Final answer:
The reference materials do not provide information on 'stagborn corals' or data to calculate their average number; instead, they discuss tabulate and Scleractinia corals' diversity and evolutionary history.
Step-by-step explanation:
The question seeks to determine the average number of stagborn corals within a specific time frame. However, there is no direct information provided in the reference materials regarding 'stagborn corals' nor an average number that could be calculated based on the information given. The provided materials discuss the diversity of tabulate corals and Scleractinia corals, their evolutionary history, and instances of mass extinctions but do not mention 'stagborn corals' or provide data from which an average could be derived. The closest information related to coral diversity through time is found in the discussions around the Late Devonian extinction event, end-Permian mass extinction, and the presence of different types of corals in the fossil record.
The average number of stagborn corals within the given time frame cannot be determined from the information provided. The provided information discusses the diversity and extinction events of different types of corals, but does not specifically mention stagborn corals. To determine the average number of stagborn corals, more specific information about stagborn corals and their population dynamics within the given time frame would be needed.