Introduction

Stegouros, a newly discovered genus of ankylosaur, lived during the Late Cretaceous period around 72 to 75 million years ago in what is now southern Chile. Its name, meaning “roofed tail,” comes from its most distinctive feature — a tail weapon shaped like a macuahuitl, a type of ancient Mesoamerican sword. This unique characteristic sets Stegouros apart from other ankylosaurs, and its discovery has shed new light on the diversity of armored dinosaurs in the Southern Hemisphere.

Physical Characteristics

Stegouros was a relatively small ankylosaur, measuring about 2 meters (6.5 feet) in length and weighing around 150 kilograms (330 pounds). Unlike the massive, heavily armored ankylosaurs of the Northern Hemisphere, Stegouros had a lighter build with less extensive body armor. Its most striking feature was its tail, which was short and wide, ending in a flat, blade-like structure with seven pairs of bony protrusions. This tail weapon resembled a macuahuitl, making Stegouros unique among ankylosaurs.

Its body armor was more modest compared to its northern relatives, with small, bony plates (osteoderms) scattered across its body. These provided some protection from predators, though the tail was likely its primary defense mechanism.

Natural History and Discovery

The fossilized remains of Stegouros were discovered in the Magallanes Basin in southern Chile in 2018, with its formal description published in 2021. This discovery was significant as it represented a previously unknown lineage of ankylosaurs from the Southern Hemisphere, distinct from the better-known ankylosaur species found in North America and Asia. The find included a nearly complete skeleton, allowing paleontologists to reconstruct much of the animal’s anatomy and gain insights into its behavior and evolution.

Stegouros is believed to be a basal ankylosaur, meaning it represents an early branch of the ankylosaur family tree. Its distinct tail structure suggests that Southern Hemisphere ankylosaurs followed a different evolutionary path from their Northern Hemisphere relatives.

Biosphere and Habitat

Stegouros lived in a forested, temperate environment near rivers and floodplains, where it would have grazed on low-lying vegetation. The climate in southern Chile during the Late Cretaceous was likely cooler and more humid than the environments inhabited by other ankylosaurs, which lived in drier, more arid regions. Dense forests and a diverse range of plants would have provided ample food for Stegouros, which likely fed on ferns, cycads, and other soft plant material.

The presence of large predators like abelisaurids and smaller theropods in the region would have made its tail weapon an essential tool for defense.

Diet and Feeding Behavior

As an herbivore, Stegouros primarily fed on plants. Its small, beak-like mouth and simple, leaf-shaped teeth were well-suited for cropping and chewing tough vegetation. It likely grazed on low-lying plants such as ferns, cycads, and horsetails, which were abundant in its forested habitat.

Stegouros may have foraged in groups or herds, which would have provided additional protection from predators. Its relatively small size and light armor suggest that it relied more on agility and its tail for defense rather than the heavy armor that characterized its larger relatives.

Stegouros shared its environment with a variety of other Cretaceous animals, including:

• Abelisaurids: These medium to large theropod dinosaurs were the top predators in southern South America. While Stegouros would have been too small to attract the attention of large abelisaurids, smaller members of the family might have posed a threat.
• Sauropods: Giant long-necked herbivores also roamed the region, feeding on higher vegetation that Stegouros could not reach. These large herbivores would not have competed directly with Stegouros for food.
• Small theropods: A variety of smaller, carnivorous theropods likely lived alongside Stegouros. These predators may have posed a danger, especially to juvenile Stegouros.
• Pterosaurs: Flying reptiles were common in the Late Cretaceous skies, and some species may have scavenged on the remains of dead dinosaurs, including Stegouros.

Conclusion

Stegouros is a fascinating example of the diverse evolutionary paths taken by ankylosaurs in different parts of the world. Its discovery in Chile reveals a previously unknown branch of armored dinosaurs in the Southern Hemisphere and highlights the unique adaptations that arose in isolation from their northern relatives. The distinctive tail weapon of Stegouros sets it apart from other ankylosaurs, offering a glimpse into the wide variety of defensive strategies employed by these prehistoric animals.