Comentário: Lincoff in Audubon Society Field Guide to North American Mushrooms, p. 427, reports this as “High up on living sugar maples; also reported on beechs.” But says nothing about dead trees.

But David Arora in Mushrooms Demystified, p. 612, states “Usually growing in overlapping, shelving masses high up on living hardwoods (especially maple); caps usually 10-30 cm broad, several arising fro a common base; fairly common in northern North America…” Photos in both sources match closely with your photos, though – I’d say they were the same thing. Another interesting similarity is the fuzzy edges of the sporocarps: rather unusual in other Polypores in my experience.