Figure 7 | Scientific Reports

Figure 7

From: Skeletal plasticity in response to embryonic muscular activity underlies the development and evolution of the perching digit of birds

Figure 7

The evolutionary transition to a twisted Mt1.

In Intermediate forms like the early short-tailed Quiliania, Mt1 has a non-twisted proximal end, but an offset (“bent”) distal end (arrow). Because Mt1 in modern birds matures from proximal to distal, we suggest that in (1) non-Pygostylian birds, Mt1 would have matured before the onset of muscular activity; (2) Maturation of Mt1 was delayed in pygostylia, allowing its distal bending by embryonic muscular activity; (3) Ornithuromorpha further delayed maturation of Mt1, allowing full torsion of its long axis by embryonic muscular activity.

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