Complex sentence: But Jean Tixier had only catalogued and reported a conjecture made in the name of the Hellenistic-era historian Berossus, by the fraudster Annio da Viterbo (1498), who had previously used the same hypothesis to postulate an ancestry for the Gauls.
Split into simpler sentences: But Jean Tixier had only catalogued and reported a conjecture made by the Hellenistic-era historian Berossus. The fraudster Annio da Viterbo (c.1432--1502) had previously used Berossus' same hypothesis to postulate an ancestry for the Gauls.