Mastodon In The Backyard

Oh! Come quick! It's a marvel!
There's a mastodon in the backyard.
I was planting Zinnias in the border,
And a bone popped up out of nowhere.
I thought it was part of something very large,
Then the penny dropped and I remembered.
I thought I'd lost it forever,
You know how it is when you put something in a safe place for later....