'Shingle' microstructure in scleractinian corals: a possible analogue for lamellar and microlamellar microstructure in Palaeozoic tabulate corals

摘要

The origin of lamellar-microlamellar microstructures in Paleozoic corals has been debated widely. Regularity and preservation with apparently well preserved fibrous microstructures suggest that the structures are biogenic, but non-fibrous, scale-like structures cannot be easily accommodated in biomineralization models of Scleractinia, the best modern analogues. However, acroporid scleractinian coral microstructure may provide an analogue for microlamellar-lamellar scales in Palaeozoic corals. We compared the microstructures of extant Acropora and the Mississippian tabulate coral Michelinia meekana. Acropora microstructure consists of aragonite fibers arranged in radiating trabeculae and in oblique bundles that are arranged in a low-relief, overlapping, shingle-like pattern. Individual 'shingles' range from 4-20 pm in width, 2-7 pm in thickness, and 20-150 pm in length. In ultra-thin sections Acropora shingles are similar in size and appearance to lamellae-microlamellae in certain Palaeozoic corals. Lamellar-microlamellar microstructure in Michelinia meekana consists of scales measuring 4-50 pm in diameter (in cross sections encountered in thin section) and 1.5-7 pm in thickness. The scales do not appear to be fibrous, but if the fibers in Acropora shingles were obscured by recrystallization, but shingles survived as recognizable units owing to the surrounding organic matrix, the resulting structures would be similar to lamellae-microlamellae. Hence, lamellar-microlamellar microstructures may have analogues in scleractinian corals, thereby supporting their fundamentally biogenic nature.