How did life emerge on Earth? How have life and Earth co-evolved through geological time? Is life elsewhere in the universe? Take a look through the 4-billion-year history of life on Earth through the lens of the modern Tree of Life! This course will evaluate the entire history of life on Earth within the context of our cutting-edge understanding of the Tree of Life. This includes the pioneering work of Professor Carl Woese on the University of Illinois Urbana-Champaign campus which revolutionized our understanding with a new "Tree of Life." Other themes include: -Reconnaissance of ancient primordial life before the first cell evolved -The entire ~4-billion-year development of single- and multi-celled life through the lens of the Tree of Life -The influence of Earth system processes (meteor impacts, volcanoes, ice sheets) on shaping and structuring the Tree of Life This synthesis emphasizes the universality of the emergence of life as a prelude for the search for extraterrestrial life.
3.3. Getting Old and Well-Preserved: Modes of Preservation
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Emergence of Life
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Week 3 - Fossilization and Precambrian Life-Earth Interaction
This week, you'll explore how scientists interpret ancient fossilized life, which yields remarkably detailed and complete reconstructions of the lifestyles of ancient organisms that have been deceased for hundreds, thousands, millions, and even billions of years. These reconstructions provide valuable information regarding the evolutionary success of organism morphology and lifestyle.
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Bruce W. Fouke, Ph.D.Director of the Roy J. Carver Biotechnology Center
Department of Geology, Department of Microbiology, and Institute for Genomic Biology
[MUSIC]
I'm Carly Miller, at the University of Illinois, and
I'd like to discuss the modes of preservation.
Which is the process that acts upon an organism from death
to present day, or when a scientist stumbles upon the fossil.
So there are very specific terms that we'd like to introduce and
be familiar with when we're looking at a hand specimen, so
that we can identify exactly what happened to that organism after death, and
how did it come to the state that it is in at the present.
So, the first mode of preservation is called original preservation.
And what that means is that actual
original material from that organism is still in tact in that fossil specimen.
So for example, we have agnatha in here, which is a Jawless fish,
which you'll learn [LAUGH], and
you can tell it's been well preserved you actually still has scales that seem to be
the same chemical composition that it was, when it was living.
And so we would call this original preservation,
it actually still has scales, a few little hard parts are still intact.
So original preservation you can have soft parts if you're lucky.
If it's frozen, if it's buried quickly you might have a shoulder [LAUGH], or
a piece of meat.
Or some, one of those soft parts of the organism's still preserved, but
more than likely, you're going to have the skeleton, or
a hard part that, survived over geologic time.
The second type of preservation, is called carbonization.
And that is basically when an organism has died, has been buried quickly and
usually pressurized, and basically reduced to a carbon signature,
which in general you can identify it by a black smudge.
That's what we're reduced to when carbon is pressurized,
it's basically just a carbon signature and it's a black smudge.
So you'll see that often with Fern plants, that's a very popular example of
a readily preserved carbonized organism, [LAUGH] is the Fern plant.
And so what you'll see is an imprint of that organism and a black smudge, and
you can start to think, okay, I don't see any actual body parts of an organism.
But, I know that it was made of carbon and
I see an imprint there, and that's the mode called carbonization.
The third one is called recrystallization, and
the tricky part about recrystallization is that you actually have to have
a little bit of knowledge about what the original composition of that organism was.
And so for example, we have a coral, and so you might say as a savvy scientist,
well I know that corals are made of calcium carbonate, and
that can form into two different minerals, aragonite and calcite, for example, and
so if you know that, you can look at a specimen and you can say well,
I know what aragonite looks like, right?
You do? I know what aragonite looks like, and
this just doesn't quite like look right.
It seems to have the same chemical composition, but the mineral has shifted,
or the needle shape is different.
And so you can start to kind of put those things together in your mind, and
what's really neat about recrystallization is it can have very specific structure.
If you look at a specimen,
you can see really specific septa, which is what they're called.
But they look like can of ribs radiating out from the mouth of the coral, and
you can get really great kind of resolution in that fossil specimen,
but it's been altered over time.
So what recrystallization is, in this case you start with calcium carbonate,
the organism dies, is buried, it's infiltrated with water.
And that water dissolves that mineral or the chemical composition there, and but
it starts to recrystallize basically right after it's been dissolved.
And so it starts recrystalizing right in place where those original
little nucleation points were, where those needles were.
And so you get the same chemical composition, but
it's been altered slightly, form, so we call that recrystallization.
So the fourth mode of preservation is called replacement, and
this is very similar to recrystallization, so
we want to make sure that we can differentiate between the two.
And again, you have to have some knowledge of that original chemical composition,
or texture, or the characteristics of that living specimen.
And so what replacement is, is when ground water infiltrates a buried dead organism,
and it dissolves and carries away those chemical components of the either
the shell or the skeleton, or something like that and it carries it away and
a completely new chemical composition infiltrates the system carried by
the ground water and it recrystallizes or fills in that void space.
That organism had died in, [LAUGH] or was buried in, and
so it is replaced completely with a different chemical composition.
So again, you have to have some knowledge of that original organism, and
what it looked like, and what it felt like, and
what it was made of when it was living to identify that as
replacement versus recrystallization versus original preservation.
So, we want to be careful with that, with replacement versus recrystallization.
So another great example of, a replacement fossil, is this Brachiopod here.
And, why it's so great, is that most people know that shells in general,
have a certain texture and a certain color that you're familiar with.
Usually kind of a white color, maybe made of aragonite or something like that, and
you'd be familiar with that.
That color just with your own observations, and
why this is such a great sample, and is such a great example of
a replacement fossil is if you look at the surface really closely
you can tell that it's covered in a shiny material, a shiny mineral.
And what that actually is is pyrite which is fools gold,
you might know it as fools gold.
And so, probably from your own experience or maybe your research or
something, that shells are not made of fool's gold.
And so you can start to think, well, what happened to this organism after it died,
and how did it get to this state where it's covered in fool's gold?
You'll start to realize that the shell wasn't originally made of that.
And so you can make that connection where the organism died.
Meteoric water went through the system, carried away the original,
maybe perhaps oreganite material, carried it away and then brought in the components
of fools gold and precipitated it on that sort of matrix.
And so, there we have a great example of a replacement fossil,
the next mode of preservation is called permineralization.
And so, usually you start with a porous part of the organism, so
for example bone is very porous on the inside.
Wood is very porous on the inside, and so
what happens is after burial ground water again infiltrates the system, and
carries in a mineral and fills in those pore spaces with a mineral.
And so what happens is that, that fossil actually ends up
quite heavy,e sometimes you can see little crystals growing in the pores.
It maybe takes on a different texture, it's shiny, it feels like a rock, and
so you can start to get some information about what this mode of preservation was,
and we call that demineralization,
when a mineral has grown in the pore space of an original part of that organism.
So the last thing we want to consider when thinking about modes of preservation is
molds and casts, a mold in a cast is basically when
you have no original chemical composition left over.
So, no actual piece of that organism is left in this area,
and you basically just have a representative of it again.
And so, a mold is basically a void space, so if you can think about it, if you push
your foot into a big squishy piece of mud, you're going to get a void space there.
And so, that is a mold of your foot,
and if that were preserved, that's a type of fossil.
And so if were to than pour plaster or pour other type of mud or
clay into that void space, where your foot was and that was hardened.
I could take out that three-dimensional structure,
and that is a cast of your foot.
The cast is kind of the positive part of that, process there.
And the mold is the indentation if you will
of that organ is when it kill over and die.
Or that part of the body that was pushed into the mud, so
we want to be able to differentiate between molds and casts.
And again you can get, how you identify that it is a mold or cast is that, again,
it doesn't necessarily have a lot of detail in that structure.
because if you think about organism, they have organisms, they have little veins,
they have little scales, they get very intricate in their structure and
the cast is sometimes just a squish in the mud.
[LAUGH] It's just kind of, a little a print of that organism and
that's not always really detailed ,so that's how you can start to clue in that
that's a mold or a cast.
So one of the themes that you probably noticed, when considering a mode of
preservation is that I've been referring to, that organism is buried,
and so, usually, if an organism has a good chance of being fossilized for
us to find later it usually needs to be buried very quickly, and if not that is
going to effect the extent to which that organism is going to be fossilized.
And so what you just want to remember is that burial the first step, in the process
of preservation after an organism dies, you just want to keep that in mind
[MUSIC].
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