''Coelosmilia'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

scleractinia Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mo ...

n coral from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

period. The specimens were found in rocks around 70 million years old dating from the Late Cretaceous of the

Mesozoic Era The Mesozoic Era is the era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Periods. It is characterized by the dominance of archosaurian reptiles such as the dinosaurs, and of gymnosperms s ...

. ''Coelosmilia'' is similar to modern-day scleractinians, except for the composition of its

calcitic Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

, non-

aragonitic Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from ...

skeleton. It is the only known scleractinian so far to have an entirely calcitic skeleton. ''Coelosmilia'' is known from several specimens collected from

carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...

deposits located in the modern-day country of

Poland Poland, officially the Republic of Poland, is a country in Central Europe. It extends from the Baltic Sea in the north to the Sudetes and Carpathian Mountains in the south, bordered by Lithuania and Russia to the northeast, Belarus and Ukrai ...

. Together, these areas would have been the continental shelves of the Late Cretaceous European continent. Polish locations known to have produced specimens of the taxon include chalk-pits and quarries in

Nasiłów Nasiłów is a village in the administrative district of Gmina Janowiec, within Puławy County, Lublin Voivodeship, in eastern Poland. It lies approximately east of Janowiec, south of Puławy, and west of the regional capital Lublin Lublin ...

Lubycza Królewska Lubycza Królewska (, ''Liubycha Korolivs’ka'') is a town in Tomaszów Lubelski County, Lublin Voivodeship, in eastern Poland, close to the border with Ukraine. It is the seat of the gmina (administrative district) called Gmina Lubycza Królewsk ...

and

Mielnik Mielnik is a village in Siemiatycze County, Podlaskie Voivodeship, in eastern Poland, close to the border with Belarus. It is the seat of the gmina (administrative district) called Gmina Mielnik. It lies approximately south-east of Siemiatycze ...

. The specimens collected from these locations, identified as the ZPAL H.II series (6,7,8) are in the collections of the Paleobiological Institute of the

Polish Academy of Sciences The Polish Academy of Sciences (, PAN) is a Polish state-sponsored institution of higher learning. Headquartered in Warsaw, it is responsible for spearheading the development of science across the country by a society of distinguished scholars a ...

. The discovery and analysis of ''Coelosmilia'' has changed the previous understanding of the evolution of the

Scleractinia Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mo ...

. Modern-day scleractinian corals have skeletons composed of

aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...

, a calcitic mineral. Analysis of the skeleton of ''Coelosmilia'' does not seem to support development from aragonitic compounds. Instead, the skeleton is primarily calcitic — a type not ever seen in

extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...

scleractinia. It is the first recorded instance of a scleractinian coral producing a non-aragonitic skeleton. Production of non-aragonitic skeletons by marine organisms, or the evolution of organisms capable thereof has been explained as a response to the ratio of the elements

Magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

(Mg) and

Calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

(Ca) dissolved in seawater. Magnesium is a component of aragonite, and an abundance of the element is necessary for the development of aragonitic structure-building organisms. The presence of ''Coelosmilia'' during the Late Cretaceous has been suggested as an indicator of a low ratio (less than 2) of Mg to Ca in Cretaceous seas. In contrast, modern-day ratios of the two elements would be more than twice the suggested Cretaceous value, at a ratio of around 5.2. This has been used as an indicator of the shifts in the ratios of the two minerals in the oceans over the geologic time scale.

References

Caryophylliidae Prehistoric Hexacorallia genera {{paleo-hexacorallia-stub