Fossil Friday!

After all the turkey, stuffing, and sweet potatoes last week, I'm a little out of sorts, not as polished as I normally am. The solution? Presenting the most polished fossil I could find!

Polished, indeed, but what organism—or organisms—left this pattern behind?  What period did it come from?

First one to correctly identify this fossil (in the comments section below), wins my glossy awe for the week.