From NOT A LOT OF PEOPLE KNOW THAT

By Paul Homewood

A small victory!!

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The BBC article “Great Barrier Reef suffers worst coral decline on record,” was published on August 6, 2025 (with some sources listing August 5). It reports on findings from the Australian Institute of Marine Science (AIMS) annual monitoring survey of the Great Barrier Reef (GBR).

Corals on the Great Barrier Reef (GBR) and in similar tropical systems survived during the Holocene Climatic Optimum (also called the Holocene Thermal Maximum or mid-Holocene Thermal Maximum, roughly 9,500–5,500 years ago, with a peak around 6,000–8,000 years BP).

The Mesozoic Era (roughly 252 to 66 million years ago), often called the “Age of Dinosaurs,” overlapped with the existence and evolution of coral groups that are ancestors to many modern reef-building corals.

Here’s a breakdown of the key points from the fossil record and evolutionary history:1. Modern-Style Reef-Building Corals (Scleractinians) Appeared in the Triassic

• Scleractinian corals—the group that includes today’s hard, reef-building corals (like Acropora, Porites, and most tropical species)—first appear in the fossil record during the Middle Triassic period, around 247–237 million years ago.
• This was early in the Mesozoic Era, shortly after the massive Permian-Triassic extinction (~252 Ma) that wiped out older coral groups.
• Dinosaurs themselves first emerged in the Late Triassic (~230–225 Ma), so scleractinians were already present when the earliest dinosaurs appeared.
• By the Jurassic and Cretaceous periods (when dinosaurs dominated land), scleractinians had diversified significantly and became important contributors to reefs.

2. Symbiosis with Algae Dates Back to the “Age of Dinosaurs”

• The symbiotic relationship between corals and photosynthetic algae (zooxanthellae), which powers modern reef growth in shallow, sunlit waters, goes back at least ~160 million years (Mid-Jurassic, around the time of many iconic dinosaurs like Stegosaurus or early sauropods).
• This partnership helped scleractinians thrive through warmer Mesozoic oceans and recover from environmental stresses, similar to how it functions today.
• Research shows this coral-algae symbiosis endured multiple climate shifts and extinctions during the dinosaur era.

3. Reef Composition During the Mesozoic

• Early Mesozoic reefs (Triassic–Jurassic) included scleractinians alongside sponges, algae, and other organisms.
• In the Cretaceous (the heyday of dinosaurs like Tyrannosaurus and Triceratops), many large reefs were actually built primarily by rudist bivalves (extinct, reef-forming clams), though scleractinian corals were still present and diverse.
• Corals were abundant in warm, shallow seas, coexisting with marine reptiles (ichthyosaurs, plesiosaurs, mosasaurs), ammonites, sharks, and fish.

4. Survival Through the End-Cretaceous Extinction

• The asteroid impact ~66 million years ago (which ended non-avian dinosaurs) caused major disruptions, including ocean acidification and mass die-offs.
• Scleractinian corals suffered losses (~45% of genera extinct in some estimates), but many lineages survived—unlike rudists, which went fully extinct.
• This allowed the modern coral groups we know today to diversify in the Cenozoic Era.

The Mesozoic scleractinian corals (from the Triassic, Jurassic, and Cretaceous periods, ~252–66 million years ago) are the direct ancestors of today’s modern corals—all living reef-building stony corals belong to the same order, Scleractinia. Despite this shared lineage, there are notable differences in morphology, ecology, skeletal details, symbiosis, and reef-building roles compared to modern ones. These stem from evolutionary refinements over time, environmental changes (e.g., ocean chemistry, extinctions), and adaptations.

In essence, Mesozoic scleractinians laid the groundwork for modern corals—they had similar basic biology (aragonite skeletons, hexameral symmetry, potential symbiosis)—but were often “proto-modern” with simpler colony integration, larger features, and different dominant reef partners. Evolution refined them through extinctions and environmental shifts into the more specialized, high-performance reef-builders we see today.If you’d like comparisons with specific fossils (e.g., Thecosmilia from Jurassic vs. modern Acropora), or visuals showing side-by-side growth forms, let me know!