Complex sentence: Romanian scholars have identified about a dozen Transylvanian sites as forts in Gelou's duchy, but none of these forts can certainly be dated to the period preceding the turn of the 9th and 10th centuries.
Split into simpler sentences: Romanian scholars have identified about a dozen Transylvanian sites as forts in Gelou's duchy. None of them can certainly be dated to the period preceding the turn of the 9th and 10th centuries.