This is Geology Bites with Oliver Strimpel.
Today, coral reefs host a wide range of ecological niches that
support the most biologically diverse ecosystems on Earth.
Did the emergence of such reefs in the Cambrian lay the
groundwork for the Cambrian explosion of life?
The answer may be yes, but it was not modern corals, which did
not arrive on the scene until the Mid Triassic, over 250
million years later. So what were the first reef
builders? Sarah Proust studies early
animal evolution and the carbonate records they leave.
She's especially interested in reef systems and how they
persist and recur through geological time, despite the
environmental stresses imposed on them.
She's a professor of geosciences at Smith College.
Sarah Preuss, welcome to Geology Bites.
Oliver, thank you so much for having me.
I said that the first reefs that emerged in the Cambrian were not
the work of modern day corals, So what was it that built them?
So the first animal reefs that show up on the scene are built
by an ancient group of sponges called archaeosciaths.
And archaeocyaths were some of these first calcareous sponges
that in many ways look like corals and resemble them.
They have branching morphologies and built structures on the sea
floor, but were very much sponges that worked in a
relationship or in a consortium with microbial organisms as well
to construct these reefs. So in some cases, the reefs
themselves can be predominantly made by microbial organisms with
archaeocias sort of tagging along.
And then in some cases, the archaeocias themselves really do
build the frame of the reef, almost like the backbone of a
reef. And there was a long debate
about what they were and who they were and whether they were
a coral or some other kind of Organism.
But I think there's broad consensus now that they are most
closely aligned with the sponges.
And how far back in the tree of life do sponges go?
Sponges go quite a ways back, although their fossil record
does not extend as far back as we think they may have been
around. We're always trying to fill in
that gap. There are chemical records
called biomarkers that suggest that sponges go quite deep into
time, hundreds of millions of years really before they show up
in the Cambrian as the Archaeosiaths.
And there are putative sponges, fossils that go back quite into
deep time, but they tend to be debated.
There aren't, I don't think, ones that sort of have universal
agreement at this point, but there is some emerging agreement
around the biomarker record and that there are these chemical
traces of sponges that go quite deep in time.
But we actually have some fossils from the late
Proteozoic, from the Ediacran period, sort of soft bodied
creatures. In a previous episode of Geology
Bites, Rachel Wood spoke a little bit about those.
These are not related to those, are they?
These archaeocyaths are probably different.
They are biomineralizing organisms, and sponges in
general are a sort of messy group where there are some
questions about their relationships to each other.
What enabled the archaeocytes to emerge during the Cambrian?
And when they did emerge, were they very widespread?
One of the interesting things that seems to be happening
broadly across different groups is this onset of
biomineralization as part of the Cambrian explosion.
We see the roots of that in the Ediacran, but really there is an
expansion that happens in Cambrian time.
And so archaeocyas were clearly a group of sponges that
developed the ability to make a skeleton to biomineralize.
And when they show up in the fossil record, they are quite
widespread. They are a very common component
of what we would consider the Cambrian fauna.
They appear in the early Cambrian and on a variety of
paleocontinents. So they were globally quite
significant, but geologically very short lived.
Did they create ecological niches that facilitated the
growth and diversity? Well, I would argue based on
some work that I've done with students over the years, and I'm
sure you could bring on others that might say something
different, but I would argue that the structures that they
built were quite important to facilitating local diversity.
And so if we think about modern reef ecosystems, we see that in
many modern reef ecosystems, diversity tends to be high and
it tends to be places where there are a variety of
environments that can be preserved because they're
structures built on the sea floor.
Animals can capitalize on those different, you know, sort of
micro environments and thrive. And it's no different in the
Archaeosiaths when we examine the Archaeosiath reefs in deep
time and we look at the components of things that we're
living within them, as well as the organisms that we're living
on and around them. The diversity and abundance of
animals tends to be locally higher in those reef ecosystems.
So I think they had an important role to play as part of this
kind of Cambrian explosion in reef environments and
particularly for organisms that made shells for biomineralizing
organisms. That's fascinating, actually.
It reminds me of the episode I did with Paul Smith, who talked
about the Cambrian explosion and the many processes that he
suggested were responsible for the very rapid rise in animal
diversity in the Middle Cambrian.
So does our understanding of the archaeosias and their reefs and
the biomineralization they were able to do give us insight into
some of these processes and really help us understand how
the Cambrian explosion came about?
It's tricky. I think that the Cambrian
explosion, the actual mechanisms of it are still debated.
I do think that there's some combination of, you know, going
back to Charles Marshall's 2006 paper, some component of
environment, ecosystem and genetic toolkit that all played
together to facilitate this moment where organisms were
interacting and occupying new ecospace in ways that they
hadn't before. I think another interesting
piece of this is the disappearance of the
Archaeosiath reefs. Because in a paper our group put
out just this last year, we documented that when these
Archaeosiath reefs go away to skeletal organisms as in terms
of their abundance and to some extent their diversity remains
low for a period of time. And that suggests that these
reefs played an important ecological role in facilitating
where these animals were living and how they they were living.
And whatever stresses caused those reefs to disappear,
whether it was low oxygen, whether it was extreme warming,
various mechanisms that we could think about, whatever stresses
those were, really did lead to this long term decline of animal
diversity and abundance. So I think they provide both
insights into the Cambrian explosion itself, but also the
broader view of environments and ecosystems within the Cambrian
and the stresses that organisms were experiencing during this
critical moment in evolutionary history.
So that's interesting. Naively, I had thought of the
Cambrian explosion as initially a rapid, but then also an
ongoing increase in animal diversity, at least until the
end of the Cambrian. But in fact, after the initial
burst of diversity in the Early Cambrian, sometime in the Middle
Cambrian, it went into reverse and there was a decline.
So when did the archaeosias actually disappear, and was it
one of the various stresses that you enumerated that finally
polished them off? Yeah, it's, it's a good
question. I think one of the things that
can be challenging about studying the Archaeosiath
extinction is that we have somewhat poor age constraints on
the Cambrian. So whether or not the reefs
disappearing in Mongolia times exactly with the reefs
disappearing in the Western US has remained A somewhat open
question because of our difficulty in correlating
between these global units. And it's something that's
playing out in a variety of different Cambrian working
groups and discussions that I'm involved in right now is just
linking the sections together can be challenging.
However, one wants to think about the ways in which these
sections may represent a synchronous extinction or a
period, an interval of extinction, during which these
reefs disappear. They do go away globally and by
Middle Cambrian time are really archosiath reefs are thought to
be gone, a thing of the past. I think there's some evidence
for oxygen stress that may have played a role in the
disappearance of these organisms.
I think sometimes with oxygen stress, there can also be stress
in how organisms make their shells.
So a related stress to organisms that's biologically relevant in
addition to low oxygen in the environment broadly, is it can
become difficult to make a shell.
And so that may have been another stress that these
organisms were grappling with. So does the relatively rapid
rise and fall, geologically Speaking, of the archaeosias
tell us that in fact ecosystems were less stable during the
Cambrian because of all the stresses you talked about?
I like to think of it as when I wrote a paper with my colleague
a few years ago, Life on the Edge was the title of the paper.
And it's this notion that these organisms and ecosystems are
developing and existing in a world where stress is pervasive,
where you have intervals of low oxygen adjacent to areas that
are more rich in oxygen, and there's environmental
heterogeneity. Rachel Wood, who you mentioned
earlier, and Doug Irwin wrote a beautiful paper about how that
environmental heterogeneity might actually be a driver of
innovation for animals because they are having to adapt to such
a wide array of stresses and environments.
So I think it's a really interesting way to think about
the Cambrian as a bunch of organisms and ecosystems that
are living on the edge of environmental stress, and
sometimes that stress gets to be too much.
I meant to ask you about what evidence we actually have for
the archaeoscience. You mentioned biomineralization
and chemical evidence. Do we have any body fossils that
enabled us to infer the morphology that you described?
We have beautiful body fossils of archaeociats, and it's one of
the great benefits of them having been biomineralizers is
they leave behind a remnant calcium carbonate skeleton, and
it can be preserved in a variety of ways.
We once reported on archaeociats that were actually phosphatized,
so appetite was a mineral that grew inside their pore spaces
preserve them. They can be preserved as
replaced calcium carbonate skeletons.
They can be solicified, but the fact that they made a shell
actually makes their record quite outstanding.
And so there are ways in which we can examine their
morphologies quite closely and really see how they lived, what
their relationship was to the microbial organisms that shared
their reefs, whether or not they were branching, whether or not
they were more as existing solitarily as individuals.
So you have the fossil record of archaeocyats is actually quite
enjoyable to study. Where do we find some of the
best preserved fossils? Oh, there are so many places
that where Archaeosiath reefs have showed up over time.
We have beautiful examples that I just walked over with my
students in the western US, beautiful Archaeosiath reefs,
particularly ones I'm familiar with in the Paleta Formation and
Harkless Formation. They're like fun old friends to
go back and visit. There are beautiful reefs that
have been preserved in Australia and Mongolia.
Mongolia has some really exceptional examples, South
China, up in Arctic Canada, there's just a whole variety of
places one can go, which again, just sort of points to
ecologically how significant these must have been because of
how widespread they were, even though they only existed for a
relatively short period of time geologically speaking.
You mentioned they had a certain kind of symbiosis with a
microbial life. Is that analogous to the kind of
symbiosis that modern corals have with the sort of polyps
that live inside them? It's a great question.
I think we don't entirely know, but there seems to be some
evidence from, it's been argued that some archaeocyas that seem
to have evidence of microbial textures inside the skeleton
that it's possible that these were living symbiotically or at
least they didn't mind each other.
If they weren't directly benefiting each other, then
perhaps they were living in a way that they didn't mind each
other. So it's a bit different because
the zozanthellae that live inside modern corals are
photosynthetic organisms, and So what they provide for those
corals is food that might be otherwise hard to get because
the corals are sometimes living in low nutrient, low food
environments and having those organisms that can provide food
for them is very beneficial. It's a little bit more difficult
to understand exactly what it would have been for sponges that
would have been beneficial. Sponges are are these kind of
heterotrophic organisms that seem to have lived in slightly
potentially more nutrient rich settings.
So did they need the microbes as a source of food or not?
I think that's not entirely clear.
After the archaeocytes went extinct, was there a gap in reef
building or did other reef builders emerge pretty much
right away? There is a gap, and certainly
calcareous biomineralizing organisms experience a
reasonably significant gap. It's not until the early or
division that you start to see more enigmatic calcareous
organisms showing up and moving into reefs.
However, there have been a few examples, I believe from the
Middle Cambrian, certainly from Upper Cambrian strata where
there are microbial reefs that people have found sponges living
in and perhaps adding some framework to.
Many of those sponges are salacious and we're not probably
originally calcareous as the archaeocyats were.
And it's unclear that they were necessarily frame building
sponges, but they were certainly living in the reefs.
And so people have increasingly been quote UN quote filling in
the reef gap by examining some of these microbial sponge reefs
that would have existed after archaeoscience disappeared and
before you start to see the more rise of sponge and coral reefs
that dominate the Paleozoic. But the calcareous organisms
that take over in the Early or Division, at least based on the
reefs that I've spent time examining, are the ones that
most remind me of the Archaeosiath structures of the
Early Cambrian. And how long did those reef
builders last, and were they then succeeded by the kind of
corals that we have today? So those reef builders, the
enigmatic ones, again, they show up for a bit, but they don't
persist for a terribly long period of time.
By the middle or division sort of later or division into the
Silurian, you have stromatopyroid sponges and
Paleozoic corals that start to show up and build reefs in the
Paleozoic. But the real modern reefs, the
scleractinian corals that build the modern reefs, do not show up
until middle triathlon time, so more like 240 million years
before present. So there are these intervals of
reef building that are populated by different reef builders.
And were those earlier reef builders rendered extinct by the
N Permian extinction that was such a massive extinction?
The stromatopyroid sponges that were a really big part of kind
of Devonian reefs, they take a big hit in the late Devonian,
the tabulate and the burgosid corals that are a part of the
Paleozoic sort of reef building fauna, they persist that.
Then the end Permian is really the thing that takes out the
last of the Paleozoic reef builders.
And so we get almost a complete reset.
And in fact, if you go look at Lower Triassic sections around
the world, what you see again is microbes.
It's microbes that's take over as the interim reef builder when
calcareous organisms are struggling.
And some of those microbial reefs might again have a
silicious sponge stuck here or there.
But real animal reef building this takes a hit for about
5,000,000 years. And what about the modern coral
then that you mentioned that arose in the Mid Triassic?
How did those fare with the various extinctions, in
particular the one at the end of the Cretaceous that destroyed
the dinosaurs? It's interesting once the
scleractinian corals show up in spite of stresses of mass
extinction. So they survived the Triassic
Jurassic mass extinction and they survived the Cretaceous
Tertiary mass extinction. They survived what we think is a
a massive global warming event at the Paleocene, Eocene Thermal
Maximum interval. So it is interesting that the
scleractinian corals, not that individuals within that group
might not have gone extinct because of some of those
stresses, but in total those sclerotinian corals persist and
are the ones that make our reefs today.
And ironically, seem very vulnerable to what's happening
to the planet right now. Unfortunately, yes, with the
variety of stresses that we are causing, both excessive heat
that the corals fare very poorly with excessive warm ocean
temperatures because it's so deeply effects their symbiotes,
but also with the acidification of the ocean with increased
dissolve CO2 acidifying the surface ocean.
It's another stress that corals are having to deal with.
So the stresses that humans are inducing is palpable and has
effects on these ecosystems. And it is, I guess, interesting
and depressing to think about whether or not these
sclerotinian corals will be able to survive humans.
It's interesting because it's the stresses you describe that
took place in geological time sound like they were at least as
extreme, at least temperature wise, during the thermal
maximum, pallocene thermal maximum you mentioned.
But the rate of temperature increase was probably orders of
magnitude slower, so maybe they had some time to adapt.
That's one hypothesis is that the rate of change would have
been slower. And then perhaps that was a
piece of why the stress that we're seeing today is just so
much more for the modern ecosystem to withstand.
What are you working on at the moment?
One of the reef questions that have been percolating since my
dissertation, where I looked at some of those odd Lower Triassic
microbial reefs, and then for my postdoc, bounced back to the
Cambrian and started working on Archaeosiath reefs.
One of the things that I've been thinking about is this continuum
where you start with this Early Cambrian experiment in animal
reef building, the topic of our podcast today, these wonderful
Archaeosiath reefs, and then they disappear, and then what
comes back and what does that look like?
I've been really increasingly interested in this early or
division interval of reef building, where again, there are
enigmatic calcifying organisms that show up.
People debate about what their exact affinity is.
Are they sponges? Are they calcarius alga?
Are they something different entirely?
Watching them move into reef ecosystems and started to
contribute to framework is really interesting.
And what does that tell us about early or division ecosystems and
oxygen and where animals are living?
Does it again occupy the same kind of ecospace that it did in
the early Cambrian, where they are providing a location and an
ecosystem that allows other calcareous organisms to thrive?
Or are they less important because perhaps conditions in
the early or division are more broadly amenable to organisms,
and so the reefs are a little less important?
These are the kinds of questions my research group and my
students particularly, are trying to untangle.
Is it possibly one way of thinking about it a kind of
example of convergent evolution where various very different
kinds of animals have kind of reinvented the reef at various
times during geological time? I think about it all the time.
What is the advantage of the reef?
What is it about these particular environments and
these ecosystems that organisms from different groups have
figured out how to do it over time?
When you see something like that happen in evolutionary history
again and again and again, kind of like the coiled shell, all
the coiled shell must provide such wonderful advantages
because things from aminoids to snails to forums have figured
out how to do it. And we, you know, go do an
entire separate podcast about the advantages of the coiled
shell. But when I when you see things
in evolutionary history that happen again and again, you have
to think to yourself what an advantage it must have been for
for animals to figure out how to make a reef.
Because if these organisms are spending energy and evolving
into that niche, then that means that there's some real
evolutionary advantage to being there.
And one can imagine that those environments, particularly ones
that are as many reefs, are in high wave agitated settings,
tropical carbonate settings. There's a lot of calcium
carbonate around to make your skeleton.
They might be particularly well mixed with oxygen.
There's a whole variety of reasons to think that they were
particularly habitable, but it is interesting that you just see
it again and again. In spite of the stresses that
have tried to take them out over and over again, Over time they
reinvent this ecosystem. Sarah Proust, thank you very
much. It's been a pleasure, Oliver.
I'll come back and talk to you anytime.
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