Figure 3
From: Avian tail ontogeny, pygostyle formation, and interpretation of juvenile Mesozoic specimens
Commonalities and differences in avian caudal vertebral morphogenesis. (A–F) Morphogenesis of parapophyses (Pp) and transverse processes (Tp) occurs by epiphyseal-plate-mediated ossification, but further extension can be achieved by fusion of additional ossified elements. (A) 4.5 month old emu transverse process from a free caudal vertebra, transverse paraffin section, stained with alcian blue and picrosirius red. The morphology of distal transverse process cartilage in the emu is indistinguishable from equivalent staining in chicken parapophyses (see Fig. 2F), but these cartilages differ from the embryonic hyaline cartilage model (Fig. 2B); (B) chicken D126 proximal pygostyle transverse section, von Kossa stained, area in black rectangle magnified in (C) showing nearly complete extension of ossified transverse process by epiphyseal plate-mediated ossification that progressed in the medial to distal direction. (D) Juvenile lesser nighthawk (LACM 111218) microCT dorsal surface view, showing separate ossified elements (yellow arrowheads), analogous to ribs, that fuse into the parapophyses (white arrow) of free caudal vertebrae (pygostyle region noted by white bar). Complete fusion of these bony elements to vertebral bodies can be seen in (E) a microCT dorsal view of an adult lesser nighthawk (LACM 73857). (F–H) MicroCT dorsal views of juvenile tails with no evidence of caudal ribs; (F) juvenile red-headed woodpecker (LACM 111600), this specimen is at a similar ontogenetic stage, but shows no evidence of the rib-like bony elements. Caudal ribs are also absent in (G) a juvenile western screech-owl (LACM 111461) and in (H) a developmentally more advanced elegant tern (LACM 116139).
