SIMILARITIES AND DIFFERENCES BETWEEN LATE TRIASSIC AND MODERN REEFS

The Late Triassic (235-201.6 mya) is known for prolific carbonate deposition and reef growth, but how similar are these reefs to their modern counterparts? Scleractinian (modern) corals evolved in the Middle Triassic and became major reef builders in the Late Triassic, but the presence of modern corals does not necessarily signify the arrival of modern reefs. Indeed many Triassic carbonate buildups host corals, but are nothing like the shallow-water tropical reefs of today.

One of the most noticeable differences is the distinct paucity of bioerosion in Late Triassic reefs. Unlike modern reefs, which are constantly destroyed by fish, microbes, boring sponges, and other macro-organisms, Late Triassic skeletal framework builders are comparatively pristine with a few small borings at best. Additionally, many key organisms that inhabit modern reefs had not yet evolved in the Late Triassic. One of the major differences is the diversity of modern large benthic foraminifera and planktonic carbonate producers, which were all but absent from Late Triassic oceans.

Nevertheless, there are definite similarities between Late Triassic reefs and modern reefs. There is now considerable evidence that Late Triassic corals had photosymbionts, like modern zooxanthellate corals, which would have helped them achieve a larger size and faster growth rate. This adaptation alone would have been a major advance in reef building and reefs built by photosymbiotic corals would have had very different ecologies to reefs that lacked this innovation. Furthermore, calcareous red algae, like modern coralline algae, are strikingly important in some Late Triassic reefs, as are encrusting and clotting microbialites. These calcareous algae and microbialites seem to play a considerable roll in certain Late Triassic buildups, similar to some modern reefs such as those near Highbourne Cay (Bahamas) and Tahiti.

There is a growing body of evidence to suggest that there were several distinct reef types growing in the Late Triassic; coral-dominated reefs, coralline algal reefs and reefs where microbialite binding was quite important. Despite the subtle differences mentioned above, our evidence suggests that these were actually quite similar to modern shallow-water framework reefs, and that the Late Triassic was the advent of modern reef building.