THE FIRST SKELETAL, PLATFORM-MARGIN REEFS OF THE MESOZOIC AND THE APPEARANCE OF SCLERACTINIAN CORALS: ENVIRONMENTAL DISTRIBUTION OF REEF-BUILDERS ACROSS A STEEP, SHALLOW- TO DEEP-WATER SLOPE

Following end-Permian extinction, skeletal, platform-margin reefs were absent from the oceans for several million years. The appearance of scleractinian reefs in the Middle Triassic has been described as sudden, but this notion results from a limited record of earlier Middle Triassic reefs. Consequently, the pattern and timing of reef development in the aftermath of extinction remain poorly constrained.

We combined field studies and petrographic analysis of polished slabs and thin sections to establish the environmental and stratigraphic distribution of reef-building organisms on the Great Bank of Guizhou (GBG), a Late Permian to Late Triassic isolated carbonate platform in the Nanpanjiang Basin of southern China. The GBG reef is the oldest-known outer-platform reef of the Mesozoic Era. More than one square kilometer of reef is exposed in tectonic cross section, beginning near the Early/Middle Triassic boundary. Recent work on platform architecture shows that the reef developed across an environmental gradient from shallow to deep water, likely extending beyond the lower limit of the photic zone.

Our study indicates that (1) Tubiphytes (= Shamovella) was present throughout the reef and was the most important volumetric contributor to the reef structure. (2) Calcareous sponges were the first framework-building metazoans to inhabit the reef. During the early Anisian (earliest Middle Triassic), sponges were relatively common on the deeper slope. (3) Scleractinian corals did not appear until the late Anisian, much later than sponges. Although scleractinians were present across a paleoenvironmental gradient from the back-reef to the upper slope, they did not contribute significantly to the reef framework.

In contrast to the modern-style reef ecosystems of the Late Triassic, early Anisian reefs developed across a wide range of environments on shallow- to deep-water slopes. This pattern likely resulted from the wide environmental tolerance of Tubiphytes. The classification of Tubiphytes has long been debated, but our study provides new constraints on their environmental distribution and biological affinities. Not only were Tubiphytes the dominant reef-builders of the Anisian, but they were also the only organisms capable of sustaining reef growth prior to the evolution of reef-building scleractinians.