Description and Classification

Hulsanpes was a very small dinosaur, estimated to be only about 40 to 50 centimeters (16 to 20 inches) long, making it one of the smallest known non-avian dinosaurs. Its known fossils consist mainly of parts of the metatarsals (foot bones) and some tarsal bones. These bones suggest a delicate and lightly built animal.

Originally, its classification was uncertain due to the limited fossil material. However, with the discovery of related dinosaurs like Halszkaraptor, Hulsanpes is now considered a member of the Dromaeosauridae family, specifically within the subfamily Halszkaraptorinae. This group is unusual among dromaeosaurids as its members, such as Halszkaraptor and Mahakala, are thought to have possessed adaptations for a semi-aquatic lifestyle, possibly resembling modern waterfowl in their habits. Hulsanpes shares features of its foot bones with these relatives.

Distinguishing Features

Since Hulsanpes is known from limited foot bones, its distinguishing features are primarily related to this part of its anatomy:

• It possessed long and slender metatarsals, suggesting it was agile.
• The second metatarsal (the bone in the foot that connects to the second toe) was the longest and most robust of the preserved elements, but it did not show the highly specialized joint for a large, retractable “sickle claw” seen in many other dromaeosaurids like Velociraptor. This suggests a different foot mechanism or hunting style compared to its more famous relatives.
• The overall structure of its foot bones aligns it with other halszkaraptorine dinosaurs.

Paleoenvironment and Diet

Hulsanpes lived in the area that is now the Barun Goyot Formation in Mongolia. During the Late Cretaceous, this region was characterized by an arid to semi-arid climate with sand dunes, intermittent streams, and small lakes. This environment supported a variety of other dinosaurs, including protoceratopsians like Bagaceratops, ankylosaurs, and other small theropods.

Given its classification as a halszkaraptorine and its small size, Hulsanpes likely fed on small animals. If it shared the semi-aquatic tendencies of its relatives, its diet might have included small fish, aquatic insects, and crustaceans found near water sources. If it was more terrestrial, it could have hunted small lizards, mammals, and insects on land. Its small size would have placed it in a different ecological niche than larger predators of its time.

Significance and Ongoing Research

Hulsanpes is significant because it represents one of the earliest recognized members of the unique Halszkaraptorinae group, even though its full importance was not understood until more complete relatives were discovered. It contributes to our understanding of the diversity of small theropod dinosaurs during the Late Cretaceous and highlights the varied adaptations within the Dromaeosauridae family.

Due to the fragmentary nature of its fossils, much about Hulsanpes remains speculative. Ongoing research focuses on comparing its anatomy with more complete halszkaraptorines like Halszkaraptor to better infer its lifestyle and characteristics. Future discoveries of more complete Hulsanpes specimens would be crucial for confirming its potential semi-aquatic adaptations and understanding its place within the ecosystem. Studying Hulsanpes helps paleontologists trace the evolutionary path of these unusual, potentially water-loving dromaeosaurs.