Identification: Unknown Oak Tree Fungus ???

I have about 20 mature oak trees in my yard -- 70-100 years old. One of the trees has a fungus growing around the base of the tree. I am concerned that it may be a sign of disease or decay at the tree root. I am attaching photos of the fungus which is flat and round, brown, about the size of a hand, spongey in appearance but actually hard. There are about 10 of these growths on the ground surrounding the base of the tree ranging from about 3 inches to 8 inches in diameter. The tree appears to be in overall good health but seems to have lost more bark than the others nearby. Any insights will be appreciated. It was suggested that these were fruiting bodies (basidiocarps) of tree decay fungi but I was not able to make a match. Thanks in advance.

 

I am not a polypore person, but may i suggest the warted oak polypore -- Inonotus dryadeus?
here are 2 photos.


http://www.forestryimages.org/images/768x512/3047003.jpg

http://www.forestryimages.org/images/768x512/3047018.jpg

 

IANAPP either, but I think the tree experts that frequent this forum will tell you that by the time you see this on a tree, it is late in the game, so to speak.