Description and Classification

Aorun was a small, two-legged predator. The known specimen was a juvenile, measuring about 1 meter (3.3 feet) in length and weighing an estimated 1.5 kilograms (3.3 pounds). Its adult size is currently unknown but would have been larger. It had a slender build with long legs, suggesting it was a fast runner. Its skull was relatively small and equipped with numerous small, sharp teeth, suitable for catching small prey.

Aorun belongs to the group Theropoda, which includes most carnivorous dinosaurs, such as Tyrannosaurus rex and Velociraptor. More specifically, it is classified as a coelurosaur. Coelurosaurs are a diverse group of theropods that also includes birds. The exact placement of Aorun within Coelurosauria is still being studied. Some research suggests it might be an early member of Alvarezsauria, a group of unusual theropods known for their short, powerful arms and often a single large claw on each hand, like Mononykus. Other studies place it as a more basal (early-branching) coelurosaur, possibly related to dinosaurs like Compsognathus. Because the only known fossil is from a young animal, determining its final classification and adult features can be challenging.

Distinguishing Features

Aorun had several unique characteristics that help scientists identify it:

• A small opening on the inner side of one of the bones in its jaw joint (the quadrate).
• A specific groove on one of its lower jaw bones (the surangular) that connected with another part of the jaw.
• The very last tooth in its lower jaw was noticeably smaller than the teeth located further forward.
• Small bony bumps or projections were present on all of its neck bones, except for the atlas (the first neck bone).
• Relatively long hind limbs for its body size, indicating agility and speed.

Paleoenvironment and Diet

Aorun lived in what is now the Xinjiang region of China, in an area known as the Shishugou Formation. During the Late Jurassic, this area was a dynamic floodplain with rivers, lakes, and forests. The climate was warm with distinct wet and dry seasons. This environment supported a rich variety of life, including many other dinosaurs.

Aorun shared its habitat with large sauropods like Mamenchisaurus, other theropods including the early tyrannosauroid Guanlong and the larger predator Sinraptor, as well as early horned dinosaurs like Yinlong. The ecosystem also included turtles, crocodile-like reptiles, and flying pterosaurs. As a small carnivore, Aorun likely hunted small animals. Its diet probably consisted of lizards, early mammals, insects, and possibly the young of other dinosaurs.

Significance and Ongoing Research

The discovery of Aorun is important for several reasons. It is one of the most complete skeletons of a small theropod found from the Late Jurassic of China. This provides valuable information about the early evolution and diversity of coelurosaurs, the group that eventually gave rise to birds. If Aorun is confirmed as an early alvarezsaurian, it would offer important clues about the origins of this peculiar group of dinosaurs.

Ongoing research on Aorun focuses on further studying its anatomy to clarify its position in the dinosaur family tree. Comparing Aorun with other small theropods helps paleontologists understand its unique features and its relationships to other dinosaurs. The fact that the specimen is a juvenile also means it can provide insights into how these small theropods grew and developed. Future fossil discoveries may reveal more about Aorun’s adult appearance, behavior, and its specific role in its ancient ecosystem.