ReefHistory

Reefs in the Fossil Record

While most people think of all reefs as being composed of coral, it is important to recognize that not all reefs are coral reefs. What exactly constitutes a reef has been the subject of much debate, but the most accepted scientific definition is ”a marine limestone structure built by calcium-carbonate secreting organisms, which, with its associated water volumes supports a diverse community of predominantly tropical affinities, at a higher density of biomass than the surrounding ocean”. This definition does not exclude structures or organisms other than coral from being considered reefs. Therefore, other types of structures will be considered in the history of reefs on earth.

Stromatolites

Stromatolites were amongst the first structures to appear in the oceans bearing any resemblance to what may be called a reefs. They were first identified in 1908 by Ernest Kalkowsky, a professor of mineralogy of Dresden University in Germany CITATION. in 1908 by Ernest Kalkowsky, a professor of mineralogy of Dresden University in Germany CITATION. The name was derived from the Greek words stroma meaning layer, and lithos meaning stone CITATION. These were layers of rocky deposits laid down by cyanobacteria, first appearing on earth about 3.4 billion years ago in the Archean era, when all sea life was incredibly primitive. However, it is important to note that these structures were not actually living, but simply inert rock laid down by biological processes, distinguishing them from coral, and not made of calcium carbonate, excluding them from truly being considered reefs. Currently, samples of stromatolites can be found in natural labs, which are made to demonstrate what life early on CITATION.

Formation

Stromatolites were constructed by the activity of microbial communities that trap and bind sediment as well as precipitate carbonate material. A microbial community, cyanobacteria, would arrange vertically and wrap around grand of sand, which resulted in a film around the sand. Heterotrophic bacteria would colonize over the previous layer, which resulted in a mucilaginous sheet over the first layer. A sulfate reducing bacteria would colonize on the formed structure and feed on the film produced, which promoted the formation of aragonite crystals. Another layer of coccoid cyanobacteria would form, then burrowed into the previously formed crystals, which produced tunnels to allow the formation of more crystals. The end result were cement domes or columns that could reach 50 meters in thickness. Most of these formed in shallow water around the borders of continents.

Cambrian Reefs

The progress of life on earth progressed quite slowly for the next 2 billion or so years, until a period beginning about 540 million years ago called the Cambrian Explosion. In 2014, scientists discovered the earliest yet structure that fits the definition of a true reef, which appeared slightly before the Cambrian Explosion at 548 million years ago. These were created by small coral-like animals called cloudina, which secreted calcium carbonate exoskeletons which accreted into massive towering cone shaped structures. These provided protection and shelter to the primitive sea life present at the time.

During the Early-Cambrian, the first metazoan reefs of the Paleozoic began to appear. This metazoan radiation of reefs did not end until the Ordovician period. One of the very first reef ecosystems were created by archeocyathids (calcified sponges), which were the most abundant metazoan reef contributor in the Early-Cambrian. However, before this there was already an abundant number of calcifying microbes such as cyanobacteria, trilobites, and coralmorphs. Additionally, stromatolites and thrombolites were common structures found throughout this era. The framework of reefs during the Cambrian era were made from the accumulation of calcareous sediments, which were generated by microbial elements encrusted by animal skeletons CITATION. In the Middle Cambrian, the archeocyathids, the most abundant until now, became extinct. Additionally, during this time a lack of overall metazoan skeletal reef framework began, and only a few small localized reef systems with poorly constructed skeletal frameworks formed. The main reef builder left during the middle-late Cambrian were Girvanella, which were tubules arranged in fingerlike strands with other strands stacked vertically

III. Coral – During this period of great new expansion of life on earth, a new species emerged around 500 million years ago, coral. These were not at all like the coral we think of today. They were all soft coral (the development of stony corals would take tens or hundreds of millions of years), and were solitary creatures which did not originally form reefs. However, over time selective pressures caused them to aggregate and form the first true coral reefs on the planet, providing homes and shelters for the now much more complex sea life present in the oceans.

IV. Extinction events and coral evolution– There have been several extinction events throughout geological history in which coral appears to disappear completely from the fossil record, only to reappear soon after. Coral was even able to survive the largest extinction event known in history, the Permian-Triassic extinction event, in which volcanic activity caused the extinction of up to 90% of the earth’s species about 250 million years ago. Coral reefs not only survived this event, but it actually opened up new niches for coral, which became much more common and took on more modern recognizable forms in the aftermath. Stony corals had evolved by this time and began to become increasingly prevalent in the ocean environment.

V. Coral Reefs in the Cenozoic Era and the Future – Within the last few tens of millions of years, some of today’s modern extant reefs began their formation. For example, the largest reef in the world, the Great Barrier Reef, began its formation around 18 million years ago, along with many other notable coral reefs beginning around this same time. Within the last several hundred thousand years, an event occurred which corals have never dealt with before, and may be the biggest threat to them, that being the emergence of humans onto the planet. Warming oceans and pollution have caused the death of nearly 27 % of the world’s coral, with another 32 % being at risk of annihilation in the near future.

Location

Map from Hopping Hotspots: Global Shifts in Marine Biodiversity by Renema et al.

The map at right shows the appearance of fossil reefs over time. Map A shows the late middle Eocene (42-49MYA), map B shows the Early Miocene (23-16MYA), and map C shows geologically recent fossil reef formations. The colors of the dots on the map indicate the alpha-diversity of the reef studied

Interpretation

The fossil record has been used to gather data about the climate of prehistoric earth. Changes in the density, structure, and species composition of ancient reefs indicate shifts in global climate. Data gathered from fossil reefs have been used to model effects of various climate factors on modern coral reefs. Some scientists have used the fossil record to make predictions concerning the effects of climate change on modern reefs by examining analogous changes in the ancient climate. A link between changes in ancient climates causing great reductions in coral health and reef coverage has been suggested. Changes in the pH of the oceans and global temperature seen in the fossil record of reefs have been paralleled to changes in the modern climate.

Evolutionary changes over time can be tracked using fossil reefs. Ancient reef biomes have been examined by using the changes in species composition of the reefs over time. Changes in not only the primary reef building species and the species that live on the reef have demonstrated a great deal of convergent evolution. Niches such as the grazing species have remained constant in the fossil record though they have shown to have been filled by different species over time.

Outline

Stromatolites

A.What are they?

1. Identified first in 1908 by Ernest Kalkowsky, professor of mineralogy of Dresden university, Germany.
2. (p.10) Name derived from the greek words stroma (layer) and lithos (stone.)
3. They are domes or columns composed of photosynthetic prokaryotes (cyanobacteria) which are also referred to as microbialites.

B.Significance

1. Form host rocks for mineralization
2. Time period in which stromatolite formations have occurred is broad.
   1. Samples are representative of organisms ranging from the earliest life forms to modern life forms. Includes samples from time periods:
3. Achaean 4,000 to 2,500 million years ago
4. Proterozic 2,500 to 540 million years ago
5. Phanerozoic 540 million years ago to now.
6. Living examples are “natural labs that show what life was like early on”

C.Formation

1. Constructed by the activity of microbial communities that trap and bind sediment as well as precipitate carbonate material.
2. Reliant on balance of sedimentation and colonies of cyanobacteria that live on the surface of stromatolites.
3. Steps of formation:
   1. Establishment of pioneer microbial community.
4. Composed of filamentous cyanobacteria arranged vertically, then wrapped around grains of sand.
5. Results in a film around sand.
   1. Heterotrophic bacteria colonize over previous layer.
6. These are “sludge-degraders”
7. Results in a mucilaginous sheet on first layer.
   1. Sulfate reducing bacteria then colonize on and feed on film produced by bacteria in the previous layer.
8. This promotes the formation of calcium carbonate crystals, specifically aragonite crystals.
   1. 4th layer of bacteria colonize.
9. Consists of spherical coccinoid cyanobacteria.
10. These bacteria burrow into the previously formed crystals, producing tunnels that allow the formation of more crystals, resulting in a cement like structure.

Cambrian Reefs

1. Building Reefs
   1. Framework made with collected calcareous sediments generated by the reef itself.
   2. Structure not restricted to structures with metazoan frame builders
   3. Cambrian reef framework was mostly the product of the accumulation of microbial elements encrusted by animal skeletons
   4. Both stromatolites and thrombolites are common microbial structures found in Cambrian reefs
   5. Made of fenestrate micrite and clotted micrite
   6. Thromboids constituted less of the structure in early Cambrian reefs, however the middle to late Cambrian age it usually more abundant
   7. Calcified Microbes
      1. Microfossils of microbial origin are an important component Cambrian reef frameworks.
      2. Calcification of filamentous cyanobacteria formed tubules/threads referred to as Girvanella.
      3. Girvanella made tangles and multifilament sheets composing laminar zones up to a meter thick in some reefs.
   8. Archaeocyaths (sponge class) are the most seen and abundant metazoan contributor in Early Cambrian reefs.
   9. Anthaspidellids with local encrustations o microbial filaments form the framework of late Middle Cambrian reefs, specifically Iran.
2. Organism living in Cambrian reefs
   1. Trilobites
      1. Observed in matrix of Cambrian reefs
      2. Absent in Girvanella reefs
      3. In early Cambrian reefs, trilobites are scattered as small bioclasts less than a few millimeters in size
      4. Thought to have grubbed on sediment surface in search of organic particles or meiofauna.
   2. Bivalved Arthropods
      1. Early Cambrian reefs contain bioclasts in reefs
   3. Brachipods
      1. Lingulate brachiopods Are rare in Cambrian reefs, probably represent stray individuals
      2. Middle Cambrian
   4. Stenothecoids
      1. Occurred mostly in reefs of Atdabanian to Amgan age. They were probably immobile, epifaunal suspension feeders
   5. Hyoliths
      1. Not thought to be associated with younger reefs,
      2. Thought to be semisessile suspension feeders.
      3. Common in Tommotian peri-reefal grainstones.
   6. Salterellids
      1. Composed largely of lamellar calcite, and were restricted to Laurentian Botoman
      2. Largely peri-reefal fossils
   7. Mollusks
      1. Millimeter-sized helcionelloids
      2. Occurred in Tommotian to Atdabanian peri-reefal gainstones
      3. Rare in reefs after Sinks Event
      4. Absent in reefs of the younger Cambrian as well
   8. Echinoderms
      1. Echinoderm ossicles present in reef from latest Atdabanian
3. Environmental settings
   1. Cambrian reefs were allocated along depositional settings in tropical and subtropical, normal marine waters at shelf-slope break along the margins of carbonate platform or shelves.
   2. They were also in the middle of platforms near shore or shallow water
   3. Also located in deeper water downslope or in intrashelf basins
   4. The dominance of microbial structures in all Cambrian reefs suggests that the water these reefs were situated at were relatively clear.

Cambrian Divisions of Corals

1. Pre-Cambrian
   1. Stromatolites are considered by some as the earliest reef ecosystem.
      1. Up-ward growing calcified masses, and could reach 50 m in thickness.
   2. Occurred in Shallow water on the borders of the continents during the late Archean and Proterozoic time.
   3. They were the earliest calcifying organisms.
2. Cambrian period start
   1. Early-Cambrian
      1. First metazoan reefs of the Paleozoic began to appear.
         1. Metazoan radiation did not end until the Ordovician time.
      2. The first reef ecosystems were made of archeocyathids (calcified sponges). However, there was already an abundant number of calcifying microbes (cyanobacteria).
         1. Other calcifiers included trilobites and coralomorphs (first coral).
      3. Archeocyathid-microbial systems went extinct during this time.
         1. Possibly due to Anoxia.
   2. Middle-Cambrian
      1. Lithsid sponges
         1. Appeared in deeper water settings during this period.
   3. Remaining Cambrian-Early Ordovician marked the longest-lasting eclipse in the history of reef ecosystems (9)

Two different types of coral a) tablulate corals, which filled same ecological niche 400 million years ago as modern scleractinian corals, and b) rugose corals, which looked like a set of horns and created its calcium carbonate skeleton at the tips of the horns, with a wide end facing up for a sea-anemone-looking animal. Scleractinians emerged in Triassic period (240 million years ago) (1)

Extinction Events

These were several major extinction events throughout the history of life on earth, and most were caused by atmospheric conditions changing more rapidly than organisms could adapt and evolve to the new conditions. The extinction event at end of Cambrian was not part of Big Five mass extinctions (1). The first reefs were wave resistant, somewhat skeletal structures, usually composed of inorganic matter, and built by organisms. These were constructed not by reef organisms of today, but by primitive sponges called archeocyathids, which appeared in the Cambrian, but since they are sponges, don’t truly count as corals. The Cambrian extinction event was presumed to be caused by warm, low oxygen waters. Most species of trilobites died out or evolved anti-predatory mechanisms as a result of this event. However, new evidence corroborated in the geological record called the SPICE event- Steptoean positive carbon isotope excursion suggests that there was cold water and high burial of organic matter in oceans, leading to much higher oxygen levels. This disturbed the carbon isotope record, which indicates carbon nutrient recycling. If rise in oxygen levels is true, coral reef ecosystems may have evolved in its wake, initiating the Ordovician period. While the Cambrian extinction event is not necessarily an extinction event for coral, the extinctions at the end of the Cambrian period paved the way for the formation of the first recognizable coral reefs. Due to its inception here, coral reef diversity exploded, being described as “long-lived superorganisms that pop up again after every mass extinction of last 540 million years” (1). The Ordovician-Devonian Expansion of Animals of 500-360 million years ago was a massive expansion in diversity centralized around these brand new reef ecosystems. The next extinction Event was the Ordovician-Silurian extinction some 360 million years ago (1). This event was presumed to have been caused by some sort of rapid cooling event likely predicated by increased volcanic activity. Tropical temperatures likely fell by at least 5-10 °C and sea levels dropped quickly, which would have adversely impacted coral development and diversity. The Permian-Triassic extinction event of 250 million years ago could have been caused by an overwhelming of the atmosphere with cyanobacteria waste, such as hydrogen sulfide (H2S), accompanied by pronounced spike in CO2 (1). There would have been enough hydrogen sulfide in the air to kill off most land animals with primitive respiratory systems because lungs are even less developed than gills and would not have been able to cope. And, since hydrogen sulfide dissolves in water, it would have killed off many animals in shallow oceanic environments, most prominently, the coral ecosystems. After this extinction, a new kind of coral, scleractinian corals, emerged and began building its reefs. This order of coral forms the basis for the evolution of modern corals, as most modern corals are scleractinian. Mesozoic Coral in the KT extinction Most oceans in Cretaceous period would have had lagoons formed by enormous coral structures surrounding most land forms, which were the first “barrier reefs” (1). Lagoons are warmer, lower in oxygen, and generally exhibit less turbidity than the open ocean. These would have grown right up to surface of ocean, and smaller corals formed as horseshoe shapes inside these lagoons. Most of these reefs in Cretaceous period were clam reefs, composed of bivalves called rudists, which grew very quickly and stacked their inorganic matter on top of each other in a race to the surface. Each clam would reach maturity in 1-2 years and were somewhat self-destructive in their competitive growth habits. A colony a few feet high and wide would take the rudist clams five years to accumulate, while true corals took over a century to reach similar sizes. When the KT meteor struck what is now Mexico, its impact force was so immense that it likely blocked out sun, and so was deleterious to phytoplankton and other plants. This would have decreased global oxygen, increased greenhouse gas partial pressures, and decreased atmospheric temperature. On top of that, ocean temperatures likely dropped by at least 7°C (3). All of these extinction events do not necessarily result in proportional changes, up or down, in reef diversity (2). Geologic events such as rapid plate movements and extraordinary volcanic activity tend to be more telling agents of change for reef attributes. Short term changes, geologically speaking, in the environment affect nearly all reef attributes (diversity, skeletal structure and composition, oxygen levels of surrounding environment) but long term trends in reef evolution

Coral Reefs in Cenozoic Era and the Future (50 million years ago-present)

Scleractinian corals were and are the most prevalent type of coral through this geologic time period, but other orders begin to emerge. The pre-Cenozoic reefs described in above sections were generally less diverse than Cenozoic reefs, and there was a diversity explosion following the KT extinction event (1).

The Great Barrier Reef began its formation around 25 million years ago as the tectonic plates of Australia began to move into tropics. 10 million years ago, significant drop in ocean levels, which helped construction of GBR (5). Earliest evidence of completed coral structures in the GBR is 600,000 years ago (6). Living coral structure likely has been growing on older coral structures since about 20,000 years ago (6).

Evidence exists for increasing diversity in the Caribbean coral ecosystems, with a maximum around 36 million years ago and has remained fairly constant ever since (7). Caribbean reefs likely started constructing the current reefs 22 million years ago, and any currently living (upright) coral structures were likely constructed in the last 20,000 years. However, carbon dioxide levels and increasing ocean temperatures in recent years could be causing another mass extinction event in the manner of previous extinction events. Caribbean reefs have experienced 50% reduction in last few decades.

Sources:

1. Ward, Peter D., and Joseph L. Kirschvink. A New History of Life: The Radical New Discoveries about the Origins and Evolution of Life on Earth. New York, NY: Bloomsbury, 2015. Print.

2. Stanley, George D. The History and Sedimentology of Ancient Reef Systems. New York: Kluwer Academic/Plenum, 2001. Print.

3. Vellekoop, J.; et al. (2013). "Rapid short-term cooling following the Chicxulub impact at the Cretaceous–Paleogene boundary". Proceedings of the National Academy of Sciences 111: 7537–41. doi:10.1073/pnas.1319253111. PMID 24821785.

4. Pope KO, Baines KH, Ocampo AC, Ivanov BA (1997). "Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact". Journal of Geophysical Research 102 (E9): 21645–21664. Bibcode:1997JGR...10221645P. doi:10.1029/97JE01743. PMID 11541145.

5. Hopley, David. Encyclopedia of Modern Coral Reefs Structure, Form and Process. Dordrecht: Springer Netherlands, 2011. Print.

6. http://www.gbrmpa.gov.au/about-the-reef

7. Budd, A.F. Diversity and Extinction in the Cenozoic History of Caribbean Reefs. Coral Reefs (2000) 19:25-35.

8. Riding, Robert. The Ecology of the Cambrian Radiation. New York: Columbia University Press, 2001. Print.

9. McNamara, Kenneth. Stromatolites. Artarmon, NSW, AUS: Western Australian Museum, 2013. ProQuest ebrary. Web. 1 March 2016.

10. Stanley, George D. The History and Sedimentology of Ancient Reef Systems. New York: Kluwer Academic/Plenum, 2001. Print.