of the things that is important to realize because we'll see in a moment that most of the hypotheses for this radiation of animals are what I call metazoan bound hypotheses. These kinds of hypotheses do not consider what's going on with algae, the protozoans, or even the trace fossils, just metazoans.


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