Figure 6: Sim1 and flight feather evolution.

(a) Evolution of flight feathers. Character optimization on a calibrated phylogeny indicates that flight feathers (remiges and rectrices) evolved at the base of the Pennaraptora, a clade including oviraptorosaurs, dromaeosaurs, troodontids, and birds, about 170 million years ago. The arrow around the Ornithuromopha branch indicates the estimated time when the Sim1 ASHCE became conserved (144.0 Myr ago). This tree is modified from the Fig. 3 in Foth et al⁴². (b–d) The expression of Sim1 in the tail of chicken embryo at HH28 (b), HH32 (c) and HH35 (d). (e–g) The expression of Sim1 in the tail of gecko embryo at 15 dpo (e), 19 dpo (f) and 23dpo (g). Black and white arrowheads indicate the expression of Sim1 in the lateral side of the tail and the cloaca, respectively. Scale bars, 1 mm. Drawing of an eagle in a is reproduced from ref. 10 with permission. All other drawings are original by the co-authors.