Early Paleozoic reef evolution: Paleoecology and environmental drivers

Reef systems provide exceptionally detailed records of ancient shallow-marine environments, combining information about community structure, food webs, carbonate production, and ocean-seafloor conditions. Because reefs grow in place, they preserve temporal records of ecological interactions and environmental changes, offering natural laboratories for studying how physical factors (habitat complexity, seafloor stability, sea-level change, sedimentation rates) and chemical factors (oxygen and carbon dioxide levels, nutrient availability, carbonate saturation state) control biodiversity patterns, ecosystem productivity, and carbon cycling through Earth's history.

The Early Paleozoic marks the major transition from microbe-dominated Precambrian oceans to the animal-dominated ecosystems. During the early Cambrian, archaeocyath–microbial reefs appeared together with the Cambrian Explosion, followed by mid–late Cambrian reef systems built mainly by sponges and microbes. These ecosystems thrived under warm greenhouse climates and low-oxygen marine conditions that limited the spread of more complex skeletal reef builders like corals. The Early Ordovician saw the first local development of animal-built reef frameworks, coinciding with global climate cooling, increasing oxygen levels on continental shelves, and enhanced seafloor hardening through early cementation—conditions that allowed reefs to establish and grow upward.

While the Middle Ordovician reef record is incomplete due to global sea-level drops and gaps in the geologic record, the Late Ordovician shows a clear evolutionary jump: diverse, complex skeletal reefs spread widely during the Great Ordovician Biodiversification Event, representing the first modern-like reef ecosystem. Together, these patterns suggest that in addition to the evolution of reef-building organisms, environmental factors including long-term climate trends, ocean oxygen levels, sea level change, and carbonate saturation state were the main controls determining when reef ecosystems started, flourished, or collapsed.

To test these environmental hypotheses more rigorously, future research should integrate: (i) large stratigraphic databases with geochemical indicators of atmospheric O₂ and CO₂ levels, ancient temperatures, and carbonate saturation; (ii) high-resolution microanalysis techniques performed directly on samples (such as SIMS and LA-ICP-MS); and (iii) advanced imaging and chemical analysis methods (including EBSD, EDS, EPMA, and Raman spectroscopy) to separate later chemical changes from original rock textures. Systematic application of these combined approaches across different continents will enable the development of clearer, testable models for how reef ecosystems evolved, changed, and survived throughout Earth's history.