Description and Classification

Dilong was a relatively small dinosaur, measuring about 1.6 to 2 meters (5.2 to 6.5 feet) in length and weighing an estimated 11 kilograms (24 pounds). Unlike its later, giant relatives, Dilong was slender and built for agility. One of its most notable features was a covering of simple, hair-like protofeathers. These were not complex flight feathers but rather filamentous structures, similar to those found on other small theropods like Sinosauropteryx. This discovery provided strong evidence that feathers were present in early tyrannosauroids.

The skull of Dilong was relatively large for its body size, a common trait in tyrannosauroids, but it was more lightly built than the massive skulls of later giants like Tyrannosaurus rex. It possessed D-shaped teeth at the front of its upper jaw, a characteristic feature of tyrannosaurs, though its teeth were generally more slender. Dilong also had relatively long arms with three-fingered hands, contrasting with the very short, two-fingered arms of later tyrannosaurids. Dilong is classified as an early and primitive member of the superfamily Tyrannosauroidea. It helps show that the earliest members of this group were small, feathered predators. It shares some characteristics with other early tyrannosauroids such as Guanlong and Eotyrannus, and also with the family Proceratosauridae, to which it is sometimes assigned. Its features provide a link between earlier coelurosaurs and the later, more specialized tyrannosaurids.

Distinguishing Features

• Relatively small size for a tyrannosauroid, significantly smaller than later relatives like Albertosaurus or Tyrannosaurus rex.
• A body covering of simple, filamentous protofeathers, indicating that feathers were an early trait in the tyrannosaur lineage.
• Relatively long arms for a tyrannosauroid, equipped with three-fingered hands.
• A more primitive and lightly built skull compared to the robust skulls of advanced tyrannosaurids.
• Presence of D-shaped teeth in the premaxilla (front of the upper jaw), a hallmark of tyrannosaurs.

Paleoenvironment and Diet

Dilong lived in what is now Liaoning, China, during the Early Cretaceous. This region, known for the Yixian Formation, was a vibrant ecosystem with a temperate climate, forests, lakes, and active volcanoes. The volcanic activity in the area often led to exceptional preservation of fossils, making it a rich source of information about life during this time. Dilong shared its environment with a diverse array of other feathered dinosaurs, including compsognathids like Sinosauropteryx, oviraptorosaurs like Caudipteryx, and dromaeosaurs like Microraptor. Early birds such as Confuciusornis, various pterosaurs, small mammals, fish, and insects also inhabited this ancient landscape.

As a carnivore, Dilong likely hunted small animals. Its diet probably included small dinosaurs, lizards, early mammals, and perhaps fish. Its agile build and relatively long arms would have made it an effective predator of smaller prey in the dense undergrowth of its forested habitat.

Significance and Ongoing Research

The discovery of Dilong was highly significant because it was one of the first tyrannosauroids found with direct evidence of feathers. This finding strongly suggested that feathers were widespread among tyrannosauroids, and that even larger members of the group, like Tyrannosaurus rex, might have had feathers at some stage in their lives, possibly when young. Dilong provides crucial insights into the early evolution of tyrannosaurs, demonstrating their small, feathered, and agile origins before some lineages evolved into giant apex predators. It highlights the incredible diversity of feathered dinosaurs during the Early Cretaceous.

Ongoing research continues to explore various aspects of Dilong. Scientists are interested in further studying the structure and function of its protofeathers. Refining its exact placement within the tyrannosauroid family tree, through comparisons with other early tyrannosauroids like Yutyrannus and Proceratosaurus, is also an active area of study. Understanding its growth patterns and how it interacted with other species in the unique Yixian ecosystem remains a key focus for paleontologists.