Coralline red algae are among the most important yet underappreciated architects of shallow-marine ecosystems. These hard, pink-encrusted organisms precipitate calcium carbonate in their cell walls, contributing substantially to coastal carbon cycles and building three-dimensional habitats that shelter a wide range of marine life. They grow from the tropics to polar seas and have persisted through dramatic swings in temperature, light availability, and water depth across the Cenozoic. In modern oceans, they are recognized as ecosystem engineers, and recent work has even suggested that crustose corallines can contribute more to reef carbonate production than the corals themselves. Their fossil record, however, has long remained in the shadow of flashier contemporaries such as reef-building corals and larger benthic foraminifera, partly because sampling bias, preservation effects, and the sheer difficulty of identifying fragmented specimens have hampered systematic study.

Meghalaya offers an exceptional window into this neglected world. The Shillong Plateau, which includes the Jaintia, Khasi, Garo, and Mikir Hills and the Upper Assam Valley, preserves remnants of the ancient eastern Tethys Ocean, the tropical seaway that once separated the drifting Indian subcontinent from Asia. What makes the region remarkable is what is missing: unlike most Tethyan shallow-marine settings of the Paleocene and Eocene, which teemed with coral-algal reefs, Meghalaya’s carbonate platforms show no signatures of in-situ coral reefs at all. Instead, coralline red algae and small to larger benthic foraminifera dominate the fossil assemblages. This ecological oddity makes the region an ideal natural laboratory for asking how coralline algae fare when corals are absent, and what that might mean for their fate under future climate stress.

The new work, carried out by Suman Sarkar of the Birbal Sahni Institute of Palaeosciences in Lucknow, focuses on two major carbonate units: the late Paleocene to earliest Eocene Lakadong Limestone and the middle Eocene Prang Formation, with the intervening early Eocene Umlatdoh Limestone providing a telling contrast. Five carbonate successions were sampled in the East Khasi and Jaintia Hills, from abandoned limestone quarries near Cherrapunji and Therria and along the Jowai-Badarpur road transect. The complete profiles were sampled at resolutions of 0.25 to 0.5 meters, and the algae were identified from petrographic thin sections under a light microscope, using diagnostic vegetative and reproductive features. Quantitative estimates of abundance came from point-counting with a 300-micrometer grid, with at least 300 points counted per section. Species-level identifications were deliberately avoided, because the taxonomic constraints of fossil coralline algae make such assignments unreliable.

The results reveal a community of striking stability. Five major non-geniculate genera dominate the assemblages: Sporolithon, Lithothamnion, Mesophylllum’s close relative Mesophyllum, Lithoporella, and the extinct Distichoplax, alongside common occurrences of Lithophyllum, Spongites, and Neogoniolithon. Apart from one dramatic exception, generic diversity and abundance remained nearly constant from the Lakadong Limestone through the Prang Formation. That exception is Distichoplax, a coralline with a unique feather-and-zipper cellular organization preserved in laminar thalli. During the late Paleocene, Distichoplax was exceptionally abundant, forming dense filament populations in the Lakadong carbonates. By the earliest Eocene, it had suffered a drastic population collapse, persisting only as rare, possibly reworked fragments in the Umlatdoh Limestone and Prang Formation before disappearing entirely. The other genera, by contrast, diversified consistently and showed no significant response to the environmental upheavals of the interval.

The sedimentary fabric of the rocks tells its own story. Five major microfacies were distinguished, including coralline algal grainstone-packstones, coralline algal-foraminiferal grainstone-rudstones, coralline algal bindstones, and foraminiferal rudstones and grainstone-packstones. The prevalence of grain-supported textures points to deposition under high-energy hydrodynamic conditions, with only occasional intervals of calmer water marked by micritic matrix. The bindstones, dominated by encrusting coralline thalli closely associated with encrusting foraminifera, cap both the Lakadong and Prang sections and carry a crucial bathymetric signal. In these topmost beds, Hapalidiales algae dominate while light-dependent larger benthic foraminifera remain common and Corallinales become rare, a combination that rules out depths beyond 60 meters but indicates a general deepening-upward trend for both formations.

By applying modern ecological distributions of coralline algae as paleobathymetric proxies, the study reconstructs the ancient seafloor in fine detail. Today, members of the Hapalidiales typically dominate coralline floras below 40 meters water depth in tropical, nutrient-poor settings, while Corallinales prevail in shallower water and become minor components below 60 meters. In the lower parts of the Meghalaya sections, however, the photophilic genus Mesophyllum appears in moderate quantities alongside Corallinales and Sporolithales at depths shallower than 40 meters, an unusual association that pins the depositional environment to very shallow, sunlit waters. Sporolithon, which ranges from very shallow habitats to the lowest reaches of the photic zone, is too ecologically flexible to serve as a depth indicator on its own, but the combined assemblages paint a coherent picture of a shallow tropical carbonate bank that gradually deepened over time.

Why were there no coral reefs here when nearby regions such as Tibet and northwestern India hosted flourishing Paleocene coralgal reefs? The study points to two intertwined factors. First, Paleocene sea-surface temperatures in the region reached roughly 34 to 38 degrees Celsius, a regime likely too hot for reef-forming colonial corals to colonize, even though coralline algae and benthic foraminifera, whose temperature sensitivity is presumably lower, continued to proliferate. Second, nutrient conditions appear to have been meso-oligotrophic rather than truly oligotrophic. By analogy with the modern Gulf of California, where coral reefs occupy nutrient-poor waters, coralline algae occupy mesotrophic zones, and bryozoans and mollusks dominate eutrophic settings, the Meghalaya assemblages fit best into the middle of that trophic spectrum. The comparatively low foraminiferal species diversity, including the absence of typical Lockhartia communities seen elsewhere in the Tethys, supports this interpretation.

Perhaps the most consequential finding concerns resilience. Every coralline genus in the Meghalaya successions except the extinct Distichoplax still lives in modern shallow-marine environments worldwide, and some lineages originated in the Cretaceous and have diversified for more than 140 million years. These genera survived the end-Cretaceous mass extinction, the Paleocene-Eocene Thermal Maximum, the Early Eocene Climatic Optimum, and the Middle Eocene Climatic Optimum, all episodes of severe global warming and, in several cases, ocean acidification. Notably, the scarcity of corallines in the Umlatdoh Limestone appears unrelated to climate; the study argues it more likely reflects competitive exclusion, with calcareous green algae and benthic foraminifera colonizing the seafloor first. A companion study using finite element analysis of Sporolithon and Lithothamnion from another Meghalaya succession found that the mechanical stresses experienced by Paleogene taxa were comparable to those of their modern relatives, suggesting a functional resistance to warming and low pH that helped prevent breakage and, by extension, major generic extinctions.

For a planet racing toward carbon dioxide levels not seen since the early Paleogene, these findings carry real weight. The Paleocene and Eocene featured multiple phases of substantially elevated atmospheric carbon, making them closer analogues to future climate than the cooler Quaternary period that dominates most climate discussions. Coralline algae today face ocean warming and acidification driven by fossil fuel emissions, yet the fossil record from the eastern Tethys shows they can endure extraordinary environmental stress over geological timescales, even while individual genera like Distichoplax and Subterraniphyllum vanish for reasons that remain enigmatic. The Meghalaya successions demonstrate that these organisms deserve far more attention, both as carbonate producers and ecosystem engineers and as archives of climate change, and that understanding their past responses may be essential to anticipating the future of shallow-marine ecosystems worldwide.

Subject of Research: Diversity and paleoenvironmental significance of Paleocene-Eocene coralline red algae from the eastern Tethys in Meghalaya, northeastern India

Article Title: Diversity and environmental implications of Paleocene-Eocene coralline red algae in the eastern Tethys (Meghalaya, northeastern India)

Article References: Diversity and environmental implications of Paleocene-Eocene coralline red algae in the eastern Tethys (Meghalaya, northeastern India). (n.d.). https://doi.org/10.1007/s44288-026-00665-5

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00665-5

Keywords: coralline red algae, Paleocene, Eocene, Tethys, Meghalaya, paleoenvironment, carbonate platform, Distichoplax, paleoclimate, benthic foraminifera, ocean warming, paleobathymetry