I'm leaning natural as well, but I'll be danged if I can really figure out how it looks the way it looks, in other words, what natural processes actually resulted in that appearance. It's a strange looking rock for sure, and I would have picked it up to puzzle over as well. I'm familiar with the type of fossils found in Ma., and I don't believe it's any kind of fossil. I do believe it's natural, and not artifactual, however.