Complex sentence: Unfortunately, the primary historical sources for this period have been lost and so the the writings of Diodorus Siculus and Arrian are relied upon to understand the period, both of whom lived centuries later than the events they describe.
Split into simpler sentences: Unfortunately, the primary historical sources for this period have been lost. As a consequnce scholarship is largely reliant on the writings of Diodorus Siculus and Arrian, both of whom lived centuries later than the events they describe.