Morgan Hegewald

"Oligocene Coral Reef Fossils from Puerto Rico."
BSES Senior Research 2005

My research investigates coral reef fossils from the Oligocene time period collected from Puerto Rico by my advisor, Claudia Johnson and a group of students. Equal sample sizes were collected from each stratigraphic unit, oldest to youngest (Figure 1). By studying these coral specimens I hoped to see how the distribution of coral genera differs throughout the layers, and through time. Along with knowing the ecology of the coral genera I may see why such genera are or are not present in certain layers.

I collected coral specimens and was able to divide them into different genera within the stratigraphic layers. I had a set of known coral types to compare and work with to aid in the sorting and identifying. I was able to distinguish these corals based on the size, form and shape of the coral and its corallites (Figure 2). Due to weathering and recrystallization of the specimens identifying them was sometimes very difficult. To aid in the identifying process I made thin sections of some coral specimens. I was also able to use previous research of Professor Ann Budd of the University of Iowa to help in identifying my specimens. I was eventually able to see what genera of corals were present during this Oligocene time period.

Looking at the different stratigraphic units I noticed there were some genera distributed throughout every unit, and others prevalent only at some. Corals such as Porites and Goniopora can be seen in nearly every unit collected (Figure 3). In contrast, corals such as Cladocora are seen predominately in Unit 9A, relatively far away from the full development of the reef itself. There are some corals, such as Antiguastrea that is only found in Units 9 G,H,I and J. These units are higher in the sampling and are closer to the actual reef. By looking at the ecology of the coral genera it becomes clear as to why the corals were found where they are. For example, Porites and Goniopora are found in nearly every layer because they are tolerant to nearly any type of environment. They strive in shallow to deeper water, low to high light conditions, and moderate to warmer waters. On the other hand, Alveopora which was found in the lower units, 9A, B and C prefers the upper reef slopes. These units are the units found before the patch reef itself. Acropora flourishes in the reef itself where it can be protected from strong surges. In my investigation I identified Acropora predominately in unit 9M which is among the patch reef section.