Cretaceous

Summary

The Cretaceous (krɪˈteɪʃəs ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, "chalk", which is abundant in the latter half of the period. It is usually abbreviated K, for its German translation Kreide. The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct marine reptiles, ammonites, and rudists, while dinosaurs continued to dominate on land. The world was ice- free, and forests extended to the poles. During this time, new groups of mammals and birds appeared. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the

Official source

https://en.wikipedia.org/wiki/Cretaceous

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The modern analogue of a Cretaceous coral with a calcitic skeleton

Anders Meibom, Jaroslaw Hubert Stolarski

It was argued that, in contrast to all known modern scleractinian corals that form aragonite skeletons, the original mineralogy of the Cretaceous "Coelosmilia" (ca. 70-65 Ma) was calcite during a period when the Mg2+/Ca2+ ratio of the seawater was presumably the lowest in Phanerozoic (Science 2007 318: 92-94). This is consistent with the idea that scleractinians extend only partial control over their skeletal mineralogy, because calcite is the preferred calcium carbonate polymorph (over aragonite) during abiotic CaCO3 precipitation in low Mg2+/Ca2+ seawater conditions. However, subsequently it was found that a number of different Cretaceous corals formed purely aragonite coralla in the same low Mg2+/Ca2+ conditions, suggesting instead that biological control over the skeletal mineralogy differs between taxonomically different corals (Geology 2017 45:319-322) and posing the question why was "Coelosmilia’s" skeletal mineralogy affected by the seawater chemistry changes and not that of the other corals. An unusual feature of "Coelosmilia" is the infilling of a significant portion of the calice with dense, banded skeletal deposits. They are crystallographically continuous with earlier formed layers of septa, wall and axial structures, but show distinct biogeochemical signatures. To date, no modern analogs of such corals were known, and it was tempting to argue that this skeletal infilling represents secondary, purely abiotic deposits (similar to marine cements, which would be calcitic in Cretaceous seas). Here we document a living scleractinian coral that exhibits a structural organization identical to Cretaceous "Coelosmilia", including dense calicular infilling that is a species-specific feature. Surprisingly, the inner parts of these skeletal elements are composed of fibrous calcite, whereas the rest of the skeleton is aragonitic. We suggest that this modern coral is phylogenetically related to "Coelosmilia" and capable of formation of strictly biologically controlled skeletal mineralogy, which can include both aragonitic and calcitic structures. This original bipartite skeletal organization may explain several diagenetic phenomena that we observe in "Coelosmilia", and is highly relevant for the proper geochemical sampling of corals with such skeletal organization.

2019

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Changes in seawater chemistry have affected the evolution of calcifying marine organisms, including their skeletal polymorph (calcite versus aragonite), which is believed to have been strongly influenced by the Mg/Ca ratio at the time these animals first emerged. However, we show that micrabaciids, a scleractinian coral clade that first appeared in the fossil record of the Cretaceous, when the ocean Mg/Ca ratio was near the lowest in the Phanerozoic (thus a priori favoring calcitic mineralogy), formed skeletons composed exclusively of aragonite. Exceptionally preserved aragonitic coralla of Micrabacia from the Late Cretaceous Ripley Formation (southeastern USA) have skeletal microstructures identical to their modern representatives. In addition, skeletons of Micrabacia from Cretaceous chalk deposits of eastern Poland are clearly diagenetically altered in a manner consistent with originally aragonitic mineralogy. These deposits have also preserved fossils of the scleractinian Coelosmilia, the skeleton of which is interpreted as originally calcitic. These findings show that if changes in seawater Mg/Ca ratio influenced the mineralogy of scleractinian corals, the outcome was taxon specific. The aragonitic mineralogy, unique skeletal microstructures and ultrastructures, and low Mg/Ca ratios in both fossil and living micrabaciids indicate that their biomineralization process is strongly controlled and has withstood major fluctuations in seawater chemistry during the past 70 m.y.

Geological Soc Amer, Inc2016

A Cretaceous scleractinian coral with a calcitic skeleton

Anders Meibom, Jaroslaw Hubert Stolarski

It has been generally thought that scleractinian corals form purely aragonitic skeletons. We show that a well-preserved fossil coral, Coelosmilia sp. from the Upper Cretaceous (about 70 million years ago), has preserved skeletal structural features identical to those observed in present-day scleractinians. However, the skeleton of Coelosmilia sp. is entirely calcitic. Its fine-scale structure and chemistry indicate that the calcite is primary and did not form from the diagenetic alteration of aragonite. This result implies that corals, like other groups of marine, calcium carbonate-producing organisms, can form skeletons of different carbonate polymorphs.

2007

The modern analogue of a Cretaceous coral with a calcitic skeleton

Anders Meibom, Jaroslaw Hubert Stolarski

2019