What it is
The origin and early diversification of birds remain heavily debated in vertebrate evolution. Estimates of the evolutionary timescale suggest a radiation during the Jurassic period, but fossils from that period are so scarce that how the distinctive bird body plan was assembled has stayed unclear. Selection for powered flight has been seen as the main driver of the early skeletal changes in avialans, the group containing birds and their closest relatives, which fits the way aerodynamic features arose independently in several paravian lineages. The authors report a Late Jurassic avialan from the Zhenghe Fauna of southeastern China, a new genus and species they name Yuezhuornis yangyuanensis. As one of the earliest diverging avialans, it shows a striking mosaic of features: cranial characters that mix avialan and troodontid traits, paired with derived postcranial traits and a hyperextensible second toe of the kind seen in deinonychosaurs. Phylogenetically informed analyses show that it has markedly short forelimbs, a condition widespread among non-avialan theropods. The authors read these findings as strong evidence that Yuezhuornis yangyuanensis lived on the ground and could not fly, and they conclude that several divergent ecological pathways were being exploited in the initial diversification of birds.
Why it matters
A newly described Late Jurassic bird relative from southeastern China had markedly short forelimbs and lived on the ground, the authors conclude, rather than flying. Powered flight has been treated as the main driver of the early skeletal changes in avialans, the group containing birds and their closest relatives, and Jurassic fossils are scarce enough that how the bird body plan came together has stayed unclear. Yuezhuornis yangyuanensis is one of the earliest diverging avialans and mixes avialan and troodontid skull traits with derived postcranial ones and a hyperextensible second toe like a deinonychosaur's, which the authors read as several divergent ways of life being tried as birds first diversified.
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Filed underPaleontology and Evolutionary Biology, Evolution and Paleontology Studies, Fossil Insects in Amber
