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
elongate, unfused frontal processes and no palatal processes. The two
other specimens are considered new taxa, for which we erect the names
Meemannavis ductrix gen. et sp. nov. and Brevidentavis zhangi gen. et sp.
nov. These new specimens confirm that the Changma locality is dominated by
ornithuromorph birds and contribute to a better understanding of this
important avifauna. The observed variation in dental morphology hints at
trophic diversity like that observed in ornithuromorphs from the
penecontemporaneous Jehol Group of northeastern China.