Recent fossil discoveries have substantially reduced the morphological gap between non-avian and avian dinosaurs, yet avians including Archaeopteryx differ from non-avian theropods in their limb proportions. In particular, avians have proportionally longer and more robust forelimbs that are capable of supporting a large aerodynamic surface. Here we report on a new maniraptoran dinosaur, Anchiornis huxleyi gen. et sp. nov., based on a specimen collected from lacustrine deposits of uncertain age in western Liaoning, China. With an estimated mass of 110 grams, Anchiornis is the smallest known non-avian theropod dinosaur. It exhibits some wrist features indicative of high mobility, presaging the wing-folding mechanisms seen in more derived birds and suggesting rapid evolution of the carpus. Otherwise, Anchiornis is intermediate in general morphology between non-avian and avian dinosaurs, particularly with regard to relative forelimb length and thickness, and represents a transitional step toward the avian condition. In contrast with some recent comprehensive phylogenetic analyses, our phylogenetic analysis incorporates subtle morphological variations and recovers a conventional result supporting the monophyly of Avialae.
The complex patterns of trace elements including Ir and isotope distributions in the three K/T sections of the Nanxiong Basin prove the existence of two environmental events in the latest Cretaceous and earliest Paleocene. The first geochemical environmental event occurred at about 2 Ma prior to the K/T boundary interval, where the dinosaur diversity was hardly reduced, except that a number of patho-logical eggshells appeared. The second one was larger and occurred just at and near the Creta-ceous-Paleogene (K/T) boundary. The extinction of the dinosaurs spread out within 250 ka with major extinction beginning at the boundary interval. This is even later than their extinction in Montana, North America and in India. The cause of the dinosaur extinction may be the result of a complex multiple events brought about by the coincidence of global environment change marked by multiple Ir and δ 18O anomalies, and environmental poisoning characterized by other trace elements derived from the local source. Successive short- and long-term conditions of geochemically induced environmental stress negatively affected the reproductive process and thus contributed to the extinction of the dinosaurs.
