THE MIDDLE TRIASSIC RECOVERY, REEFS, CORALS AND “NAKED” LAZARUS TAXA

The delayed Middle Triassic biotic recovery of the marine environment was an important bioevent, long after the end-Permian mass extinction. It seems clear that the recovery was a response to shifts in the physical and chemical makeup of the oceans of the early Mesozoic. This followed a protracted Early Triassic time of marine perturbations reflected by the metazoan reef gap. The reappearance of high diversity among hard-bottom, benthic communities was followed by extensive carbonate sedimentation and later by the first metazoan reefs of the Mesozoic. The late Anisian (Pelsonian) records the radiation of bivalves and other shelly biotas as well as the appearance of scleractinian corals, a group unrelated to extinct Permian corals. Middle Triassic corals were taxonomically diverse, paleoecologically complex and might have been photosymbiotic but they do not seem to have been reefbuilders. Their unusual pattern of appearance and diversity, so different from that of other calcified invertebrates in the recovery interval, may be explained under the concept of “naked corals”. This posits the existence of soft-bodied anemone-like forms which survived the end-Permian mass extinction in isolated refugia during the Early Triassic and early part of the Middle Triassic (a total of 6-8 million years), without leaving a fossil skeletal record. This most likely occurred under conditions of extensive marine eutrophication and adverse pCO₂ and pH. The geologically lengthy absence and then sudden appearance of such higher-level taxa as scleractinians (here termed the “Naked Lazarus Effect”), is unusual but is explained by physio-biochemical responses to ameliorating conditions of global oceans during the recovery phase.