Paul Hoffman's argument for Snowball Earth
So, first off, I'm just a guy with access to JSTOR and an interest in geology, so perhaps I'm not really qualified to assess "how good" the evidence is. But, I can read through the papers and summarize some stuff for you, which perhaps is sufficient to answer the question.
So, as suggested in the title, serious ...
It looks very much like a shark tooth, but it looks like it’s weathered and worn. It may have spent a good deal of time buried below the sand, being slowly ground by the sand and tide action. Eventually it became dislodged and after a while of being worn smoother it washed up and you found it.