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July 5, 2012: A research summary
Symbiosis built the pyramids
In fact, searching for Nummulites in Egypt isn't so different from searching for sand in the Sahara Desert; nearly the entire country is made of them or more specifically, from limestone rock composed of their shells. Every set of stairs, countertop and bathroom wall I encountered was full of Nummulites shells. But I couldn't use just any shells for my test. I needed to find shells that hadn't changed at all in the last 45 million years. As you can imagine, all kinds of things can happen to a shell in that amount of time, so while I had a wonderful time searching the desert outcrops of Egypt in search of pristine shells, I never actually found any.
That's where my friends and colleagues, Dr. Strougo and Dr. Boukhary of Ain Shams University come in. These guys have been studying Nummulites longer than I've been alive, and they had the collection of shells to prove it! After another fruitless expedition, Dr. Strougo pulled some very dusty boxes from his private collection, and after rummaging through more than a few he held up a small, perfect sample he and Dr. Boukhary collected from a clay pit in 1984. It contained a species of Nummulites called Nummulites gizehensis, named for the Giza plateau where the Pyramids are located. It's one of the most common Nummulites found in the Pyramid building blocks. But that wasn't all! There were also several much smaller types of shells that could tell me about the chemistry of the water the Nummulites were living in. These were critical for the analysis I wanted to do because shell chemistry doesn't just change if there are symbionts doing photosynthesis inside, it also changes depending on the chemistry of the water it grows in. So without information about the water chemistry, it would be impossible to see if there was also a chemical difference because of symbionts doing photosynthesis.
Once I got this precious sample back to Berkeley, it was time to get to work. The first task was to really make sure that the shells I wanted to test were the same as when the Nummulites gizehensis were living on the bottom of the Tethys Sea 45 million years ago. I did this by using a scanning electron microscope to look closely at the physical structure of the shells, and by making maps of the different elements that were in them. If they were as pristine as I hoped they were, then I wouldn't see any changes to the shell shape even at the smallest scale, and there wouldn't be anything except calcium, carbon and oxygen present in the shell. Once I knew I had original shell material on my hands, I was able to do an analysis of the carbon and oxygen atoms in the shell. If algal symbionts had lived in N. gizehensis then the numerical results would differ from the results for the other shells by a very particular amount. I ran the test multiple times on multiple specimens to make sure, and every time my results were the same; they fit the expected results for having algal symbionts living inside them.
Does this mean that symbiosis was the driving force for evolutionary changes in the size, complex shape and/or reproductive habits of Nummulites? We don't know, because this study only addresses one very specific question: did Nummulites gizehensis have algal symbionts? But through answering that question a whole set of new questions about the role of symbiosis in the evolution of this lineage can be asked.
Life after the coral-apocalypse
Other researchers studying fossil coral reefs have found that in fact, coral reef communities don't really change across these glaciations. However, they were looking at reefs in Papua New Guinea and the Caribbean, which didn't experience the same extreme conditions that occurred in the Red Sea. The environmental catastrophe (from the perspective of a coral!) that took place during low sea level meant that corals went extinct over at least two-thirds of the Red Sea. Even in the areas closest to the narrow channel connecting the Red Sea to the Indian Ocean, conditions would have been deadly for most corals; only a few hearty species could have survived there. That means almost the entire Sea would have to be recolonized from scratch surely there would be some big differences each time a recolonization took place.
But my results surprised me. At first glance the younger fossil corals looked different than the older ones, but being exposed to wind and rain for such a long period of time can alter the appearance of a fossil so much they aren't recognizable. Luckily most corals have massive skeletons, and if you cut into them you'll find areas that have been protected from the elements and still have the features that make them recognizable. Once I was able to accurately identify the older, weathered corals, it turned out the older fossil coral reefs weren't really any different than the younger ones, and both of them looked like the coral reefs that grow in the Red Sea today. There were differences between the sites I surveyed, but those differences had to do with the kind of reef environment they preserved, not the time period they lived during. For example some of the sites were shallow lagoons with calm water, while others were steep slopes exposed to lots of waves, and different kinds of species live in those different areas. But once I compared a group of fossils that lived in a lagoon to the corals that live in Red Sea lagoons today, they were the same.
What lessons can we learn from the tumultuous history of coral reefs in the Red Sea? The good news is that if seas return to the warm, clear, nutrient-poor conditions corals love and there are any corals to do the recolonizing coral reefs will return to their former glory. The key is that there have to be some places where corals can persist. In biology, we call these places refugia small, isolated areas where populations can survive until conditions change again and they can spread back out. In the Red Sea, the far south may have acted as a refugium for some of the heartiest species, which also happen to be some of the most important founding species for settling new reefs. We still don't really know how big these refugia need to be, or how many of them would be needed to preserve all or most of the world's coral species during a prolonged period of extreme climate change. Even so, these marvelous animals are more resilient over geologic time than they sometimes seem on shorter times scales, and I hope that means they will have a place in our future's oceans.
All photos by Lorraine Casazza.
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