Complex sentence: Altdorfer's most significant contemporary influence was Matthias Grünewald (c.1470--1528); art historian Horst W. Janson remarked that their paintings "show the same' unruly' imagination".
Split into simpler sentences: Altdorfer's most significant contemporary influence was Matthias Grünewald (c.1470--1528). Art historian Horst W. Janson remarked that their paintings "show the same' unruly' imagination".