Corals can grow in shapes specific to the environmental conditions of their immediate surroundings. Marine biologists have repeatedly demonstrated this by growing corals on various parts of a reef and noting the differences in growth that result. For example, a coral species growing in deep water on the front edge of a reef can assume an entirely different shape than that same species growing in shallow water on the reef crest. These two locations have different amounts of average sunlight and wave activity, which causes the coral to grow in different shapes to take optimal advantage of those environmental properties. This “plasticity” in corals has always been a challenge for paleontologists because it makes identifying and classifying corals extremely difficult. It also makes determining the evolutionary relationships among closely related corals using morphological characteristics nearly impossible.
One solution to this problem is to focus on very small-scale features, rather than large-scale features like colony shape, to determine how different coral species are related to one another. Small-scale coral skeletal features are usually less than a millimeter in size and are visible only by visualization techniques such as scanning electron microscopy (SEM) or petrographic thin sectioning. Scanning electron microscopes allow the precise observation of surface features, whereas petrographic thin sections can reveal internal growth structures. Sahale Casebolt, currently a PhD student at Virginia Polytechnic Institute and State University, looked at such small-scale features in modern and fossil corals collected at various localities throughout the Caribbean with Dr. Nancy Budd at the University of Iowa. Many of the fossilized corals come from the Dominican Republic, which has beautifully preserved ancient reefs exposed along modern streambeds. These fossil reefs, and others like them throughout the Caribbean, have been fundamental in shaping the way that paleontologists understand past tropical marine ecosystems. Casebolt’s research project involves comparing groups of corals based on micro-features to groups based on DNA sequences, and has confirmed that small-scale features can be useful indicators of evolutionary relationships. Reconstructing evolutionary relationships among fossil corals helps paleontologists understand why some reefs are more resilient than others, and how coral diversity has fluctuated over time. Examining these small-scale skeletal features has the potential to reveal evolutionary patterns that were previously unknown, and has implications for coral evolution, marine ecology, and modern reef conservation.
SEM images show small-scale features like bumps on the surface of the coral skeleton, which are usually simple (A), but can be branched (B) in some species.
Sahale Casebolt was the 2011 recipient of the J. Thomas Dutro Jr. Student Award In Systematic Paleontology, awarded annually by Paleontological Research Institution in Ithaca, New York, in recognition of important basic systematic paleontological research by a graduate student. See PRI’s Museum of the Earth Research website for more information about this annual award.