14mm long, and as it's symmetrical (as near as dammit, both inside and out) it really has to be from the midline of something. Individual hip bones aren't symmetrical, and it certainly doesn't look to me remotely like something from the front of the chest of anything, so it's skull or a bit of vertebra. The front and sides taper off nicely in cross section so it doesn't look like it's broken off anything, except perhaps at the back. The author says it looks just like what parrot lower jaws look like, with all the external holes and internal channels in the right place, which also distinguish it from lower jaws of other dinos and turtles. He suggests maybe a 'lory'.
Weight for weight it must be the most significant fossil of its kind. Presumably we now have enough confidence in our calibration of molecular dating to confirm that modern birds diverged in the Cretaceous; we can also get an idea of just how good small arborial forms were at hiding from us!
Stidham Thomas, A. "A Lower Jaw from a Cretaceous Parrot" Nature v 396 pp29-30 5 Nov 98.