During the dinosaurs-birds transition, the most conspicuous morphological change is the abbreviation of the long bony tail. Due to the lack of transitional fossils, little is known about how that modification took place. In living birds, the tail ends in a compound ploughshaped element termed pygostyle that attaches the fan-shaped tail feathers, forming an indispensable flight apparatus. Based on comparative anatomical, histological and electrical scanning analyses, Wang et. al. suggested that the plough-shaped pygostyle and the tail fanning is evolutionally decoupled in the early avian history, which challenges the long-standing theory that these two features are coevolved. A plough-shaped pygostyle is distributed widely in Ornithuromorpha. Therefore, the rare occurrence of this structure in some enantiornithines is the result of convergence, as confirmed by a few other fossil birds, further highlighting that the early avian evolution is characterized by parallelism.

bird fossil  /  evolution pygostyle  /  fan-like tail feathers