The last two decades have witnessed great advances in reconstructing the transition from non-avian theropods to avians,but views in opposition to the theropod hypothesis still exist.Here we highlight one issue that is often considered to raise problems for the theropod hypothesis of avian origins,i.e.the "temporal paradox" in the stratigraphic distribution of theropod fossils - the idea that the earliest known avian is from the Late Jurassic but most other coelurosaurian groups are poorly known in the Jurassic,implying that avians arose before their supposed ancestors.However,a number of Jurassic nonavian coelurosaurian theropods have recently been discovered,thus documenting the presence of most of the major coelurosaurian groups in the Jurassic alongside,or prior to,avians.These discoveries have greatly improved the congruence between stratigraphy and phylogeny for derived theropods and,effectively,they reject the "temporal paradox" concept.Most importantly,these discoveries provide significant new information that supports the relatively basal positions of the Tyrannosauroidea and Alvarezsauroidea among the Coeluro-sauria.Indeed,they imply a new phylogenetic hypothesis for the interrelationships of Paraves,in which Archaeopteryx,the Dromaeosauridae,and the Troodontidae form a monophyletic group while the Scansoriopterygidae,other basal birds,and probably also the Oviraptorosauria,form another clade.Mapping some of the salient features onto a temporally-calibrated theropod phy-logeny indicates that characteristics related to flight and arboreality evolved at the base of the Paraves,earlier than the Late Jurassic.
The Middle-Late Jurassic transition period is a critical period for the evolution of terrestrial vertebrates,but the global fossil record from this time is relatively poor.The Shishugou Fauna of this period has recently produced significant fossil remains of dinosaurs and other vertebrate groups,some representing the earliest known members of several dinosaurian groups and other vertebrate groups and some representing the best-known specimens of their group.These discoveries are significant for our understanding of the origin and evolution of several vertebrate lineages.Radiometric dating indicates that the fauna is aged approximately 159-164 Ma.Comparisons with other similarly-aged terrestrial faunas such as Shaximiao and Yanliao show both taxonomic similarities and differences between these faunas and indicate that the Junggar deposits might have preserved the most complete vertebrate fossil record for a Middle-Late Jurassic Laurasian terrestrial fauna.
