The Miocene epoch (especially the Early to Middle Miocene, ~23–11.6 million years ago) saw one of the most significant expansions of coral reefs in the Cenozoic era, particularly in the Indo-Pacific region. This boom helped establish the foundations of modern reef biodiversity in the Coral Triangle.

Researchers describe it as driven by a confluence of environmental, tectonic, and biological factors.

1. Tectonic and Geological Drivers (Major Role)

Plate movements and shallow marine habitat creation: The northward drift of the Australian plate and its convergence with Eurasian/Philippine plates increased tropical shallow-water shelf areas. This provided more stable, suitable substrates for reef growth (e.g., broad continental shelves).

Indo-Australian Archipelago (IAA) formation: Tectonic activity created complex island arcs, atolls, and varied topography, boosting habitat heterogeneity and connectivity for larval dispersal.

Regional examples: In areas like Australia’s North West Shelf, precursor barrier reefs formed around 20 million years ago, acting as foundations for later systems. Tectonic subsidence, uplift, and sea-level interactions influenced local growth and drowning cycles.

2. Environmental and Climatic Drivers

Warm temperatures during the Miocene Climatic Optimum (MCO): Relatively high sea surface temperatures (SSTs) supported coral calcification and symbiosis with zooxanthellae. Reefs thrived in tropical to subtropical zones before later cooling events.

Sea-level fluctuations and eustasy: Transgressive-regressive cycles provided accommodation space for vertical reef accretion. Repeated exposures and flooding helped shape lagoon and reef systems.

Ocean circulation: Enhanced currents (e.g., proto-Leeuwin Current) transported warm water, larvae, and nutrients, supporting growth along margins like Western Australia.

Nutrient and productivity regimes: Oligotrophic (low nutrient) conditions in many areas favored zooxanthellate corals, while localized upwelling or terrestrial inputs may have varied by region. Aridification in some coastal areas reduced sediment stress.

Later in the Miocene, cooling (Late Miocene Cooling ~7–5.4 Ma) contributed to reef declines or “gaps” (e.g., Pliocene Reef Gap), with ~2°C SST drops, shifts in monsoons, increased terrestrial runoff, and changes in currents acting as stressors.

3. Biological and Ecological Drivers

Coral evolution and growth forms: Diversification of reef-building corals (Scleractinia), including more robust, wave-resistant structures (e.g., certain Acropora and other genera). Shifts toward modular, fast-growing species enabled larger, thicker reefs.

Symbiosis and functional innovations: Strengthened coral-algal symbioses improved energy efficiency. Concurrent radiations in reef fishes (e.g., labrids/wrasses, planktivores, herbivores) created feedback loops—fishes helped control algae, promote coral recruitment, and enhance ecosystem resilience.

Reef-fish co-evolution: Trophic innovations and increased reef association fueled diversification, with reefs providing new niches.

There is a major scientific study published in 2026 links a massive expansion of coral reefs around 20 million years ago (Early Miocene) to the origins and explosive diversification of modern coral reef ecosystems, particularly in the Indo-Pacific Coral Triangle—the world’s richest marine biodiversity hotspot.

The 2026 Science Advances paper highlights that unique combinations of these factors allowed Indo-Pacific reefs to reach unprecedented size and thickness, surpassing prior Cenozoic systems. This habitat expansion then amplified biodiversity by creating more space and complexity.

Earlier work (e.g., Jones et al. 2022) similarly stresses the interplay of global cooling trends (post-Optimum) with tectonic increases in tropical substrate as key to modern distributions.

Miocene reefs contrast with earlier (e.g., Eocene Tethys) or later systems. Post-Miocene declines involved cooling, sea-level drops, increased sedimentation, and tectonic changes (e.g., Tethys closure). The Miocene represents a “sweet spot” of warmth, substrate availability, and biological innovation.

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The rise and fall of the world’s greatest marine biodiversity hotspot

“The rise and fall of the world’s greatest marine biodiversity hotspot” is the title of a 2026 paper published in Science Advances by Alexandre C. Siqueira and colleagues.

The study shows that the Indo-Pacific Coral Triangle (the planet’s richest marine biodiversity hotspot) arose primarily due to a dramatic expansion of coral reefs in the Early to Mid-Miocene (roughly 23 to 11.6 million years ago). This reef boom created unprecedented habitat area and complexity, which in turn drove the diversification of corals, reef fishes, and associated marine life.

Unprecedented reef scale: During this period, Indo-Pacific reefs reached sizes and thicknesses greater than any in the previous 66 million years (post-Cretaceous). These were massive systems, likely facilitated by a combination of tectonic shifts (e.g., continental positioning), environmental conditions (warm tropical waters, suitable sea levels), and biological factors (e.g., coral growth forms and symbioses).

Biodiversity linkage: Reef expansion directly correlated with increased species richness and functional diversity in marine assemblages. Larger, more structurally complex reefs provided new ecological niches, promoting speciation and supporting higher biomass and variety of fish and corals.

“Rise”: This Miocene peak established the foundations for the modern Coral Triangle hotspot, which today spans a relatively small ocean area but hosts a disproportionate share of global marine biodiversity (e.g., ~76% of reef coral species and thousands of fish species).

“Fall” (or implications): The paper highlights that these ancient massive reefs eventually declined or shifted, and it draws parallels to current threats. Ongoing reef loss due to climate change, pollution, and other human impacts risks reversing the biodiversity gains built over millions of years, with major consequences for tropical marine ecosystems.

It emphasizes habitat area and condition as primary drivers of marine biodiversity over deep time, beyond just climate.

It provides historical context for why the Coral Triangle is so special and underscores the fragility of these systems—reefs aren’t static; they rise and fall with geological and environmental changes.

Related research shows fast-growing corals and certain fish groups (e.g., wrasses, parrotfishes) radiated around this time, filling new roles in the expanding ecosystems.

This work builds on earlier studies of reef history in the Tethys Sea, Australian precursor reefs, and global Cenozoic patterns.

Published: Science Advances Vol 12, Issue 18

DOI: 10.1126/sciadv.aec7264 www.science.org/doi/10.1126/sciadv.aec7264

Authors: Alexandre C. Siqueira, Wolfgang Kiessling, Nussaïbah B. Raja and David R. Bellwood

Abstract

Habitat condition and area shape global species distributions, with shallow-water reefs hosting a disproportional share of marine biodiversity. Although reef area is a well-established predictor of marine species richness, its historical context is less well understood. Here, we show that the rise of tropical marine biodiversity is closely tied to reef expansion in space and time. During the Early/Mid Miocene (23 to 11.6 million years ago), Indo-Pacific reefs reached unprecedented size and thickness, surpassing any reef systems in the past 66 million years. These massive reefs, likely driven by unique environmental, biotic, and tectonic conditions, fostered the expanding diversity and functional evolution of marine fish and coral assemblages. Our findings underscore the importance of historical reef contexts and the implications of ongoing reef losses for tropical marine biodiversity.