This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: 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 Middle Triassic, over 250 million years later. So what were the first reef builders? Sara Pruss studies early animal evolution and the carbonate records animals 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. Sara Pruss, welcome to Geology Bites.

Sara Pruss: Oliver, thank you so much for having me.

Oliver Strimpel: 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?

Sara Pruss: The first animal reefs that show up on the scene were built by an ancient group of sponges called archaeocyaths. Archaeocyaths were some of the first calcareous sponges, and in many ways they look like corals and resemble them. They had branching morphologies and built structures on the seafloor, but they were very much sponges that worked in a relationship, or in a consortium, with microbial organisms to construct these reefs. In some cases, the reefs themselves can be predominantly made by microbial organisms, with archaeocyaths sort of tagging along. And in other cases, the archaeocyaths themselves really do build the frame of the reef, almost like the backbone of a reef. There was a long debate about what 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.

Oliver Strimpel: And how far back in the tree of life do sponges go?

Sara Pruss: 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 in time, really hundreds of millions of years before they show up in the Cambrian as the archaeocyaths. And there are putative sponge fossils that go back quite deep in time, but they tend to be debated. I don't think there are ones that 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.

Oliver Strimpel: But we actually have some fossils from the late Proterozoic, from the Ediacaran 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?

Sara Pruss: 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.

Oliver Strimpel: What enabled the archaeocyaths to emerge during the Cambrian? And when they did emerge, were they very widespread?

Sara Pruss: One of the interesting things that seems to be happening broadly across different groups is the onset of biomineralization as part of the Cambrian explosion. We see the roots of that in the Ediacaran, but really there is an expansion that happens in Cambrian time. And so archaeocyaths were clearly a group of sponges that developed the ability to make a skeleton, to biomineralize. 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 on a variety of paleocontinents. So they were globally quite significant, but geologically very short-lived.

Oliver Strimpel: Did they create ecological niches that facilitated the growth of diversity?

Sara Pruss: Well, based on some work that I've done with students over the years, and I'm sure you could bring on others who might say something different, I would argue that the structures that they built were quite important in facilitating local diversity. If we think about modern reef ecosystems, we see that in many of them, diversity tends to be high, and they tend to be places where there are a variety of environments, because they're structures built on the seafloor. Animals can capitalize on those different, you know, sort of microenvironments and thrive. And it's no different with the archaeocyaths. When we examine archaeocyath reefs in deep time, and we look at the components of things that were living within them, as well as the organisms that were 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, particularly for organisms that made shells, for biomineralizing organisms.

Oliver Strimpel: 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 archaeocyaths 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?

Sara Pruss: It's tricky. I think that the actual mechanisms of the Cambrian explosion are still debated. I do think that there's some combination, going back to Charles Marshall's 2006 paper, 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 archaeocyath reefs. In a paper our group put out just this last year, we documented that when these archaeocyath reefs go away, skeletal organisms, in terms of their abundance and to some extent their diversity, remain 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 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 insights both into the Cambrian explosion itself and into the broader view of environments and ecosystems within the Cambrian, and the stresses that organisms were experiencing during this critical moment in evolutionary history.

Oliver Strimpel: 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 archaeocyaths actually disappear, and was it one of the various stresses that you enumerated that finally polished them off?

Sara Pruss: Yeah, it's a good question. I think one of the things that can be challenging about studying the archaeocyath extinction is that we have somewhat poor age constraints on the Cambrian. So whether or not the reefs disappearing in Mongolia is timed 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. It's something that's playing out in a variety of different Cambrian working groups and discussions that I'm involved in right now: just linking the sections together can be challenging. However one wants to think about whether these sections represent a synchronous extinction, or an interval of extinction during which these reefs disappear, they do go away globally, and by middle Cambrian time, archaeocyath 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 that it can become difficult to make a shell. And so that may have been another stress that these organisms were grappling with.

Oliver Strimpel: So does the relatively rapid rise and fall, geologically speaking, of the archaeocyaths tell us that ecosystems were in fact less stable during the Cambrian, because of all the stresses you talked about?

Sara Pruss: I like to think of it the way I did when I wrote a paper with my colleague a few years ago; "Life on the Edge" was the title of the paper. 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 richer in oxygen, and there's environmental heterogeneity. Rachel Wood, who you mentioned earlier, and Doug Erwin wrote a beautiful paper about how that environmental heterogeneity might actually be a driver of innovation for animals, because they have 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.

Oliver Strimpel: I meant to ask you about what evidence we actually have for the archaeocyaths. You mentioned biomineralization and chemical evidence. Do we have any body fossils that enable us to infer the morphology that you described?

Sara Pruss: We have beautiful body fossils of archaeocyaths. It's one of the great benefits of their having been biomineralizers: they leave behind a remnant calcium carbonate skeleton, and it can be preserved in a variety of ways. We once reported on archaeocyaths that were actually phosphatized, so apatite was a mineral that grew inside their pore spaces and preserved them. They can be preserved as replaced calcium carbonate skeletons. They can be silicified. 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, or whether they were existing solitarily, as individuals. So the fossil record of archaeocyaths is actually quite enjoyable to study.

Oliver Strimpel: Where do we find some of the best-preserved fossils?

Sara Pruss: Oh, there are so many places where archaeocyath reefs have shown up over time. We have beautiful examples that I just walked over with my students in the western US, beautiful archaeocyath reefs, particularly ones I'm familiar with in the Poleta Formation and the 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 how ecologically 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.

Oliver Strimpel: You mentioned they had a certain kind of symbiosis with microbial life. Is that analogous to the kind of symbiosis that modern corals have with the sort of polyps that live inside them?

Sara Pruss: It's a great question. I think we don't entirely know, but there seems to be some evidence. It's been argued that some archaeocyaths seem to have evidence of microbial textures inside the skeleton, so it's possible that these were living symbiotically, or at least that 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 zooxanthellae that live inside modern corals are photosynthetic organisms, and so what they provide for those corals is food that might otherwise be hard to get. 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 would have been beneficial for sponges. Sponges are these kind of heterotrophic organisms that seem to have lived in potentially slightly more nutrient-rich settings. So did they need the microbes as a source of food or not? I think that's not entirely clear.

Oliver Strimpel: After the archaeocyaths went extinct, was there a gap in reef building, or did other reef builders emerge pretty much right away?

Sara Pruss: There is a gap, and certainly calcareous biomineralizing organisms experience a reasonably significant gap. It's not until the early Ordovician 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, and 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 siliceous and were probably not originally calcareous, as the archaeocyaths 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-unquote, filling in the reef gap by examining some of these microbial sponge reefs that would have existed after the archaeocyaths disappeared and before you start to see the rise of the sponge and coral reefs that dominate the Paleozoic. But the calcareous organisms that take over in the early Ordovician, at least based on the reefs that I've spent time examining, are the ones that most remind me of the archaeocyath structures of the early Cambrian.

Oliver Strimpel: And how long did those reef builders last, and were they then succeeded by the kind of corals that we have today?

Sara Pruss: Those reef builders, the enigmatic ones, again show up for a bit, but they don't persist for a terribly long period of time. By the middle Ordovician, or later Ordovician into the Silurian, you have stromatoporoid 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 Triassic time, so more like 240 million years before present. So there are these intervals of reef building that are populated by different reef builders.

Oliver Strimpel: And were those earlier reef builders rendered extinct by the end-Permian extinction that was such a massive extinction?

Sara Pruss: The stromatoporoid sponges, which were a really big part of Devonian reefs, take a big hit in the Late Devonian. The tabulate and the rugose corals that are a part of the Paleozoic reef-building fauna persist past 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. In fact, if you go and look at Lower Triassic sections around the world, what you see, again, is microbes. It's microbes that take over as the interim reef builders when calcareous organisms are struggling. And some of those microbial reefs might again have a siliceous sponge stuck here or there. But real animal reef building takes a hit for about 5 million years.

Oliver Strimpel: And what about the modern corals, then, that you mentioned arose in the Middle Triassic? How did those fare with the various extinctions, in particular the one at the end of the Cretaceous that destroyed the dinosaurs?

Sara Pruss: It's interesting: once the scleractinian corals show up, they persist, in spite of the stresses of mass extinctions. They survived the Triassic–Jurassic mass extinction, and they survived the Cretaceous–Tertiary mass extinction. They survived what we think was a massive global warming event at the Paleocene–Eocene Thermal Maximum. 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, the scleractinian corals persist and are the ones that make our reefs today.

Oliver Strimpel: And, ironically, they seem very vulnerable to what's happening to the planet right now.

Sara Pruss: Unfortunately, yes, with the variety of stresses that we are causing. There's both excessive heat, as the corals fare very poorly with excessively warm ocean temperatures because it so deeply affects their symbionts, and also the acidification of the ocean, with increased dissolved CO2 acidifying the surface ocean. It's another stress that corals are having to deal with. So the stress 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 scleractinian corals will be able to survive humans.

Oliver Strimpel: It's interesting, because the stresses you describe that took place in geological time sound like they were at least as extreme, at least temperature-wise, during the Paleocene–Eocene Thermal Maximum you mentioned. But the rate of temperature increase was probably orders of magnitude slower, so maybe they had some time to adapt.

Sara Pruss: That's one hypothesis: that the rate of change would have been slower. And then perhaps that's a piece of why the stress that we're seeing today is just so much more for the modern ecosystem to withstand.

Oliver Strimpel: What are you working on at the moment?

Sara Pruss: There's one of the reef questions that has been percolating since my dissertation, when 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 archaeocyath 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 archaeocyath reefs, and then they disappear. And then what comes back, and what does that look like? I've been increasingly interested in this early Ordovician interval of reef building, where, again, there are enigmatic calcifying organisms that show up. People debate what their exact affinity is. Are they sponges? Are they calcareous algae? Are they something different entirely? Watching them move into reef ecosystems and start to contribute to framework is really interesting. And what does that tell us about early Ordovician ecosystems and oxygen and where animals are living? Do they again occupy the same kind of ecospace as in the early Cambrian, where they provide a location and an ecosystem that allows other calcareous organisms to thrive? Or are they less important, because perhaps conditions in the early Ordovician 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.

Oliver Strimpel: Is one way of thinking about it, possibly, as a kind of example of convergent evolution, where various very different kinds of animals have reinvented the reef at various times during geological time?

Sara Pruss: 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... The coiled shell must provide such wonderful advantages, because things from ammonoids to snails to forams have figured out how to do it, and we could do an entire separate podcast about the advantages of the coiled shell. But 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 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 as many reefs are in high-wave-agitated settings, tropical carbonate settings, have a lot of calcium carbonate around to make your skeleton, and 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, organisms reinvent this ecosystem.

Oliver Strimpel: Sara Pruss, thank you very much.

Sara Pruss: It's been a pleasure, Oliver. I'll come back and talk to you anytime.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.