Gansus yumenensis (Early Cretaceous; Hou and Liu 1984, You et al. 2006) is known from several fossils, none of them complete and none of them including the skull, mandibles or anterior neck. Both You et al. and the large reptile tree (LRT, 1063 taxa) nest Gansus between Hesperornis and Ichthyornis among the Mesozoic toothed birds. The long posterior ilium and long pedal digit 4 are the first traits shared with Hesperornis, appearing some 35 million years later.
The LRT used far fewer bird traits
than did You et al. And I never observed the fossil firsthand. Nevertheless the LRT was able to confirm the nesting of Gansus in You et. al.
Hou L and Liu Z 1984. A new fossil bird from Lower Cretaceous of Gansu and early evolution of birds. Sci. Sin. Ser. B. 27:1296−1302.
Li Y et al. (5 co-authors) 2011. New material of Gansus and a discussion on its habit. Vertebrata PalAsiatica 49:435–445.
You et al. (12 co-authors) 2006. A nearly modern amphibious bird from the Early Cretaceous of Northwestern China. Science 312:1640–1643.