Complex sentence: Historian András Róna Tas says that the title may be connected to the Bulgars' old Hungarian exonym, "nándor", which implies that the palatine was originally the head of the Bulgar groups in Hungary.
Split into simpler sentences: Historian András Róna-Tas says that the title may be connected to the Bulgars' old Hungarian exonym "(nándor)". If his theory is valid, the palatine was originally the head of the Bulgars in Hungary.