Understanding the murky origins of coral diversity in the Coral Triangle

Studies on coral communities living under marginal
conditions such as low light, high turbidity, or high nutrients are important
to understand how coral reefs will respond to current global changes. The Kutai
Basin of East Kalimantan (Indonesia) contains a rich and well-preserved Miocene
fossil record of small patch reefs that developed under the influence of high
siliciclastic inputs from the Mahakam Delta. Conversely, palaeontological and
molecular studies suggest that the Miocene was an important period for
diversification in the Coral Triangle. As part of the Throughflow ITN, the aims
of this study are to determine the diversity corals during this period and to
understand how environmental factors controlled coral diversification on both
temporal and spatial scales. A total of 150 morphospecies from 70 genera have
been identified. Coral morphologies seem to respond to the gradient of
siliciclastic sediments and nutrients input. Platy-coral assemblages were
common in the vicinity of the delta, characterized by a higher turbid-water
regime, and mainly from the Early to Middle Miocene. Interestingly, about 85
percent of the studied coral genera were already present by the Middle Miocene.
In contrast, communities of branching corals mixed with scattered massive
corals became more frequent during the Late Miocene in settings located towards
the north characterized by less deltaic impact. Our findings suggest that
despite the high influence of sediments in this region during the Miocene,
coral assemblages could not only cope, but also build reefs and undergo
speciation under presumably suboptimal environmental conditions.