Complex sentence: Marie Severin worked for Marvel Comics in the early 1960s and into the 1970s, where she began to stretch as an artist and began penciling the early adventures of Dr Strange, The Sub-Mariner and The Incredible Hulk.
Split into simpler sentences: Marie Severin worked for Marvel Comics in the early 1960's and into the 1970's. She began to stretch as an artist and drew the early adventures of Dr Strange, The Sub-Mariner and The Incredible Hulk.