Understanding the murky origins of coral diversity in the Coral Triangle

Studies on coral communities living in marginal
conditions (i.e. low light, high turbidity, extreme temperatures, high
nutrients) are important to understand how coral reefs will respond to current
global changes. The Kutai Basin (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, this research aims to know how diverse corals were during this
period and which environmental factors controlled their diversification on
both, temporal and spatial scales. Our new collections include tens of
thousands of specimens, and preliminary results suggest that species diversity is
comparable to modern coral settings living under similar environmental
conditions for distinct platy and branching coral assemblages. About 150
morphospecies (70 genera) have been identified, including nine genera known as
extinct. Coral morphologies seem to respond to the gradient of siliciclastic
sediments and nutrients input created by the progradation of the Mahakam Delta
since the Early Miocene. 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. On the other hand, communities of branching corals
mixed with scattered massive corals were more frequent during the Late Miocene
in settings located towards the north characterized by less deltaic influence. Future
research, including sedimentological and geochemical analysis, will focus on
disentangling the influence of major global environmental changes and the
regional progradation of the Mahakam Delta on species turnover.