Wel, it is made of flint, and flint comes in many an odd shape. Quite commonly, flints are shaped like short lumpy sausages. Some of those cases can be ascribed to the siliceous material filling burrows before it turned to flint. In a smaller number of those, the burrow was branched - so the resulting flint would be branched as well. I think this is probably the explanation for your specimen.
...Which is very dull compared with your pixie's foot idea!