| A chunk of colonial coral--a hexacoral--from the Upper Triassic Luning Formation, Coral Reef Canyon, Nevada. Those small roundish compartments on the surface of the rock were constructed by the living coral organism, which secreted a polyp with six-fold symmetry; beginning in the Triassic Period of the Mesozoic Era, such hexacorals became the dominant form of coral on Earth after the rugose and tabulate varieties went extinct at the close of the Paleozoic Era. Whitish diagonal streak in center of specimen is the mineral calcite. |