Geologic Era: Late Cretaceous
Location Found: Hell Creek Formation (Montana)
Size: Unknown, but probably on the larger side
Extinction Date: Unknown, but assuming dinosaur remains, 65 million years ago
I have tentatively identified this bone fragment as being a partial tail chevron. Tail chevrons acted as support for the tail itself and also as an anchor for some of the larger muscles. These structures also protected the blood vessels and nerves that ran on the underside of the tail. Chevrons were one of the reasons dinosaurs could hold their tails lateral to the ground, instead of letting them drag.
This fragment is a little over 3 inches long, and would have been from about the middle of the bone. The dip at the far right side in the middle would be where the bone would have split into the characteristic Y shape of a chevron. The bone also begins to narrow off at the far left side, which I would also expect from a chevron. If this bone were complete, I would estimate it would be between 9-12 inches long.
If this estimate is correct, this bone came from a rather large animal such as Edmontosaurus, Triceratops, or Tyrannnosaurus. Identifying isolated bones, especially fragments, is very difficult, but I can at least narrow down the species to the larger ones.
Full Tail Chevron: https://vmnhpaleontology.wordpress.com/2008/05/07/sauropod-chevron/
Chevron Location: “Herrerasaurusskeleton” by Zach Tirrell from Plymouth, USA – Dino origins. Licensed under CC BY-SA 2.0 via Commons – https://commons.wikimedia.org/wiki/File:Herrerasaurusskeleton.jpg#/media/File:Herrerasaurusskeleton.jpg