Complex sentence: Most of the subject cities were probably Oenotrian towns in the interior, but we know that Sybaris had extended its dominion across the peninsula to the Tyrrhenian Sea, where it founded the colonies of Poseidonia (Paestum), Laüs (Laus), and Scidrus.
Split into simpler sentences: Most of the subject cities were probably Oenotrian towns in the interior. Sybaris also extended its dominion across the peninsula to the Tyrrhenian Sea, where it founded the colonies of Poseidonia (Paestum), Laüs (Laus), and Scidrus.