I wrote to a couple of mushroom experts whose sites I found while trying to find an answer to the question of what this is. Here's the response I got from David Fischer at American Mushrooms.com Thank you to David for his quick response to my question.
This is the Black-staining Polypore (Meripilus sumstinei = M. giganteus). It's a common species. It is edible* but not among the most popular.
Some additional input from Stan at Mid-Missouri Morels and Mushrooms :
I think I would agree with Fisher that it is a black staining polypore. Tear off a few pieces and if it bruises or turns black, then it will be confirmed. People do eat these but usually when they are younger. They get kind of tough with age. If it doesn't turn black, then it is a Hen of the Woods mushroom, which are mighty tasty and one of my favorites.
In either case, it is definitely a type of polypore mushroom, and no polypore is poisonous so you could try it if you wanted. However, some people do have minor allergic reactions to some polypores, so if you do, only cook a very small batch and try a couple of bites. If no effects by the next day, you can try more.
Thanks for the sharing your find.Stan