The Evolution of Indo-West Pacific Reefs through Space and Time

The Coral Triangle is a global marine biodiversity hotspot located in the Indo-West Pacific (IWP) (Fig. 1). Over 75% of the world's total coral species live in this region, making it a top conservation priority (1). Most of these corals are scleractinian (stony corals), which provide the structural framework for other reef organisms (2). Therefore, scleractinian corals are vital for the structural integrity of the Coral Triangle.

A global decline in coral cover has occurred over the last 30 years due to anthropogenic factors (3, 4). This is related to global carbon dioxide emissions (5) and local development that increases ocean turbidity (a measurement of
sediment load in water) (6). The combination of these threats and other stressors make corals less resilient to coral bleaching events (7). Despite relatively high levels of turbidity in the IWP, coral cover is declining at a slower rate compared to other regions (3). Moreover, most of the reefs in the IWP reside in relatively high levels of turbidity (8). The tolerance of reef corals to turbidity will likely play a significant role in the future of reef ecosystems. Although an increase in sediment load coincides with slower coral recovery after bleaching events (9), there might be a turbidity-tolerance threshold that allows corals to reduce the impact of bleaching stress (8). Previous studies suggest turbid reef environments may be more resilient to coral bleaching during thermal stress (8,10), including in the IWP (11). The fossil record indicates the first reefs in the Coral Triangle were subject to high siliciclastic inputs, which suggests these corals developed under high turbidity levels (12, 13, 14). Turbid reefs in the Coral Triangle will likely increase due to coastal development, deforestation, pollution and nutrient run-off (6), making it important to understand the taxonomic and functional diversity of these communities in the past and present.

Coral reefs are commonly perceived as living in clear, warm, and oligotrophic waters. Consequently, coral communities living under turbid conditions have been overlooked. Turbid reef communities seem to be more common than previously thought, both in the past (12, 13) and present (8, 15). This suggests there are two types of turbid reef communities: naturally-occurring (pre-human contact) and anthropogenically-induced (post-human contact). Previous comprehensive work has been done for Miocene Coral Triangle reef assemblages (12, 13, 16); however, little is known about the IWP Scleractinian community in the Oligocene (17, 18). Studying these pioneering corals will help establish a baseline for the ancient Coral Triangle community. This can then be compared to contemporary coral reef communities.