That's right! These fossils preserve the hardest parts of the crinoids—the stem. As stem plates decayed, they were slowly replaced by minerals like quartz, pyrite or calcite.

What other kinds of fossils are there? Click to find out!

Fossilized hard parts are the most plentiful kind of fossil. Shells, bones, and teeth are hard enough to be preserved in this way, so they are the most common fossils to find.

Of course, you never find crinoid fossils that look this good just sitting in a fossil field! The crinoids in the museum's collection have been carefully cleaned and preserved by skilled technicians.

Here are several rocks that still look as they did when found in the field. Which of these contains a crinoid fossil?