Today: a little, long-legged bird from the Green River formation
Eozygodactylus americanus (Weidig 2010; Eocene; 53mya; FMNH PA726) Originally considered a perching songbird related to Passer (the sparrow), this long-legged, lakeshore wading bird (think: sandpiper), nests with Geococcyx (the roadrunner) in the large reptile tree (LRT, 1224 taxa). Like Geococcyx (the roadrunner), pedal digit 4 is also retroverted (= zygodactyl). The right (lower) femur appears to be broken in two.
As a basal taxon,
Eozygodactylus was derived from the predatory bird clade, the toothed bird clade and, more distantly, Megapodius (the mound builder). Eozygodactylus has a relatively large skull and short rostrum, which might suggest a possible juvenile stage. No related adult taxa (Figs. 4, 5) have similar proportions.
Closely related basal neognath birds include
long-legged screamers (genus: Chauna) at the base of the sparrow/chicken/parrot clade and long-legged stone curlews (genus: Burhinus) at the base of crows + jays + woodpeckers + swallows. By convergence, woodpeckers also have a zygodactylus pes. Apparently this trait comes and goes, without a single long zygodactyl lineage.
The nesting of Eozygodactylus
at the base of a basal bird clade supports the antiquity of long-legged shore birds (Figs. 4, 5) in the evolution of birds, distinct from traditional and DNA tree topologies.
Weidig I 2010. New Birds from the Lower Eocene Green River Formation, North America. Records of the Australian Museum 62:29-44.