Making a parrot zygodactyl foot: Osteology and morphogenesis of the tarsometatarsus in the monk parakeet (Myiopsitta monachus).

The tarsometatarsus conformation and foot types in birds are unique traits within vertebrates. We investigate how the tarsometatarsus and the zygodactyl foot are formed during development in the monk parakeet (Myiopsitta monachus). Using bones, whole mount specimens stained for cartilage and bone, and histological sections, we focus on the osteology and morphogenesis of the tarsometatarsus. We also compare the tarsometatarsus development between the altricial monk parakeet with the precocial chicken. The results and conclusions we reached are: (1) the hypotarsus, a character of phylogenetic significance, is monocanaliculate in the adult; (2) digit I retroversion is consequence of the displacement of the articulation site of the metatarsal 1 and its torsion; (3) digit IV retroversion is linked to the development of the trochlea accesoria; (4) in ovo, the ossification and fusion of the metatarsals 2-4 begin in their mid-diaphysis and extends cylindrically to both proximal and distal directions; and (5) the differences in the development of the tarsometatarsus between the monk parakeet and the chicken evidence heterochronies, probably related with their different types of development.