Article: Original structure and composition of Permian rugose and Triassic scleractinian corals
Rugose corals from the Permian of Timor (Indonesia) and scleractinian corals from the Triassic of northern Italy are both exceptionally well preserved. The scleractinians, as shown previously by Montanaro Gallitelli, are preserved as the original aragonite. They have skeletal structures virtually identical to those in living corals, with some minor diagenetic alteration but without change in mineralogy. Microprobe scan lines show strontium as the common minor element in the Triassic skeletal aragonite. The rugosans studied were selected as the best-preserved available Permian corals, from Basleo, Timor. Small crystal sizes within calcitic skeletal micro-structure and perfect development of trabecular septal structure, as shown by scanning electron microscopy in polished and etched sections and also in broken sections, suggest little diagenetic alteration in some specimens. Where diagenetic structures are recognizable (as in walls of Polycoelia) they are obviously discordant with primary biogenic structures. Skeletal carbonate with little or no observable diagenetic alteration at the electron microscopic level is lacking in both strontium and magnesium as minor elements, suggesting that low magnesian calcite, rather than aragonite was the primary skeletal mineral composition in these Permian corals.