The Evolution of Scleractinian Corals

Scleractinian Corals’ Fossil Records and Evolution

Scleractinian corals’ fossils record can be inferred from as far as the middle Triassic period. It is however very difficult to understand their evolution from as back as then due to the fact that deep seas’ corals’ representation in the molecular phylogenetic analyses has been poor. In this essay, however, I am going to look at their evolution through the Permian, Triassic and Jurassic periods. Looking through the prevailing environmental conditions in those periods, dominant organisms of those times, and the continental arrangement of the time as well as fossils locations.

Dominant Organisms of the Permian Age

In the late Palaeozoic time (Permian age), the dominant organisms in the reefal setting were a mixed biota of sponge, calcareous algae, bryozoans molluscs, echinoderms, and calcisponges. Then came the middle Triassic time where most of the dominant organisms were still of the Permian age. This age also marked the emergence of scleractinian corals although they were not actively involved in reef-building until the late Triassic period. Hydrozoans also became dominant in the late Triassic period with several of first reefal organisms such as calcareous algae and calcisponges sinking into oblivion.

Evolution through the Triassic Period

The evolution went through a lot of events and processes through the periods. During the early Triassic period (Scythian), there were no reefal environments known. This was straight after the Permian extinction and we have to wait until the middle Triassic for the reestablishment of the Permian-type reef community. This community was reorganized during the late Triassic period. The reorganization was brought about mainly by the change of the heat conditions in the sea. The major coral radiation, experienced from late Triassic to the Jurassic period stimulated scleractinian linages into positions of dominance. During the early Triassic, some reptiles returned to the oceans (of the order ichthyosauri). Their vertebrae suggest that they were likely rolling like eels while moving through the waters. They later adapted better swimming techniques and became dominant giants before the plesiosaurs of the Jurassic period.

Environmental Conditions and Continental Arrangement

The climate during the start of the Triassic was dry with extremely hot summers and cold winters in the interior of the mono-continental world (Pangea). Late in the Triassic, the continental plates in the Tethys Sea started spreading leading to the formation of the northern and southern continents (Laurasia and Gondwana). The oceans were very depopulated by the extinction due to the fact that levels of carbon dioxide were very high.

Survival and Diversification of Marine Life

Looking at how uniform fossil fish from the Triassic period are, one can easily infer how little marine life survived the extinction. During the late Triassic into the Algal symbiotic were acquired in some species of corals in the warm waters of the Tethys seaway, this was as a result of the major coral radiation. In the later stages of the Jurassic period, the split between Laurasia and Gondwana was complete and many plants and animals were recovered from the Permian extinction. On land, the dinosaurs and flying pterosaurs dominated the ecosystems. This was the period that marked the cameo of birds. In the oceans, the plesiosaurs that were smaller and weaker became a predominant and formidable force in the sea.

Fossil Distribution

The fossil distribution of the Triassic scleractinian faunas is divided into two. The first is the eastern front that occupied the Tethys seaway (cutting through modern mid-Asia all the way from the Mediterranean to the Far East). The other front being in the west along the coast of the United States and Canada.

Modern Threats to Scleractinian Corals

Stony corals evolution since the Triassic has been massive. In the modern world’s oceans, most of coral reefs are formed by the scleractinian corals. They are mostly in the shallow waters where sunlight rays penetrate although a little chunk of the species can also be found deep into the oceans. Human activities in modern times have been affecting the aquatic ecosystem in great depths. This together with the effects of global warming poses a great threat to the species and if nothing is done to resolve this, the number of scleractinian corals will decline significantly in the coming years.

References

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