Cladistic matrix targeted at resolving relationships between fossil birds from the Changma locality of the Lower Cretaceous Xiagou Formation

We describe six specimens consisting of cranial remains and associated partial presacral axial series belonging to ornithuromorph birds from the Changma locality of the Lower Cretaceous Xiagou Formation of northwestern Gansu Province, China. Comparison among specimens is limited by the paucity of overlapping elements, their differing exposed views, and, in some specimens, poor preservation. Despite this, three separate taxa are represented, evidenced by differences in their dentary dentitions: one specimen is edentulous, another has sharp, closely spaced, relatively high-crowned and peg-like teeth, and a third preserves blunt, relatively low-crowned teeth placed in a communal groove, a morphology previously reported among adult birds only in Hesperornithiformes. We propose that the high-crowned specimen may be referred to Gansus yumenensis based on shared similarities with the closely related Iteravis huchzermeyeri, including a very similar dentition and an edentulous premaxilla with el...