How many pairs of pneumatic foramina do *Xiongguanlong*'s cervical centra possess compared to *Tyrannosaurus*?
A single pair, whereas *Tyrannosaurus* exhibits two pairs.
A significant point of distinction in the post-cranial skeleton involves the cervical vertebrae—the bones forming the neck. *Xiongguanlong* possesses only a single pair of pneumatic foramina, which are air openings, located on each cervical centrum. This configuration is markedly different from that found in derived, massive tyrannosaurids such as *Albertosaurus*, *Daspletosaurus*, and *Tyrannosaurus*, all of which consistently display two pairs of these openings in their cervical vertebrae. This comparison clearly illustrates that derived features did not evolve simultaneously across the entire skeleton, with *Xiongguanlong* retaining a more primitive vertebral trait alongside advanced skull characteristics.
