Possibly an immature stemless Ganoderma. While the mature fruitbody is shelf-like, it starts out as a roughly hemispherical outgrowth from its woody substrate. At that stage there are no differentiated upper and lower surfaces. Over time the fruitbody expands, becomes shelf-like and produces distinctly different upper and lower surfaces. Yours has gotten to the stage where the pores on the underside have started developing.