Siemomysł was the third pagan Polans

Polans

Polans may refer to two Slavic tribes:* Polans , in the area of Dnieper river* Polans , in the area of Warta. The tribe unified most of the lands of present-day Poland under the Piast dynasty....

 duke, listed by Gallus Anonymous in his Gesta principum Polonorum, of Piast dynasty, and the father of Poland

Poland

Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...

's first historical ruler, Mieszko I. He was son of the second known Polans duke Lestek. Following Gallus' account and historical research, Siemomysł has been credited with leaving the lands of Polans, Goplans

Goplans

Goplans, Goplanes or Goplanie is a name of an early medieval West Slavic tribe, later to become part of the Polish nation. They were mentioned by Bavarian Geographer as 'Glopeani' and believed to have lived around Lake Gopło, with its capital in Kruszwica, between 7th-9th century AD.Medieval...

 and Masovians

Masovians

The Masovians or Mazovians are a Lechitic tribe or an ethnic group associated with the region of Mazovia. They were first referenced by Nestor the Chronicler in the 11th century. Originally, their main settlements were in the area of Płock, Łomża, Wizna, Czersk, Ciechanów, Płońsk, Zakroczym and...

 to his son Mieszko I, who further expanded them during his reign. His name in German was Ziemomysl. . According to Henryk Łowmiański he aided the Ukrani

Ukrani

thumb|250px|right|Burgwallinsel, a former Ukrani [[burgh]] on an isle in Lake [[Oberuckersee]]The Ukrani were a West Slavic Polabian tribe in the Uckermark from the 6th–12th centuries. Their settlement area was centered around the lakes Oberuckersee and Unteruckersee at the spring of the...

uprising against the Germans in 954 AD.