The smallest known non-avian theropod dinosaur

Abstract

Non-avian dinosaurs are mostly medium to large-sized animals, and to date all known mature specimens are larger than the most primitive bird, Archaeopteryx. Here we report on a new dromaeosaurid dinosaur, Microraptor zhaoianus gen. et sp. nov., from the Early Cretaceous Jiufotang Formation of Liaoning, China. This is the first mature non-avian dinosaur to be found that is smaller than Archaeopteryx, and it eliminates the size disparity between the earliest birds and their closest non-avian theropod relatives. The more bird-like teeth, the Rahonavis-like ischium and the small number of caudal vertebrae of Microraptor are unique among dromaeosaurids and improve our understanding of the morphological transition to birds. The nearly completely articulated foot shows features, such as distally positioned digit I, slender and recurved pedal claws, and elongated penultimate phalanges, that are comparable to those of arboreal birds. The discovery of these in non-avian theropods provides new insights for studying the palaeoecology of some bird-like theropod dinosaurs.

A short-armed dromaeosaurid from the Jehol Group of China with implications for early dromaeosaurid evolution

“…As one of the most bird-like groups, they share numerous derived similarities with birds, including small size, a feature considered critical for the origin of avian flight (Novas & Puerta 1997;Sereno 1999;Xu et al 2000;Xu & Norell 2004;Turner et al 2007b). Basal dromaeosaurids are all small in size and this is particularly true for Liaoning dromaeosaurids (Xu et al 1999(Xu et al , 2000(Xu et al , 2003Hwang et al 2002;Xu & Wang 2004;Xu 2006). So far, five dromaeosaurid taxa have been reported from the Early Cretaceous Jehol Group of western Liaoning, China, all being shorter than 150 cm in total body length.…”

“…Most phylogenetic analyses posit the Dromaeosauridae as the sister taxon to the Troodontidae, forming a monophyletic Deinonychosauria, which in turn is the sister taxon to the Avialae (Ostrom 1969;Gauthier 1986;Sereno 1999;Norell et al 2001;Xu 2002;Senter et al 2004;Senter 2007;Turner et al 2007b), although some analyses suggest that the dromaeosaurids themselves are the closest relatives of birds (Xu et al 1999;Holtz 2000). As one of the most bird-like groups, they share numerous derived similarities with birds, including small size, a feature considered critical for the origin of avian flight (Novas & Puerta 1997;Sereno 1999;Xu et al 2000;Xu & Norell 2004;Turner et al 2007b). Basal dromaeosaurids are all small in size and this is particularly true for Liaoning dromaeosaurids (Xu et al 1999(Xu et al , 2000(Xu et al , 2003Hwang et al 2002;Xu & Wang 2004;Xu 2006).…”

Adaptation to the sky: Defining the feather with integument fossils from mesozoic China and experimental evidence from molecular laboratories

“…The smallest feathered dinosaur, Microraptor Zhaoianus, has developed posterior teeth that have a less compressed crown and a constriction "waist" beneath the crown. The heterodont dentition pattern represents the transition from the non-avian theropod type of dentition to that of Mesozoic birds (Xu et al, 2000). Interestingly, an Oviraptorosaurian, Incisivosaurus, had more diverse tooth types than other feathered dinosaurs.…”

“…One is similar to avian downy feathers, and another one similar to avian pennaceous feathers, but lacking identifiable barbules. Another dinosaur, the smallest known non-avian theropod dinosaur, Microraptor zhaoianus (Xu et al, 2000), displayed a more advanced filament pattern near the femur. The filaments are long and contain a rachis.…”

Confusing dinosaurs with mammals: Tetrapod phylogenetics and anatomical terminology in the world of homology

“…Thus, it was consequently equally simple to decide where and where not to apply NAA terminology. With the discovery of very bird-like, though phylogenetically nonavian, theropods (Burnham et al, 2000;Xu et al, 2000Xu et al, , 2003Norell et al, 2002), the systematic line blurred, necessitating an arbitrary definition of bird (Chiappe and Witmer, 2002;Witmer, 2002). Again in parallel, it became difficult to decide where to begin and where to cease utilizing NAA nomenclature.…”