Co-author Jerry Harris kindly sent me a PDF of his latest paper:
Abstract: Fossil footprints are important in understanding Cretaceous avian diversity because they constitute evidence of paleodiversity and paleoecology that is not always apparent from skeletal remains. Early Cretaceous bird tracks have demonstrated the existence of wading birds in East Asia, but some pedal morphotypes, such as zygodactyly, common in modern and earlier Cenozoic birds (Neornithes) were unknown in the Cretaceous.
We, herein, discuss the implications of a recently reported, Early Cretaceous (120–110 million years old) trackway of a large, zygodactyl bird from China that predates skeletal evidence of this foot morphology by at least 50 million years and includes the only known fossil zygodactyl footprints. The tracks demonstrate the existence of a Cretaceous bird not currently represented in the body fossil record that occupied a roadrunner (Geococcyx)-like niche, indicating a previously unknown degree of Cretaceous avian morphological and behavioral diversity that presaged later Cenozoic patterns.
Fig. 2 Tracks of Shandongornipes muxiai. a Map of holotype trackway (LRH-DH01). b–e Photographs and schematics of individual tracks in trackway (digit numbers labeled), including b photograph of first (left foot, LRH-dz70), c outline drawing of third (left foot, LRH-dz68), d outline drawing of fifth (right foot, LRH-dz66), and e photograph of fourth (left foot, LRH-dz67). f Segment of a trackway made by extant roadrunner G. californianus near St. George, Utah; segment includes same left–right sequence preserved in LRH-DH01 (a) and shown at same scale. g Schematic of fourth Shandongornipes print (LRH-dz67). h Individual G. californianus right footprint [from Elbroch and Marks (2001); used with permission] rotated with hallux in approximately same orientation as Shandongornipes print in (g)