[Youngblood]

"The degradation process coated the animal with a thin veneer of pyrite, also known as 'fool’s gold,'

This answers a problem I was meaning to investigate. Last week I found a fossilized bivalve or brachiopod (couldn't tell which since it was only partly exposed) in some limestone. It was coated in a golden substance, presumably pyrite. I was wondering how this might have happened.

[Bernard Marx]

Was just the shell preserved? I have some very interesting pyritized ammonite psueomorphs from Germany (whole shells only). They occur in limestone concretions and have obviously been cleaned and polished with the sandblasting method described in the article. They've been cracked open carefully so several show the ammonite "mold" in which the pyrite formed.