Sinornithosaurus millenii

The original fossils of Sinornithosaurus millenii, described in 1999, preserved filamentous integumentary structures around the skeleton, making it one of the first dromaeosaurids found with clear evidence of feather-like coverings. This discovery from the Liaoning deposits of China helped confirm that feather-like structures were present in non-avian dinosaurs long before the origin of modern birds.

Sinornithosaurus millenii was described in 1999 by the Chinese paleontologist Xu Xing and colleagues, based on fossils from the Early Cretaceous Yixian Formation in Liaoning Province, China. The species name "millenii" was chosen to mark the approaching millennium.

A 2009 study proposed that grooved teeth and features of the skull might indicate Sinornithosaurus was venomous, using its fangs to deliver toxins to prey. However, this hypothesis has been controversial and several subsequent studies have challenged the interpretation, arguing that the dental and cranial features do not constitute convincing evidence of a venom-delivery system.

Sinornithosaurus was a relatively small dromaeosaurid, estimated at roughly 1 meter in length and weighing only a few kilograms, making it comparable to or slightly smaller than Velociraptor. Both genera share the sickle-shaped claw on the second toe characteristic of dromaeosaurids, but Sinornithosaurus is notable for its much better-preserved evidence of feathering.