Many coral species die as soon as the water temperature reaches 30° C.
16 percent of the world coral reefs died in just one year (1998), due to El-Niño.
So? It only takes a few to survive through any excursion to assure the continuation of coral or any other species.
In 1998 this mass death was still a local phenomenon. Example : Chagos Reef ind the Indian Ocean lost 90% of its coral. By 2000, with the continuation of abnormal high water temperatures, 27 percent pf the world coral reefs had died
And this is your evidence that there never has been an iceage or warmer periods??
Coral continues to survive regardless of local phenomenon or even the excursion of a global order for there have obviously been areas on the earth for species to survive regardless of the extremes in localities. 10% made it Chagos Reef, in only takes a few to assure a survival adaptation across geological time.