CORALS EVOLUTIONARY HISTORY

This week I’ve been reading some papers on coral evolutionary history from the first fossil of the Permian through to their current day biodiversity. I’ve looked at the dips and troughs along the way and the environmental causes. This is a brief over through the 500mya or so....

The Permian

Corals first appeared in the Cambrian period (Pratt et al. 2001) some 542 mya. However fossils were rare until the Ordovician some 100 mya later where rugose and tabulate corals began their period of great success.  Like there modern day counter parts, Rugosa and Tabulata formed reefs which are now held in sedimentary rocks.  They are characteristic of the shallow waters of the Silurian and Devonian (figure 1).

http://www.stratigraphy.org/upload/ISChart2009.pdf
(just in case the figure is poor quality here is a link)

Wang & Sugiyama (2000) showed two decreases in coral diversity during the Permian from China. The first shows a loss of 80% of coral species and is associated with the eruption of Omeihan basalt. The second caused the total demise of rugose and tabulate corals along with many marine and terrestrial species. This occurred at the end of the Changhsigian (figure 1) and is related to the Permian global regression (the Permian mass extinction).  

Evolution of modern day corals.

Following this extinction scleractinian corals evolved to fill this niche, with a complete fossil record that dates back to 240my (mid Triassic). These consist of the corals we all know and associate with the modern reefs. They are a group of calcified anthozoan corals that populate shallow water in tropical to sub tropical reefs.  They stand out as being one of the few calcified metazoan to arise in the Triassic as the majority of orders originated in the Palaeozoic.  The origin of this coral group has remained an unsolved problem in palaeontology and has several proposed hypotheses. These include: the idea that Scleractinia evolved from Palaeozoic rugose corals that somehow survived the Permian mass extinction (Stanley 2000), However Scrutton (1997) does not consider Rugosa to be ancestral to Scleractinia. Alternatively the idea that Scleractinia were derived from soft-bodied, ‘‘anemone-like’’ ancestors that survived the Permian mass extinction, has become a widely considered hypothesis (Stanley 2002). Paleozoic scleractiniamorphs also have been presented as possible ancestors (Stanley 2000).

After their resounding successes in the Silurian and Devonian, corals experienced further periods of high diversity in the Triassic, Cretaceous and the Eocene to recent. Each phase of high diversity is separated by a period of lower diversity that followed high extinction rate (Copper 1994). The extinction episodes corresponded with those of other marine shelf communities and were associated with global climatic cooling and oceanic regression.  The recovery period of corals are much longer than other marine species and further emphases there specificity and suggest that reef ecosystems are especially sensitive to large-scale environmental perturbations (Copper 1994).

The Cenozoic Era.

The Cenezoic era has seen three main extinction highs for coral species (Middle to Late Eocene, Late Oligocene to Early Miocene, and Plio-Pleistocene) (figure 2) each coincided with large scale environmental perturbations. Global drops in temperature due to the Oi-1 glaciation (Southern Hemisphere Glaciation) have been widely cited as the cause of Middle to Late Eocene extinctions in both terrestrial and marine biota (Budd 2000). Increased upwelling, associated turbidity and cooling have been proposed causes of the Early Miocene extinction (Budd 2000) and drops in sea surface temperature associated with the onset of northern hemisphere glaciation affected many different marine organisms during the Late Plio-Pleistocene (Budd 2000). The intensity of the extinction was highest during the Plio-Pleistocene where is occurred over a million years or less.

The creation of coral reefs during the Holocene (the last 10ka) is sea level dependent. At the peak of the Wisconsin glaciation (21ka) sea levels had dropped to about 110-120m below their current levels, as sea levels rose during de-glaciation and approached the critical depths of modern reefs (30m) coral reefs took hold and began to grow in suitable habitats on many island shelves.

.... in my next entry I’ll look specifically at the history of the Caribbean coral reefs.

Pratt et al. (2001) ISBN: 0231106130

Budd (2000) DOI: 10.1007/s003380050222

Copper (1994) DOI: 10.1007/BF00426428

Scrutton (1997) DOI:10.1144/pygs.51.3.177

Wang & Sugiyama (2000) DOI: 10.1080/002411600750053853

CORAL REEFS: AN INTRODUCTION.

Everybody knows what a coral reef is, right? Well just in case here is a recap.....

Coral reefs are the rainforests of the ocean. Coral Reefs are biologically and ecologically important ecosystems that have a very high biodiversity. They account for less than 1% of the earth’s surface but host 25% of all marine species (Mulhall 2007). Most coral reefs were formed after the last glacial period  when snow melted, sea levels rose and continental shelves were flooded.

A coral is marine organism from the class Anthozoa. Typically they live in compact colonies of many identical individual ‘polyps’. Each polyp secretes a calcium carbonate shell which acts as protection from the sea and its inhabitants. These shells form a raised mound called a reef. Corals have a mutual relationship with zooxanthellae (single celled algae) which provide energy from photosynthesis in return for nutrients, protection and being held closure to the ocean surface, and therefore the sun.   

Reefs are typically found in latitudes between 30˚N and 30˚S as they are limited in geographical distribution to the clear, warm and sunlit waters of the tropical ocean. Reef forming corals flourish only in water under 30 metres deep (the photic zone, where light penetrates the surface layer of water) and warmer than 22 ˚C. Consequently coral reef hot spots include the Indo-Pacific around Indonesia, the Philippians and Papua New Guinea. The Caribbean (Atlantic Ocean), although less of a hot spot, still contains a number of common species.

This specificity means corals are highly susceptible to current climate change through ocean acidification, rising sea levels and warmer ocean temperatures.

Over the next ten weeks I will explore the scientific literature, the news and follow fellow coral bloggers to try and piece together a comprehensive understanding of how corals have coped in the past, the present and what the future might hold for them.

Join me later in the week for a review of the corals’ evolutionary history.

Mulhall, M. 2007. Saving rainforests of the sea: An analysis of international efforts to conserve coral reefs. Duke Environmental Law and Policy Forum. 19: 321-351.