Zelenkov, N. V. 2007. The structure and probable mechanism of
evolutionary formation of the foot in piciform birds (Aves:
Piciformes). Paleontological Journal 41 (3): 290-297.
"The foot structure of piciform and coraciiform birds is examined in
detail. Certain similarity in foot structure between woodpeckers and
woodhoopoes is shown and provides the basis for the model of the
evolutionary formation of the zygodactyl foot. It is shown that
African mousebirds and the Madagascan Leptosomus discolor are
probably the closest relatives of piciforms. A brief review of fossil
groups presumably belonging to the piciform stem is provided."