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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”[1]. 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 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 at Dresden University in Germany [2]. The name "stromatolite" was derived from the Greek words "stroma," meaning layer, and "lithos," meaning stone [2]. These layers of rocky deposits laid down by cyanobacteria first appeared on Earth about 3.4 billion years ago in the Archean Era[3], 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 early life on earth was like [2].


Stromatolites were constructed by the activity of microbial communities that trap and bind sediment as well as precipitate carbonate material[2]. A microbial community, cyanobacteria, would arrange vertically and wrap around grains 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 [2]. Another layer of coccoid cyanobacteria would form, then burrow into the previously formed crystals, which produced tunnels to allow the formation of more crystals[2]. The end results were cement domes or columns that could reach 50 meters in thickness. Most of these formed in shallow water around the borders of continents[2].

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 true reef structure, which appeared only 8 million years prior to the Cambrian Explosion. Cambrian reefs were created by small coral-like animals called cloudina, which secreted calcium carbonate exoskeletons which accreted into massive towering cone shaped structures that provided protection and shelter to the primitive sea life present at the time [4].

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 was 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 [5]. 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[5]


During the Cambrian era, there was also an explosion of many different organisms which were present in many of the reefs. Trilobites were observed as small bioclasts in the matrix structure of Cambrian reefs. These small creatures were thought to eat organic particles on the sediment surface of reefs. However, trilobites were absent in Girvanella reefs. Other organisms found as bioclasts on Cambrian reefs were Bivalved Arthropods, Stenothecoids, and Hyoliths, and Echinoderm's ossicles[5].


During this period of great new expansion of life on earth, a new species emerged around 500 million years ago, coral[6]. This was not at all like the coral we think of today. The first corals were soft corals (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[6].

Ordovician Period

Reef communities underwent many transitions and reorganizations during the Early Ordovician period [5]. The reef builders consisted of much more diverse, higher taxa than the Cambrian era. Additionally, there was no longer single genera dominance in reef roles. The earliest reef-building corals, Lichenaria, first appeared during this period. The reefs created had large, well-calcified skeletons. There was an increased diversification in macroscopic calcareous skeletons, which took place in level bottom and pelagic communities with some corals in reefs [5]. In complex reefs, the organic framework consisted of thrombolites and Renaclis clusters as well as the Lichenaria. They were structured into columns with hemispherical tops and tunnels containing skeletal sediment. The shift from Early Ordovician to Middle Ordovician is considered one of the greatest changes in reef communities, as the rare Bryozoa corals became highly abundant and diverse. In the Late Ordovician period, different corals and stromatoporoids replaced Bryzoa corals as the major reef-builders [5].

Extinction Events

Shifting atmospheric conditions contributed to five major extinction events throughout earth's history. Corals are resilient, and were event able to survive the largest known extinction event, 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. Rather than destroying the coral populations, the event opened up new niches for coral, allowing it to became common and take on the modern, recognizable forms in the aftermath. By this time, stony corals had evolved and they became increasingly prevalent in the ocean environment. The first extinction event was at the end of the Cambrian period, and while it is not considered one of the major five, it marked a turning point in coral development [7].

Cambrian Extinction- SPICE Event

The first reefs were wave-resistant, partially-skeletal structures, usually composed of inorganic matter. They were built by primitive sponges called archeocyathids which first appeared in the Cambrian period. The Cambrian extinction was presumed to have been caused by warm, oxygen-deficient waters that caused most species of trilobites to die out or develop anti-predatory mechanisms through evolutionary processes [7].

However, new evidence in the geological record 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” [7]. The Ordovician-Devonian Expansion of Animals spanning 500-360 million years ago was a massive expansion in biodiversity centralized around these brand new reef ecosystems.

Ordovician-Silurian Event

The next extinction Event was the Ordovician-Silurian extinction some 360 million years ago [7]. 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.

Permian-Triassic Event

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 [7]. 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 [7].

KT Extinction and Meteor Impact

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” [7]. 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[8]. On top of that, ocean temperatures likely dropped by at least 7°C [9]. All of these extinction events do not necessarily result in proportional changes, up or down, in reef diversity [10]. 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)

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.[11].

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 [7].

Foundations and Problems for Modern Reefs

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, there was a significant drop in ocean levels, which helped construction of the Great Barrier Reef [12]. 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 [13].

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 [14]. 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.


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 [15]


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[16]. 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[11]. 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[17].

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[17]. 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[18].


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