I have been talking snarge with GeschWhat, and sent her some pictures of two coprolites I have collected over the years in the Niobrara Chalk of western Kansas.  I have shown these to a few people, and nobody seems to know what caused these strange marks.  The one that is marked a lot was the first I found.  It was sticking out of a chalk cliff about 5 feet above the floor of the valley.  The marks were on the parts of it still in the chalk, so there is no way they were added after fossilization