In order to study the radiation of these metazoans
after this time, we went to the Aldan River in Siberia,
courtesy of Russian helicopters and 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. Here we are on the Aldan River looking at
rocks . 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, 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.
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?
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. 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, 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 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, another
kind of single-celled protozoan. These made tubes and they're
worldwide in their distribution as all of the other things are.
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. 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. 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. 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. 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
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. 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. 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. 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. 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. 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. More of them here.
Then of course, about the same time, we have
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.
They 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.