Complex sentence: There is a gap in the archaeological record between the most recent Fort Ancient sites and the oldest sites of the Shawnee, who were recorded by European (French and English) explorers as occupying this area at the time of encounter.
Split into simpler sentences: There is a gap in the archaeological record between the most recent Fort Ancient sites and the oldest sites of the Shawnee. The latter were recorded by European (French and English) explorers as occupying this area at the time of encounter.