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1988 Functional Morphology of the Tail Apparatus of the Pigeon (Columba livia) [1] →ISBN 13 :The propygostylar joint of the pigeon differs from the other intercorporal joints; it is primarily a hinge joint. 2013 October 4th, Ryan N. Felice and Patrick M. O’Connor, “Ecology and Caudal Skeletal Morphology in Birds: The Convergent Evolution of Pygostyle Shape in Underwater Foraging Taxa ” in PLoS ONE IX, № 2: e89737 (publ. 26th February 2014), § 2: ‘Materials and Methods’, sub-§ 2.2: «Skeletal Morphology and Analytical Approaches»:
These metrics were collected at three serial positions within the caudal vertebral series. The first (i.e. post-synacral) free caudal vertebra, the vertebra halfway along the length of the caudal series, and the last (i.e. propygostylar ) free caudal vertebra. 2015 January 5th, Han Hu et al., “A New Species of Pengornithidae (Aves: Enantiornithes) from the Lower Cretaceous of China Suggests a Specialized Scansorial Habitat Previously Unknown in Early Birds ” in PLoS ONE X, № 6: e0126791 (publ. 3rd June 2015), § 4: ‘Results’, sub-§ 4.7: «Description», sub-sub-§ 4.7.2: ‹Vertebral Column and Ribs›:
A caudal vertebra appears partially incorporated into the pygostyle as the propygostylar vertebra; the transverse processes are still identifiable.
