If you take sample of paleontologists and ask them how they became interested in the field of paleontology, some of them will doubtlessly tell you that they first got hooked on dinosaurs as a kid and things proceeded from there. I am definitely an example of this phenomenon: as far back as I can remember I was fascinated by dinosaurs, and “paleontologist” was almost always the answer when people asked what I wanted to be when I grew up. Even though my research now focuses on ancient mammal relatives that lived before the dinosaurs, I've been very fortunate to be able to realize my childhood dream and be a paleontologist. Recently, however, I've been involved in a research project that has literally taken me back to my childhood.
The Asteroid and the Dinosaurs
In June, 1980, Luis Alvarez, Walter Alvarez, Frank Asaro, and Helen Michel published a seminal paper in the scientific journal Science, in which they presented evidence of an asteroid impact at the end of the Cretaceous Period of Earth History (about 65 million years ago). They also hypothesized that the after-effects of this impact caused the end-Cretaceous mass extinction, whose victims famously include the non-avian dinosaurs. Extensive research over the last three decades has led to the impact hypothesis becoming the generally accepted explanation for the ultimate cause of the end-Cretaceous extinction (Schulte et al., 2010), but in 1980 it was an extremely controversial idea and it generated a lot of attention in the popular media. A fascinating, first-hand account of the story behind this discovery and the scientific debates that ensued can be found in Walter Alvarez's book T. rex and the Crater of Doom.
Walter Alvarez's book T. rex and the Crater of Doom describes how he and his collaborators discovered that an asteroid impact was the likely cause of the end-Cretaceous extinction and the scientific debates that followed. All scientists are naturally curious about the world, but Walter is probably the most curious person I've ever met, which doubtlessly was one of the things that contributed to this discovery. Walter inscribed my copy of the book when I was a graduate student at the University of California, Berkeley.
The 1982 Williamstown Elementary School Science Fair
In 1982 I was six years old and in first grade. Being a kid that loved dinosaurs, when the yearly science fair rolled around, I naturally wanted my project to focus on that topic. I don't remember where exactly I got the idea, I must have seen it on Nova or in a magazine, but I decided I would do my project on the recent discovery that an asteroid killed the dinosaurs. You can see the two parts of my poster in the photos below (thanks to my parents for saving them all these years!). The main point of the poster was describing how the asteroid impact could have killed the dinosaurs: dust and debris blasted into the atmosphere by the impact would have blocked sunlight from reaching the Earth's surface. Without sunlight plants would die off, and the the loss of this food source would cause plant-eating dinosaurs to go extinct. In turn, meat eating dinosaurs also would go extinct as the herbivores that were their food source disappeared. I didn't think of it in these terms at the time, but the process described in my poster reflects how disturbances to one part of a community of organisms can spread along the links in the community's food web, causing animals and plants that were not involved in the initial disturbance to nonetheless become extinct. After the science fair, I didn't think much about these ideas for quite some time.
My first grade science fair project, describing the process by which an asteroid impact could have resulted in the extinction of dinosaurs. Note that originally plastic dinosaurs were glued to the poster where it mentions plant-eating and meat-eating dinosaurs. The dinosaurs were too important as toys to be left on the poster after the science fair and were removed. Looking back, my handwriting in first grade was much more legible than it is now.
The Circle Begins to Close
We'll now fast-forward to about the year 2001. At the time, I was a graduate student working on my Ph.D. at the University of California, Berkeley. UC Berkeley has a strong paleontology program, and I was a student in Dr. Kevin Padian's lab working on a group of ancient mammal relatives called dicynodonts. My research project focused on reconstructing how the various dicynodont species were related to each other, and using those relationships as a framework to study the evolutionary history of the group. Dicynodonts lived in the Permian and Triassic periods of Earth history, and as such they were witnesses to and survivors of the end-Permian mass extinction. This extinction event is the largest in Earth history, and it occurred about 250 million years ago. Dr. Walter Alvarez was (and is) a professor in the Department of Earth and Planetary Science at UC Berkeley, and by the time I was a graduate student his interests in mass extinctions had expanded from just the end-Cretaceous extinction to include the end-Permian extinction as well. My work didn't focus on how the end-Permian event affected dicynodonts, but Walter's knowledge of things Permian made him a good choice to have as a member of my dissertation committee. I ended up taking a couple of classes with him and in 2003, some 21 years after my first grade science fair project, he was one of the people who read and signed off on my Ph.D. dissertation (thanks Walter!). I'm not sure what my six-year-old self would have thought if you told him that someday he would know the person who played a key role in discovering that an asteroid impact caused the end-Cretaceous extinction and have him as a mentor in the process of becoming a paleontologist. But that's not the end of the story.
Food Webs and Extinction
After I graduated from UC Berkeley, I spent about two and a half years working as a postdoctoral researcher at the California Academy of Sciences with Dr. Peter Roopnarine, one of the museum's curators. Peter is a paleoecologist (i.e., he is interested in understanding the ecological relationships of past organisms and how and why those relationships change over time). During the time that I was at the Academy, Peter became interested in network theory and how it could be applied to scientific questions that he was interested in. We tried a few different network-based projects, but the one that really got going was using network theory to model how disturbances can propagate through a community's food web, causing plants and animals to become extinct. Peter developed a model called CEG (Cascading Extinction on Graphs) to examine this problem, and I contributed data about the food webs of several Permian and Triassic communities from South Africa that included many dicynodonts. One of the things that we discovered in the course of that work is that the earliest Triassic community in South Africa, which existed just after the end-Permian extinction, had a very usual food web structure that made the community relatively unstable (Roopnarine et al., 2007; Roopnarine and Angielczyk, 2012). Much of Peter's research has subsequently focused on figuring out why the structure of the community made it unstable, and I've continued to work with him on the implications of the model results for dicynodonts and other animals that were alive at the time. This research also inadvertently brought me a step closer to my first grade project: I was now studying the link between food webs and extinctions, although I wasn't yet paying attention to the end-Cretaceous extinction.
Guild-level food webs for a Late Permian community (left) and an Early Triassic community (right) from the Karoo Basin of South Africa. The differences in the structures of the food webs means that disturbances can spread more easily through the Triassic community, resulting in more extinction for a starting disturbance of a given magnitude.
Since 2007, I've worked at the Field Museum of Natural History. My research continues to focus on topics such as how dicynodonts are related to each other, and how food web structures might have affected the end-Permian extinction. About two years ago, I started working with a Ph.D. student at the University of Chicago named Jonathan Mitchell. Jon is interested in the paleoecology of birds, and as part of his Ph.D. project he is planning on studying how birds affect the food web structures of certain ancient and modern communities. When he started at U of C, Jon was interested in incorporating the CEG model into some of his work, and he began working on a project looking at the food web structures of some Cretaceous communities that included birds and dinosaurs. As things progressed, we decided that it would be interesting to try to determine if the food web structures of communities at the end of the Cretaceous were such that they might have made the extinction resulting from the asteroid impact worse than it might otherwise have been. In other words, one of the expected consequences of the impact is a die off of plants, so did the food web structures of latest Cretaceous communities allow that disturbance to easily spread to other members of the communities, causing additional extinctions? As we show in a paper that was published in the journal Proceedings of the National Academy of Sciences (Mitchell et al., 2012), the short answer is yes: the food web structures of latest Cretaceous communities in North America did make them more vulnerable than communities from a few million years earlier. So if the asteroid hit 75 million years ago instead of 65 million years ago, it probably would have still caused a mass extinction, but that hypothetical extinction likely would not have been as severe as the one that actually happened.
Plot showing a measure of how much extinction can be caused in various Late Cretaceous communities from a die-off of plants; communities from within two million years of the end-Cretacoues mass extinction are shown in red and communities from the preceding 13 million years are shown in blue. The CEG model predicts that on average the older communities will experience less extinction than the younger ones for a given disturbance.
In participating in this work, and in writing this paper, my research has truly come full circle. My first science fair project was about how the effects of the asteroid impact at the end of the Cretaceous could have spread through food webs, causing extinction, and now I'm a co-author of a research paper that addresses essentially the same question (albeit in a somewhat more sophisticated way). Even as a paleontologist, I wouldn't have predicted that this would happen given the focus of my research (ancient mammal relatives that lived before the dinosaurs). However, one of the rewarding things about being a scientist is that you get to follow where your curiosity leads, and sometimes it takes you to unexpected places. And now when someone asks me about why the dinosaurs went extinct at the end of the Cretaceous Period, I can tell them that it's a question I've been working on for practically my whole life...
Alvarez, L.W., W. Alvarez, F. Asaro, and H. V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Tertiary Extinction. Science 208: 1095-1108.
Alvarez, W. 1997. T. rex and the Crater of Doom. Princeton: Princeton University Press.
Mitchell, J.S., Roopnarine, P.D., and Angielczyk, K.D. 2012. Late Cretaceous restructuring of terrestrial communities facilitated the end-Cretaceous mass extinction in North America. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1202196109
Roopnarine, P.D. and Angielczyk, K.D. 2012. The evolutionary palaeoecology of species and the tragedy of the commons. Biology Letters 8: 147-150.
Roopnarine, P.D., Angielczyk, K.D., Wang, S.C., and Hertog, R. 2007. Food web models explain instability of Early Triassic terrestrial communities. Proceedings of the Royal Society Series B 274: 2077-2086.
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