Sinornithosaurus, meaning “Chinese bird-lizard,” is a genus of feathered dromaeosaurid dinosaur from the early Aptian age of the Early Cretaceous Period (120 – 125 million years ago). It lived in what is now China and was the fifth non-avian feathered dinosaur discovered by 1999. It was discovered in the Jianshangou beds of the Yixian Formation, from the Sihetun locality of western Liaoning. Xu Xing, Wang Xiaolin and Wu Xiaochun, of the Institute of Vertebrate Paleontology, Beijing are credited with its discovery.

The bone beds where this specimen was found are the same beds where four other feathered dinosaurs (Protarchaeopteryx, Sinosauropteryx, Caudipteryx, and Beipiaosaurus) were previously discovered. The type specimen, IVPP V12811, is in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China.

Xu Xing described Sinornithosaurus and performed a phylogenetic analysis which demonstrated that it is primitive among the dromaeosaurs. He also showed that features of the skull and shoulder are very similar to Archaeopteryx and other Avialae (winged dinosaurs). These two facts demonstrate that the earliest dromaeosaurs were more like birds than later dromaeosaurs.

Many Sinornithosaurus specimens have been preserved with feather impressions. The feathers were composed of filaments, and showed two features that indicate they are early feathers. The first feature shows several filaments were joined together into “tufts” like the structure of down feather. The second feature shows a row of barbs that were joined together to a main shaft, making them similar in structure to normal bird feathers. However, they do not have the secondary branching and tiny little hooks that modern feathers have, which allow the feathers of modern birds to form a discrete vane.

A 2010 study indicated that Sinornithosaurus may have had feathers that varied in color across the body, based on analysis of microscopic cell structures in preserved fossils.

Sinornithosaurus has also been described by Empu Gong in 2009 as the first-identified venomous dinosaur. Gong and his colleagues noted that the unusually long and fang-like mid-jaw teeth had prominent grooves running down the outer surface, towards the rear of the tooth, a feature seen only in venomous animals. They interpreted a cavity in the jaw bone that was the possible site for the venom gland. Gong and colleagues suggested that these unique features indicated that Sinornithosaurus may have specialized in hunting small prey such as birds, using its long fangs to penetrate feathers and stun the prey with its venom, like a modern snake.

However, a team of scientists in 2010 published a paper casting doubts on the claim that Sinornithosaurus was venomous. They noted that the grooved teeth are not unique to this genus, and in fact grooved teeth are found in many other theropods and dromaeosaurids. They also noted the teeth were not as long as Gong and his team claimed, but rather had come out of their sockets, a common occurrence in crushed and flattened fossils.

Gong submitted a rebuttal of the contradictory study casting doubt on their findings. They admitted that grooved teeth were common among theropods –though they suggested they were really only prevalent among feathered maniraptorans — and hypothesized that venom may have been a primitive trait for all archosaurs if not all reptiles, which was retained in certain lineages.

Sinornithosaurus was among the smallest of dromaeosaurids with a length of about 36 inches. It was predatory and very agile. It had a sickle-shaped toe claw. It is known from at least two species: S. millenii (the type species) and S. haoiana. The latter was described in 2004 based on another specimen that differed from the type species. A well-preserved specimen, named “Dave,” has been suggested to represent a species of Sinornithosaurus, possibly a juvenile. However, phylogenetic analyses have suggested that Dave is in fact more closely related to Microraptor.