period of the Paleozoic Era, marked by the appearance of the first vertebrates. The fragmentation of Paleogondwana continued as well as the separation of the continents of Paleolaurentia, Baltica and Siberia, due to the expansion of the Iapetus Ocean. A cooling of the planet occurs, evidenced by ice caps on South Pole lands and a lowering of about 100 m of the sea level. The drifting of the continental plates leads to the formation of an island-arc between the existing continents and Paleogondwana, resulting in a new ocean – the Paleothetys. Geological register of this geological period is conserved in the Mount Saint Gabriel (Castelo Melhor).