Final answer:
The sentence in question explains the process of coral reef formation and sets the stage for understanding the ecological and evolutionary significance of coral reef ecosystems. It outlines how the accumulation of calcium carbonate skeletons from living and deceased corals results in the establishment of a reef structure over time.
Step-by-step explanation:
The main role of the sentence "As coral thickets build upward on the skeletal remains of older colonies, a reef is established." in the passage is to describe the process by which coral reefs are formed. It emphasizes that the calcium carbonate skeletons of corals are fundamental to reef formation, highlighting the significance of both the living and dead corals in providing structure.
This sentence sets a foundational premise for understanding how coral reef ecosystems are built over time through the accumulation of skeletal material, which then becomes the habitat for a diverse ecosystem.
Additionally, the sentence points to the ecological importance of corals as they not only build reefs but also offer protection to various marine life forms by providing a barrier against waves and ocean currents. The evolutionary aspect is also implied, suggesting that the reef-building process has occurred over incredibly long time periods, during which these organisms have adapted to their warm, tropical environment.