title: On the absence of sternal elements in Anchiornis (Paraves) and Sapeornis (Aves) and the complex early evolution of the avian sternum.
abstract: Anchiornis (Deinonychosauria: Troodontidae), the earliest known feathered dinosaur, and Sapeornis (Aves: Pygostylia), one of the basalmost Cretaceous birds, are both known from hundreds of specimens, although remarkably not one specimen preserves any sternal ossifications. We use histological analysis to confirm the absence of this element in adult specimens. Furthermore, the excellent preservation of soft-tissue structures in some specimens suggests that no chondrified sternum was present. Archaeopteryx, the oldest and most basal known bird, is known from only 10 specimens and the presence of a sternum is controversial; a chondrified sternum is widely considered to have been present. However, data from Anchiornis and Sapeornis suggest that a sternum may also have been completely absent in this important taxon, suggesting that the absence of a sternum could represent the plesiomorphic avian condition. Our discovery reveals an unexpected level of complexity in the early evolution of the avian sternum; the large amount of observable homoplasy is probably a direct result of the high degree of inherent developmental plasticity of the sternum compared with observations in other skeletal elements.
authors.affiliation: Institute of Geology and Paleontology, Linyi University, Linyi, Shandong 276000, China; Shandong Tianyu Museum of Nature, Pingyi, Shandong 273300, China; and.
authors.forename: Xiaoting
authors.initials: X
authors.lastname: Zheng
authors.affiliation: Key Laboratory of Vertebrate Evolution and Human Origins of the Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China zhouzhonghe@ivpp.ac.cn jingmai.oconnor@gmail.com.
authors.forename: Jingmai
authors.initials: J
authors.lastname: O'Connor
authors.affiliation: Institute of Geology and Paleontology, Linyi University, Linyi, Shandong 276000, China;
authors.forename: Xiaoli
authors.initials: X
authors.lastname: Wang
authors.affiliation: Key Laboratory of Vertebrate Evolution and Human Origins of the Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China.
authors.forename: Min
authors.initials: M
authors.lastname: Wang
authors.affiliation: Shandong Tianyu Museum of Nature, Pingyi, Shandong 273300, China; and.
authors.forename: Xiaomei
authors.initials: X
authors.lastname: Zhang
authors.affiliation: Key Laboratory of Vertebrate Evolution and Human Origins of the Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China zhouzhonghe@ivpp.ac.cn jingmai.oconnor@gmail.com.
authors.forename: Zhonghe
authors.initials: Z
authors.lastname: Zhou
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001717:Birds / Q000033:anatomy & histology / Q000502:physiology*; D010012:Osteogenesis; D013249:Sternum / Q000033:anatomy & histology / Q000502:physiology*
keywords: Jehol; Maniraptora; Mesozoic; flight; histology