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The Evolutionary History of Reefs
Reefs are the oldest ecosystems on Earth, one that has held a great diversity and complexity with plants and animals throughout EarthÕs history. The plants and animals that have formed these communities for billions of years have left their mark across the surface and subsurface of the planet. They have done so through the limestone remains of the organisms of the reef community along with fossil evidence within these stone structures. Not only does the fossil record within these remains tell us about the environment and organisms present in the local area, but they tell a story of what was happening on our planet at the time of growth. Given that the reefs of yesterday and today are very sensitive to environmental changes, they are great indicators of climate and sea level changes along with plate movement (Newell, 54.) Throughout these changes, the plants and animals that make up these reefs have changed and evolved, leaving the evidence of their ever changing communities behind for us to discover and analyze.
The first reefs made their dˇbut about two billion years ago during the late Precambrian. These reefs were strictly made from the growth of algae, a type of plant, within EarthÕs tropical seas at the time (Newell, 58.) These ancient algae made their reef structures in the form of upward growing trunks known as stromatolites (Melezhink, 311.) They did this by taking in bits of calcium carbonate and creating their own lime to form limestone. These structures grew tens of feet tall, without the aid of any reef-building animals such as sponges or corals. These structures continued to flourish in this manner for millions of years.
During the Cambrian time period the stromatolites saw their first animal counterpart, a type of sponge known as archaeocyathids. This was the first appearance of a trend that would continue to the reefs of present day, reef communities. These sponges rooted themselves to the stromatolites, giving a perfect environment for other sea animals to live in and around. It was at this time, 540 million years ago, that the fossil record indicates the first mass extinction event to effect life on Earth, including the reef organisms. The archaeocyathids died off, leaving only the blue-green algae to again produce reefs for the next 60 million years on their own (Newell, 60.)
It was not until the Mid- Ordovician that animals were reintroduced into the ecosystem. Algae continued to thrive with the addition of new algae, bryozoans, sponges, and most importantly, the first corals (Newell, 61.) The introduction of coral into the ecosystem is one of the most vital pieces in relating ancient reefs to our own. It is one organism of the reef that has continued to be part of the reef even into the present. This addition created a tripartite of algae, coral and sponges that can still be seen today.
It is believed that corals appeared at this time because of plate movement. The areas that the reefs were growing in were becoming warmer because of the continental drift towards the equator. Oceanic circulation, nutrient flow, and availability of habitats also held a part of this time of coral growth. Glaciations, along with sea level fall, made it easier for corals to survive (Kaljo, 233.)
The second mass extinction event occurred roughly 350 million years ago during the Devonian. This caused a rift in the tripartite that had previously appeared in the Ordovician that had remained a constant in reef history for nearly 130 million years. Only some stromatolites remained after the event (Newell, 61.) It is believed that this event was caused by environmental changes that changed the climate of the reef building locations. However, the exact reasoning for the environmental change is not concrete.
Stromatolites continued to thrive through the carboniferous period into the Paleozoic along with stromatolites, bryozoans and brachiopods; however there was a decline in the existence of coral at the time, and no stromatoporoid sponges, sponges that had previously been present within the reef community, were present. New types of green algae along with new sponges and crinoids entered the system during the Paleozoic. These crinoids along with the brachiopods thrived at this time with thousands of species (Newell, 61.)
The rise of reef communities can only exist for so long before they fall. The end of the Paleozoic, 225 million years ago brought a major extinction event, larger than ones previously seen on Earth. This event not only affected marine life, but life on the surface as well, in a major way. It is believed that in the formation of the super continent Pangaea, all shallow marine seas, where the reefs strive, were drained to some extent (Newell, 61.) This in turn killed all reef-building organisms at that time, causing reef systems to stop growing.
The disappearance of reefs lasted only 10 million years before reappearing. At the time of the Triassic, corals began to become more dominant in a world that had previously seen algae as the front runner in reef formation. These corals are ancestors of the coral of today. The Jurassic proved to bring back the stromatoporoid sponges that had fallen out during the Carboniferous. It has been discovered that the Jurassic was a time of global rise in sea level (Benito, 47.) This likely contributed to more favorable environments for reefs to grow in.
During the Cretaceous two thirds of the continental land mass of Earth was under shallow sea. It was at this time that coral families were at their peak. Many new species of coral appeared during this time, including the Blastozopsammia. This type of coral has been found to hold many characteristics of modern coral, but has not been found to be connected to many types of the past. It shows a key point in how far coral evolution had come at that time (Fikorn, 501.) However, the new organisms joining the reef community along with the corals could not survive in the cold water that was slowly accumulating in the Atlantic regions. At this time the Atlantic was growing wider, as well as deeper. As the ocean grew deeper, the water grew colder, cooling down reef environments. This in turn disrupted the growth and survival of many of the organisms. However, some were able to adapt. Limestone deposits in the Florida Keys prove the idea of plate spreading at this time, showing that the area experienced a time of sea level regression (Lidz, 845.)
As the climate became more and more differentiated as the continents drifted apart in the Cenozoic, so did the reef systems. As the continents rode apart climate variations became more distinct and the areas in which the reefs could grow become smaller and more spread out. As reef systems moved apart, differences within the systems became more distinct as well. This can most easily be seen the coral species found in the Caribbean and Europe (Newell, 62.) It is this movement that has set the limits for todayÕs reef structures.
By studying reefs today we can also begin to understand what the reefs of the past are telling us. Reefs today that experience subsidence or migration due to tectonic movement will react in much the same way that reefs of the past did. In this case of plate movement, they will drown or die from other similar processes. When the amount of light or Oxygen that a reef receives decreases, the ability of the reef to live efficiently decreases as well (Webster, 129.)
The fossil record holds evidence for hundreds of mass extinction events across single continents as well as globally. Not only were the plants and animals on land affected by these events, but the ones at sea as well (Newell, 57). The delicate environments that are reefs hold the key to what could have caused these events. Scientists have long been struggling to answer the question of what caused the extinction driven gaps in the fossil record, especially the fossil record found in ancient reefs. Although scientists now have a better understanding what the record shows, there are still unanswered questions (Kunznetsov, 159.) It is now believed that a lot of the gaps within the fossil record of the reefs are due to climate change based on tectonic plate movement. These tectonic changes have affected climate, which in turn alters the chemical composition of the limestone, and have also affected location of reef building organisms throughout time (Motaggioni, 426.) Luckily for us, this activity has been stored in the rock across the world, allowing us to get a better understanding of our EarthÕs history form both an ecological geological standpoint.
Work Cited
Benito, M.I.; Mas, R. "Sedimentary evolution of the Torrecilla Reef Complex in response
to tectonically forced regression (Early Kimmergian, Northern Spain)." 2006 Sedimentary Geology 183: 31-49.
Filkorn, H, Alor, J.P. "A New Early Cretaceous Coral ( Anthozoa;
Scleractinia;Dendrophylliina) and its Evolutionary Significance." 2004. The
Paleontological Society 78(3): 501-512.
Greenstein, B.J.; Pandolfi, J.M.. "Taphonomic Alteration of Reef Corals: Effects of Reef
Environment and Coral Growth Form II: The Florida Keys." 2003 Palaios 18:
495-509.
Kaljo, D. "Diversity of late Ordovician rugose corals in Baltoscandia: role of
environmental changes and comparison with other areas." 2004. Proc. Estonia
Acad. Sci. Geol. 53(4): 233-245.
Kuznetsov, V. "The Evolution of Reef Structures through Time: Importance of Tectonic
and Biological Controls." 1990. Facies 22: 159-168.
Lidz, B. H.; Reich, C. D.; Shinn, E. A. "Regional Quaternary submarine geomorphology
in the Florida Keys." 2003. GSA Bulletin 115(7): 845-866.
Melezhik, V.A.; Fallick, A.E.; Makarikhin, V.V.; Lyubtsov, V.V.. "Links between
Palaeoproterozoic palaeogeography and rise and decline of stromatolites:
Fennoscandian Shield." 1997 Precambrian Research 87: 311-348.
Montaggioni, L.F; Le Cornec, F; Correge, T; Cabioch, G. "Coral barium/calcium record
of mid-Holocene upwelling activity in New Caledonia, South-West Pacific."
2006. Palaeogeography, Paleoclimatology, Palaeoecology 237: 436-455.
Newell, N.D. ŅThe Evolution of Reefs.Ó 1972. Scientific American. 226:54-65
Webster, J.M.; Braga, J.C.; Clague, D.A.; Gallup, C; Hein, J. R.; Potts, D.C.; Renema,
W; Riding, R; Riker Coleman, K; Silver, E; Wallace, L.M.. "Coral reef evolution
on rapidly subsiding margins." 2009. Global and Planetary Change 66(1-2): 129-
148.
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