An extra leaf?

I found this little plant March 15 growing on a west-facing hillside in the woods. It is a springtime woodland wildflower native to North America called a trillium. There are many kinds and colors of North American trilliums. The one most common in Southeast Missouri has a purple bloom.

Trilliums, as well as many other kinds of flowers, have small leaf-like outgrowths called sepals immediately beneath the flower. Trilliums have three oversized sepals that extend from a single stem. The trillium on the left is a surprise of nature. It has a fourth sepal sticking straight up.

I hope to go back to the place I found this trillium in a week or so to see if the flower, which normally has three purple petals, will have four.