Figure 5: Evolution of ankle anatomy in early birds.

(a) Dorsal/cranial (left) and plantar/caudal (right) views of the right pes (and, more obliquely and proximally, the tibia) of Confuciusornis using scan data from Fig. 1, showing where virtual sections (blue-green planes perpendicular to bones) were taken for use in b–d. (b) Inferred distribution of the preserved tendons/ligaments, fibrocartilages, articular cartilages, cartilaginous cristae and ridges in the right ankle joint of Confuciusornis. Medial view emphasizing the wrap-around tendons of the digital flexors, associated fibrocartilage and mineralization. (c) Cross-sectional reconstructed scan data from the proximal tarsometatarsus as shown in a and used in d. (d) Drawing of histological horizontal cross-sections through the proximal end of the tarsometatarsus (or metatarsus) of squamate Iguana³⁸, pygostylian bird Confuciusornis, enantiornithine bird Pengornis, ornithuran bird Yixianornis and crown-group bird Columba³⁹ shows our hypothesis for the evolution of the hypotarsus (hy) and other derived features of avian ankles. Pink colours are tendinous/ligamentous tissue; white is bone; grey is muscular tissue and yellow is (fossilized) fibrocartilage. From left to right, as the number of main toes was reduced and the pes narrowed and consolidated, becoming more robust, it became more plantarly projected (that is, a larger hypotarsus) and formed fibrocartilages, mineralizations and ultimately bony ridges and grooves to enhance the leverage of as well as enclose and guide the pedal tendons. See Figs 1 and 2 for abbreviations. Not to scale. Virtual ‘histological’ sections for the simple sketches of Pengornis (IVPP V15336) and Yixianornis (IVPP V12558) were done digitally using microCT scan data from ref. 3, at the approximate same proximodistal position as the other images, following the procedure shown in a and c.