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.

I. Stromatolites – Stromatolites were amongst the first structures to appear in the oceans bearing any resemblance to what may be called a reefs. 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.

II. Earliest 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.

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.(p.12)
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. (p.1.)

B.Significance

1. Form host rocks for mineralization (p.12)
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(p.10)
4. Proterozic 2,500 to 540 million years ago(p.10)
5. Phanerozoic 540 million years ago to now.(p.10)
6. Living examples are “natural labs that show what life was like early on” (p.11)

C.Formation

1. Constructed by the activity of microbial communities that trap and bind sediment as well as precipitate carbonate material. (p.15)
2. Reliant on balance of sedimentation and colonies of cyanobacteria that live on the surface of stromatolites.(p.15)
3. Steps of formation (p15):
   1. Establishment of pioneer microbial community.
4. Composed of filamentous cyanobacteria arranged vertically, then wrapped around grains of sand. (p.15)
5. Results in a film around sand.
   1. Heterotrophic bacteria colonize over previous layer.
6. These are “sludge-degraders”(p.15)
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.

Sources:

1. Riding, Robert. The Ecology of the Cambrian Radiation. New York: Columbia University Press, 2001. Print.
2. McNamara, Kenneth. Stromatolites. Artarmon, NSW, AUS: Western Australian Museum, 2013. ProQuest ebrary. Web. 1 March 2016.