It is a basidiocarp, aka 'conk' or shelf or bracket fungus, which is
actually the fruiting body of of a fungus that typically affects the root
or heartwood of trees. Lots of different fungi or root rots can generate
these conks and if as well developed and as visible as your photo portrays,
they usually indicate a tree that is well on its way to death.
pam - gardengal
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