Final answer:
Corals were the major reef builders during the Ordovician that took a severe hit in the Permian mass extinction but did not go extinct. They were crucial to marine ecosystems and demonstrated resilience by surviving the most significant loss of species in Earth's history, adapting and persisting to the present day.
Step-by-step explanation:
The major reef builders of the Ordovician period that suffered significantly during the Permian mass extinction but did not go extinct are corals. Specifically, this refers to the tabulate corals and rugose corals. Both were essential to reef building in paleozoic marine ecosystems and reached a peak in diversity during the Devonian period.
During the Ordovician period, tabulate corals originated and became significant reef constructors. Additionally, the rugose corals became particularly diverse in the Devonian period, contributing greatly to reef building before going extinct in the end-Permian mass extinction. Notably, while many species were lost during this extinction event, the coral lineage itself continued beyond it, distinguishing corals from groups like trilobites, which did go extinct.
The end-Permian extinction was remarkably severe, resulting in the loss of approximately 96 percent of marine species and 70 percent of terrestrial species. This event reshaped Earth's biodiversity profoundly. The survival of corals after such a catastrophic event underscores their resilience and evolutionary success, despite the major losses they endured.