Description and Classification

Xuwulong was a moderately sized herbivorous dinosaur, estimated to have reached about 5 meters (approximately 16 feet) in length. Like other early ornithopods, it likely moved primarily on its two hind legs (bipedal) but could also walk on all four legs when needed, perhaps when searching for food low to the ground. Its skeleton featured a relatively long neck, a long tail for balance, and a distinctively rectangular-shaped skull that measured around 38 centimeters (15 inches) in length. The skull was low and elongated, with a noticeable gap, called a diastema, between its beak-like front mouth and the battery of chewing teeth further back.

Xuwulong belongs to the group Hadrosauroidea. This group includes the famous “duck-billed” dinosaurs (Hadrosauridae) and their closest relatives. Xuwulong is considered a basal (early) member of Hadrosauroidea, meaning it lived before the more specialized duck-billed dinosaurs like Edmontosaurus or Parasaurolophus evolved and shows some of their early features. Paleontologists place it as more advanced than very early hadrosauroids such as Equijubus, but not as advanced as later forms like Probactrosaurus or the true hadrosaurids. This position helps scientists understand the step-by-step evolution towards the complex feeding mechanisms of later hadrosaurs.

Distinguishing Features

Xuwulong can be identified by several unique anatomical characteristics:

• A relatively long neck for a hadrosauroid, composed of 13 cervical (neck) vertebrae.
• A skull that is low, elongated, and somewhat rectangular in profile.
• The presence of a prominent diastema (toothless gap) in the upper jaw, separating the front beak from the cheek teeth.
• Maxillae (main upper jaw bones) that each held 18 tooth positions, contributing to its dental battery for grinding plants.
• The ilium (the largest hip bone) had a relatively straight upper margin and other specific features that differentiate it from related dinosaurs.

Paleoenvironment and Diet

Xuwulong fossils were discovered in the Xinminpu Group in the Yujingzi Basin of Gansu Province, China. During the Early Cretaceous period, this region was characterized by a floodplain environment with numerous rivers and lakes. This landscape would have supported a rich variety of plant life, forming the primary food source for herbivorous dinosaurs like Xuwulong. The climate was likely warm and relatively humid, allowing ferns, conifers, cycads, and early flowering plants to thrive.

As a hadrosauroid, Xuwulong was an herbivore. Its dental battery, with multiple rows of replaceable teeth, was well-suited for grinding and processing tough plant material. It probably fed on the abundant low-growing vegetation. Xuwulong shared its environment with other dinosaurs, including other ornithopods such as the more primitive Equijubus and another early hadrosauroid, Jintasaurus. The broader fauna of the Xinminpu Group also included sauropods, theropods, turtles, and pterosaurs, painting a picture of a diverse Early Cretaceous ecosystem.

Significance and Ongoing Research

The discovery of Xuwulong is scientifically significant because it provides crucial insights into the early evolution and diversification of hadrosauroid dinosaurs, a group that became extremely successful later in the Cretaceous. The exceptionally complete nature of its fossil skeleton is rare for early hadrosauroids and offers paleontologists a detailed look at their anatomy, helping to bridge an important gap in the fossil record, particularly in Asia, which is a key continent for understanding ornithopod evolution. Xuwulong helps to clarify the evolutionary steps leading from earlier iguanodontians to the highly specialized hadrosaurids.

Ongoing research related to Xuwulong and its relatives may include further detailed anatomical comparisons with other early hadrosauroids like Equijubus, Jinzhousaurus, and Probactrosaurus. These studies aim to refine its precise placement within the Hadrosauroidea family tree and to better understand the development of key hadrosaurian features, such as their advanced chewing mechanisms. Future discoveries of additional specimens could also shed light on its growth patterns, behavior, and the specific ecological role it played within its Early Cretaceous environment.