The identification of these two specimens has been bugging me for a few years now. These are the only two specimens like this that I have ever seen and they were collected from pretty much the same locality and horizon (Sonsela Member, Chinle Formation) in the Petrified Forest. They are thick bones, characterized by their triangular shape (one is missing a corner) and by a short but deep notch in what presumably is the dorsal end. It is bilaterally symmetrical, thus represents a mid-line element, and the lateral edges show possible open sutures. The photos show the tops and updersides of the two elements.
I have a working hypothesis on what I think they are (at least what bone) but I'd like to hear your opinions.