Figure 9. Plot of the number of vertebral units throughout chicken development and avialan evolution. As individual vertebrae fuse throughout ontogeny, the number of segments within the axial columns decreases. Starting with 45 individual segments at stage 28, adult chickens have a total of only 23 segments. Avialan evolution is plotted as a phylogeny with representative taxa plotted with their respective vertebral units. Included taxa are Mi, Microraptor; Ar, Archeopteryx; Je, Jeholornis; Co, Sa, Sapeornis; Lo, Longipteryx; Pa, Parapengornis; Or, Orienantius; Pr, Protopteryx; Pi, Piscivoravis; Ap, Apsaravis; He, Hesperornis; Ba. Baptornis; Ca, Calciavis; Dr, Dromaius. Silhouettes for Microraptor and Archaeopteryx are from Dececchi & Larsson (2013), Piscivoravis from www.phylopic.org, and Longipteryx and Hesperornis are new.