The hand of birds revealed by early ostrich embryos.

The problem of resolving the homology of the digits of the avian hand has been framed as a conflict between paleontological and embryological evidence, the former thought to support a hand composed of digits I, II, III, because of similarity of the phalangeal formulae of the earliest known bird Archaeopteryx to that of Mesozoic pentadactyl archosaurs, while embryological evidence has traditionally favored a II, III, IV avian hand. We have identified the critical developmental period for the major features of the avian skeleton in a primitive bird, the ostrich. Analysis of digit anlagen in the avian hand has revealed those for digits/metacarpals I and V, thus confirming previous embryological studies that indirectly suggested that the avian hand comprises digits II, III, IV, and was primitively pentadactyl. The identity of the digits of the avian tridactyl hand was first addressed in 1821 and has been hotly debated since then (Meckel 1821; Holmgren 1955). Early avian embryos have three central digital condensations and a condensation for a reduced digit/metacarpal V which disappears late in development; typically a condensation for digit/metacarpal I is absent. By incubating, preparing and comparing bracketed embryos of a paleognathous bird, the ostrich, we identified the critical period for digital anlagen, providing the first direct and demonstrable evidence to support previous embryological studies that implied that the avian hand comprises digits II, III, IV (Burke and Feduccia 1997; Hinchliffe 1997). A deep split between neognathous and paleognathous birds is evidenced by Lower Cretaceous fossils of the paleognaths Ambiortus and Otogornis from Mongolia and China (Kurochkin 1999). Because paleognaths are the most likely primitive living birds, their embryology presents an opportunity to test hypotheses of skeletal homology obscured by derived modifications in other taxa.