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Comment on the absence of ossified sternal elements in basal paravian dinosaurs

In a recent paper, Zheng et al. (1) studied the presence and absence of sternal elements within the pectoral girdle of three paravian theropods (Anchiornis, Jeholornis, and Sapeornis) from the Jehol Biota of China, which are each known from a number of specimens. Based on the consistent absence of any traces of sternal elements in Anchiornis and Sapeornis, but the frequent perseveration of soft tissues (e.g., feathers) in the sediments of the Jehol Biota, the authors conclude that sternal elements were completely absent in these taxa, even as chondral elements. Because of the lack of ossified sternal elements, Zheng et al. conclude the same for Archaeopteryx, and finally propose the complete absence of sternal elements as plesiomorphic condition for Avialae and that the sternal elements of basal birds are not homologous to those of Oviraptorosauria or Dromaeosauridae.