BERLIN—Angela Merkel took a step closer to political retirement Tuesday after receiving her formal dismissal certificate from the post of chancellor after 16 years in office, though she will lead a caretaker government until her successor is sworn in. Hours earlier, Merkel attended the opening session of Germany’s new parliament in Berlin, taking a seat in the VIP gallery as the recently elected lawmakers met for the first time. Merkel, who first won a seat in the Bundestag 31 years ago, did not run again in the Sept. 26 election. Lawmakers elected Baerbal Bas, a member of the Social Democratic Party, as speaker of the 736-member lower house. She succeeds Wolfgang Schaeuble, a 79-year-old conservative and former finance and interior minister, who was also the longest-serving member of the last parliament, having first been elected in 1972. Bas, whose party narrowly beat Merkel’s Union bloc, has been a member of …