Some classics were just made to go together. Peanut butter and chocolate. Thanksgiving dinner and stretch pants. Scleractinian corals and dinoflagellate algae. And boy, do those two go way back—scientists looking at fossils say the two have been cohabiting since at least about 212 million years ago, in the Late Triassic Period. The researchers published their report today in the journal Science Advances.

Happy, healthy coral is essential for a happy, healthy reef. To stay happy and healthy, many modern corals have forged super close relationships with teeny algae called zooxanthellae. The corals give the algae a safe place to live and the chemical components for photosynthesis, while the algae make oxygen, keep the water clean, and produce all kinds of helpful nutrients. The pair really have a good thing going.

But just how long it’s been going on has been anybody’s guess. Previous studies on the pair’s relationship have been largely speculative, using data from modern-day corals to imagine their ancestors’ world.

Now, two new scientific techniques, one visual and one chemical, have allowed us to get a far more accurate picture of coral history.

Earlier this year, lead author Katarzyna Frankowiak and a number of her co-authors reported that they’d figured out how to tell if a fossilized hard coral had been in a relationship with algae. The trick, they said, is to look very closely at the coral’s skeleton to see how it had grown and aged. Even when the algae itself was long gone, its presence had left irrevocable (if microscopic) changes in the coral’s life.

For the new study, the researchers applied this technique to tiny samples of fossilized hard corals found near the former Tethys Sea in Turkey. They used a variety of high-powered microscopes to examine the fossils in the most minute of detail and found that the skeletons of these ancient, ancient samples looked a lot like those of modern symbiotic hard corals.

Algae activity (brown dots in the tissue, upper left image) is recorded in the coral skeleton as structural (growth bands; upper right image) and geochemical signatures. Such regular growth bands occur in Upper Triassic (ca. 220 Ma) scleractinian corals (lower images) as well. Image Credit: Isabelle Domart-Coulon (upper left), Jarosław Stolarski (upper right, and lower images)

The second new method concerned the corals’ chemical composition. The experience of living with algae alters a coral’s very molecules, changing the ratio of various oxygen, carbon, and nitrogen isotopes. And just as with the visual inspection, analysis of the fossil corals’ isotopes suggested that they’d been sharing their lives with zooxanthellae.

Analyzing the coral isotopes yielded another insight: the sea in which these buddies lived was likely in pretty poor condition. The fossil corals shared a similar ratio of nitrogen isotopes with modern symbiotic Bermuda corals, which are currently struggling in nutrient-starved waters. It’s possible, the researchers say, that these difficult conditions were what inspired the algae and the corals to band together in the first place.