ABSTRACT In extant turtles and lepidosaurs, the astragalus forms from a single ossification center in a tarsale proximale resulting from fusion of originally separate chondrogenic condensations in the proximal tarsus. In crocodiles, the astragalus also ossifies from a single center although the cartilaginous precursors of astragalus and calcaneum do not fuse to form a tarsale proximale. Developmental stages of the parareptilian Mesosaurus document a similar ossification pattern, as do ontogenic series of a number of fossil diapsid reptiles. There is no consistent evidence of ontogenetic fusion of a “tibiale,” intermedium, and centrale to form the astragalus of Captorhinus. The development of the amniote astragalus contrasts with variable patterns of fusion of originally separate proximal tarsal ossifications in anthracosaurian amphibians and in Diadectes. It is concluded that the reptilian (and probably also the mammalian) astragalus is a neomorph, which resulted from ontogenetic repatterning and is diagn...