Birch polypore fungus, also known as the birch bracket or razor strop fungus, is found quite often growing on the trunks or main branches of birch trees. It’s a parasite of the trees, and it’s not unusual for the fungus to have finished the tree off and to be growing on fallen wood, as here.
The fruiting body had various uses: sharpening razors, as tinder for starting fires, and medicinal. Ötzi the iceman, whose five thousand year old frozen body was found in an Alpine glacier, was carrying some some.