Bryozoan Diagenesis: Bearing on the Nature of the Original Skeleton of Rugose Corals

Diagenetic alteration of Pleistocene specimens of the bimineralic cheilostome bryozoan Schizoporella floridana from the Miami Limestone of southern Florida has resulted in replacement of the aragonite layers with coarse neomorphic calcite containing numerous relic skeletal aragonite crystals as soli...

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Bibliographic Details
Published in:Journal of paleontology 1975-07, Vol.49 (4), p.587-606
Main Author: Sandberg, Philip A.
Format: Article
Language:English
Subjects:
Aragonite
Calcite
Carbonates
Cements
Crystallites
Crystals
Diagenetic processes
Limestones
Mineralogy
Skeleton
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Summary:Diagenetic alteration of Pleistocene specimens of the bimineralic cheilostome bryozoan Schizoporella floridana from the Miami Limestone of southern Florida has resulted in replacement of the aragonite layers with coarse neomorphic calcite containing numerous relic skeletal aragonite crystals as solid inclusions. In addition, the calcite primary skeleton has been recrystallized from a lamellar to a transverse bladed texture in conjunction with a change from 4-5 mole percent MgCO3 (in modern specimens) to about 0.5 mole percent. The latter textural change is not clearly discernible by means of the light microscope. This further supports earlier contentions that the supposed preservation of fine texture during Mg loss from calcites is largely a function of level of observation. The nature of the textural alterations of these bimineralic bryozoan skeletons and the characteristic mode of alteration of known aragonites throughout the geologic record argue most strongly that rugose corals must have been originally calcite, and that lamellar calcite is one of several original ultrastructures of rugosan skeletons. The biocrystallization model based on scleractinian corals, if restricted to trabecular aragonite, must be rejected for rugose and tabulate corals. Expansion of the model to include the diversity of calcite ultrastructures of rugose and tabulate corals is not unreasonable in view of the diversity of ultrastructures and mineralogy which exists in molluscs and bryozoans, even within single skeletons.
ISSN:0022-3360
1937-2337