Which earlier Greek historian did Diodorus partly draw on for his account? He partly drew on Ephorus of Cyme.
Ephorus was a fourth-century BC Greek historian from Cyme in Aeolis. He wrote a universal history in 29 books, tracing Greek history from the return of the Heracleidae to the era of Philip II of Macedon. The work is lost, but later authors, particularly Diodorus, preserve fragments and historical material that may derive from it.
For Diodorus’s account of mainland Greece, including the Persian Wars and the events around Thermopylae, scholars generally regard Ephorus as the principal underlying source. Diodorus was not simply copying one book mechanically, however; he may also have consulted Herodotus, Thucydides, and other historians while shaping his own chronological narrative.
A common mix-up is Timaeus of Tauromenium, another important source associated with Diodorus. Timaeus was especially relevant to Diodorus’s Sicilian material, whereas Ephorus is more closely connected with the mainland Greek narrative. The word “partly” matters because ancient historians commonly combined several earlier works.