The occurrence of zooxanthellae in Recent scleractinian corals is strongly correlated with their growth form, corallite size, and degree of morphological integration of corallites. The great majority of zooxanthellate corals are multiserial with small, highly integrated corallites, whereas most corals lacking zooxanthellae are solitary or uniserial colonial forms with large, poorly integrated corallites. Beginning in the Jurassic, fossil scleractinian faunas are morphologically similar to Recent faunas dominated by zooxanthellate species, strongly implying that most scleractinians contained zooxanthellae by that time. Evidence for Siluro–Devonian tabulates and Triassic scleractinians is equivocal but still suggests the presence of zooxanthellae in these corals. In contrast, morphological evidence suggests that rugosan corals lacked zooxanthellae.
Most populations of Recent zooxanthellate corals contribute to reef formation, but many do not. Similarly, fossil corals interpreted to contain zooxanthellae on morphological grounds did not always form reefs. Recent reef formation depends upon a host of environmental factors that have little to do with the possession of zooxanthellae per se. Coral morphology should be a better predictor of the presence of zooxanthellae in fossil corals than their association with reefs.