When looking at an Allosaurus skull in profile, specifically through the fenestra a bone can be seen sticking up inside the skull (eg http://www.dinostoreus.com/allosaurus-skull-lg.jpg). What is this bone? What is its function? And is it specific to Allosaurus? (I do not see it in Tyrannosaurus). Thank you.

The bone is the palatine: one of a pair of bones that meet along the midline of the palate and are connected along their outer edges to the side walls of the skull. They are in fact present in Tyrannosaurus, and in fact in all dinosaurs and most tetrapods. As to their function, they both form part of the roof of the palate and form part of the border around the internal nostrils on the palate, and help support various soft-tissue structures within the skull cavity. There is a long history of including a three-dimensional set of palatines within Allosaurus skulls, but many pictures and models showing theropod skulls do not depict them (often because the palate is poorly known due to the delicate anatomy of the bones here).