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. 2018 Jun 13;8:9014. doi: 10.1038/s41598-018-27336-x

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Some Mesozoic birds formed a relatively longer pygostyle than extant birds. (A) The percent of adult pygostyle length to femur length was plotted for four different Cretaceous (red) and three different extant (orange) bird groups. The data shows great variation in relative pygostyle lengths overall, and suggests that confuciusornithiformes and enantiornithes incorporated more vertebrae into their pygostyle (n: number of specimens measured). (B) MicroCT scan of a Confuciusornis pygostyle (NGMC 98-8-2); distal to the left, dorsal to the top. Red arrows indicate intervertebral foramen from incompletely fused neural arches which theoretically correlate to the original number of pygostyle vertebrae before fusion. The break in the middle is due to a thin bone section removed for histology¹⁷, with the two pygostyle halves digitally reconstructed. Silhouettes were either taken from public domain images on phylopic.org (Protopteryx, Matt Martyniuk; Gallus, Steven Traver; and Buteo, Lauren Anderson), or drawn by D. Rashid (Confuciusornithiformes, Sapeornithiformes, Stem Ornithuromorpha, and Palaeognathae silhouettes). NGMC is the abbreviation for the National Geological Museum of China.