Shallow marine mixed siliciclastic–carbonate shoals, a carbonate platform, and the subsequent development of a reefal buildup occur in sequence from the late Chattian to the Aquitanian in the Niah area of Sarawak. They document the transition from larger foraminifera-dominated, calcitic environments to scleractinian coral–dominated, aragonitic environments in SE Asia, which correspond to a significant increase in biodiversity. A late Chattian to early Aquitanian phase of carbonate sedimentation was initiated by larger foraminifera on shallow marine argillaceous shoals raising from the seabed at about 60 m in water depth occasionally up to near sea level. Carbonate production is almost entirely the result of the accumulation of larger foraminiferal shells dominated by Eulepidina dilatata , a species that could thrive thanks to its photosymbiosis with microalgae. Such mixed carbonate–clastic shoals formed repeatedly on a muddy shelf during a period stretching from about 23.5 Ma to 22.3 Ma. Following a period of siliciclastic deposition, a roughly circular carbonate platform with an area of some 25 km 2 was formed at around 21.2 Ma in stratigraphic continuity with the underlying shallow marine sandstones of the Nyalau Formation. Known as the Subis Limestone, it consists at first of bedded carbonates characterized by the presence of red algae, a high diversity of free benthic and sessile endosymbiotic sessile foraminifera, and a variety of organisms typical for reefal environments, including colonial corals. A reefal buildup started forming on the carbonate platform as early as 21.1 Ma. This phase of growth was likely initiated by low-relief patch reefs, 150 to 200 m in diameter and 60 to 80 m in height, such as those exposed in a southern quarry. Analogous with same-age reefal development models from the Java Sea, it is proposed that the patch reefs coalesced through time to form a larger isolated carbonate buildup that grew up at least until the end of the Aquitanian at 20.4 Ma. This Subis buildup reached an area of 16 km 2 ; it has a preserved thickness of 260 to 280 m and had a paleo-relief of about 100 m above the surrounding sea floor. It is asymmetrical, with a reef wall forming high, west-facing cliffs and another reef wall likely extending on the NE edge of the buildup, beyond the Niah Great Cave. A further increase in faunal and floral diversity occurred during this phase, concomitant with the diversification of ecological niches within the buildup. Reefal and peri-reefal environments are dominated by red algae; solitary and colonial corals (domal, branching, and platy), with subordinate foraminifera (large and small benthic); and associations of foraminifera and algae forming laminar foralgal binding tissues, sponges, hydrozoans, bryozoan, bivalves, echinoderms, and serpulids. The reef rim consists of coral framestone and algal-foraminiferal bindstone. The backreef facies is characterized by rudstones and floatstone with coral debris, and the lagoon facies includes microbial crusts, green algae, articulated and nonarticulated red algae, benthic foraminifera ( Miliolids ), ostracods, gastropods, and up to 4-m-high platy corals pillars. Forereef deposits include grain- and mud-supported reef debris; a debris apron present some 2.5 km away from the western edge of the buildup consists of debris flows and calciturbidites embedded in outer neritic shales. The upper part of the buildup is missing as a result of recent subaerial erosion. Three successive steps in the development of carbonate ecosystems are identified, which are linked to a series of innovative symbiotic relationships established during the late Chattian and the Aquitanian. During an early phase (23.5–22.3 Ma), monospecific populations of endosymbiotic larger benthic foraminifera thrived on shallow marine muddy shoals. At around 21.2 to 21.1 Ma, new species of endosymbiotic larger benthic foraminifera, sessile-encrusting foraminifera, and coralline algae colonized shallow marine grounds and created a carbonate platform. From about 21.1 to at least 20.4 Ma endosymbiotic scleractinian corals, red algae, and a diverse association of organisms created patch reefs and a buildup.