What key skeletal features of *Xiongguanlong* allow tracking of later tyrannosauroid evolution?

The reduction in the number of fingers or structure of nasal bones

*Xiongguanlong*'s relatively complete skeleton is invaluable because it retains primitive characteristics that later evolved into defining traits of giant tyrannosaurs. Specifically, studying its nearly complete remains allows scientists to trace the origins of key features that became exaggerated later on. These include characteristics like the reduction in the total number of fingers found on the forelimbs and the specific structure of the nasal bones in the skull. By observing these features in their earlier, less exaggerated state on *Xiongguanlong*, researchers can precisely map the evolutionary steps that led toward the morphology seen in giants like *T. rex*.