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Scuba diving in the Caribbean may not sound like a day at the office to most of us, but for Mairi Best, a researcher in McGill's Department of Earth and Planetary Sciences, it's all in a day's work. She is interested in how organisms become part of the fossil record, a discipline known as taphonomy - literally the study of the grave.
Since researchers rely heavily on the fossil record to reconstruct the past, decipher evolution and understand changing climates, it is imperative that they are aware of how environment affects fossil preservation. Best believes that researchers are not extracting all the information that is available in the fossil record. She hopes that her work will help researchers access the information offered by fossils more wisely and clearly. "When we look at the fossil record, we should realize that we can't read that record straight. We have to filter our reading," says Best. "You may have one environment that preserves 50 percent of the total community, and another environment that only preserves 20 percent."
Her interest in this field was piqued in geology class; she was intrigued with the process involved in transforming colourful living reefs, like those found in the Caribbean, into the 430 million-year-old fossilized reefs on the shores of Lake Huron. Best concentrates on how different marine environments affect the preservation of shells as they fossilize. She has discovered that reef environments are more damaging than any other when it comes to the overall condition of marine fossils.
Best's research focuses on mollusk shells; these shells are abundant and made of the most common material found in watery environments, making them an excellent model to study general fossilization. Fossilization begins as soon as the organism dies, while the shell is in the surface layers of sediment. With time the shell becomes deeply buried and cannot be exhumed by storms or other animals - at this point it can be considered a fossil, Many factors determine whether a shell becomes a fossil including shell material, water quality, sedimentation rate and even how the organism lived. However, the single most important reason that shells deteriorate before fossilization is biological interactions - the actions of other animals. "These can be anything from a crab moving into or cracking open an empty shell, to organisms that bore onto and destroy the shell surface," says Best.
Tropical reefs are the high-rises of the sea, both in terms of biodiversity and sheer numbers, which is why the condition of fossils in reef areas is so poor. There, just like Montreal, good property is valuable. The minute a shell is vacated, it is likely to be snatched up by something else. A shell that manages to survive while in the first few metres of sediment stands a good chance of surviving later events and becoming part of the fossil record. Shells from reef environments, where water is clear and life is abundant, are much less likely to fossilize compared to shells from muddy areas, which are buried quickly, safe from attack by other organisms.
Best's method for examining fossil preservation begins with a detailed description of the marine environment. She then collects existing shells to get an idea of their typical appearance. "We describe the shells in terms of approximately 25 variables, such as whether the shell is broken, dissolved, bored into or encrusted," explains Best. To determine how quickly shells acquire these characteristics, she plants fresh shells into the environment. She returns one, two or three years later to retrieve and characterize these shells. Best is currently running experiments that will continue for up to 20 years, but she has shown that shells deteriorate rapidly in the first few years, making this a critical period in the fossilization process. Even aided by Global Positioning System, elaborate marking systems and detailed mapping of the sites, finding the shells again is not easy. Fishing tackle can tangle up in the arrays and pull them up and the posts she uses to mark her sites have been removed by people surface diving for conch. Still, her recovery rate is about 80 percent. "It can get a bit complicated," laughs Best, "when you are in a coastal village and spot a PVC post with shells displayed on it for decoration and recognize it as part of an experiment that has been removed from your site, but that's part of the game."
Best started out in the tropics around the world and is currently conducting research on both the east and west coasts of Canada. She is now starting to study fossilization in the Arctic coast and Antarctica. Her experience working in such a diverse range of environments makes her uniquely qualified to see patterns and compare conditions of fossils. She is using the same method to systematically study fossils at different latitudes to construct a global latitudinal comparison of how fossils are preserved.
McGill's SPARK program (Students Promoting Awareness of Research Knowledge) is funded by NSERC and run by the Faculty of Education, VP Research Office and the University Relations Office. See www.spark.mcgill.ca for more information and articles.