Complex sentence: Parfitt proposed that the Ark of War may have been taken across the Jordan and citing Islamic sources proposed that it had then moved south, in the possession of Arab tribes, until it reached Yemen, where certain families claimed to have kept it in their possession.
Split into simpler sentences: Parfitt proposed that the Ark of War may have been taken across the Jordan River and, citing Islamic sources, proposed that it had been taken with the people as they migrated south, possessed by Arab tribes. After they reached Yemen, certain families claimed to have kept it in their possession.