Stearn, Colin W. Department of Earth and Planetary Sciences, McGill University, Montreal, Ontario, Canada.
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An extinct group of genera of Mesozoic fossils. Informally known as the spongiomorphs, they were established as a family of scleractinian corals by Fritz Frech in 1890 on the basis of specimens from the Zlambach beds of late Triassic age in Austria. He grouped the fossils into four genera (Spongiomorpha, Heptastylopsis, Heptastylis, and Stromatomorpha). The fossils consist of closely spaced longitudinal rods of calcite or aragonite that are perpendicular to the growth surface and that give off lateral expansions which join the rods and in some forms unite to form laminae parallel to the growth surface (see illustration). Frech noted the similarity of his group of fossils to the stromatoporoids, particularly to the genus Actinostroma, and named one of the species Heptastylis stromatoporoides. In tangential section, the rods of the spongiomorphs have a central dark dot, if round, or a line, if oval, and concentric microstructure. Their fibrous microstructure suggests that they were originally aragonite, and their water-jet appearance in longitudinal section is similar to that of the septa of scleractinian corals. Some spongiomorphs from the middle Triassic Cassian beds still preserve the aragonite mineralogy of the living corals.
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