Four-winged dinosaurs or sexual dimorphism of dinosaurs.
In recent years, several theropod dinosaurs with pennaceous feathers attached to the metatarsus have been reported from Western Liaoning, China. The findings are considered as new evidence to support the "tetrapteryx" hypothesis. By examining hundreds of specimens of Microraptor zhaoianus, ″M. gui″, Anchiornis huxleyi and Confuciusornis sanctus, and with mathematical statistics, we find that: ( 1) ″M. gui″ is synonymous with M. zhaoianus, and M. zhaoianus is a valid name. ( 2) Only some studied specimens show clear pennaceous feathers attached to the metatarsus or tarsometatarsus. ( 3) It is surely considered as sexual dimorphism, not the so-called four-winged dinosaurs. ( 4) These species have a distant relationship with birds, and they may be evolutionarily extinct collateral branches. ( 5) During the evolution from dinosaurs to birds, there should be a "gliding" stage, rather than a "four-winged dinosaur" stage.