Description and Classification

Chilesaurus was a relatively small dinosaur, estimated to be about 3.2 meters (10.5 feet) long, though some specimens suggest it could grow larger. It walked on two legs (bipedal) and possessed a small head, a moderately long neck, and sturdy hind limbs. Its forelimbs were also robust, but its hands were unusual for a theropod, featuring only two well-developed, blunt fingers.

The classification of Chilesaurus has been a subject of considerable scientific discussion due to its mosaic of traits. While its general body plan is similar to theropod dinosaurs (the group that includes carnivores like Tyrannosaurus rex and Velociraptor), many of its other features are reminiscent of different dinosaur groups. For example, its teeth were leaf-shaped and adapted for grinding plants, similar to herbivorous ornithischian dinosaurs or sauropodomorphs, rather than the sharp, blade-like teeth of most theropods. Its pelvic structure, with a pubis bone pointing backwards, is also characteristic of ornithischian dinosaurs, even though Chilesaurus is considered a saurischian dinosaur (the group theropods belong to).

Initially, its bizarre anatomy led to uncertainty about its precise placement within the dinosaur family tree. Some studies suggested it was a very primitive theropod, while others placed it closer to the base of Tetanurae, a large clade of theropods. There was even a proposal to place it as a very early ornithischian. However, most current research supports its classification as an early diverging tetanuran theropod or possibly related to the enigmatic Megaraptora. Its uniqueness has even led to the proposal of a new dinosaur group, Chilesauridae, to accommodate it and potentially other similar forms. Chilesaurus is often described as a “Rosetta Stone” for understanding dinosaur evolution because of its unexpected mix of characteristics.

Distinguishing Features

Chilesaurus possessed a unique set of features that distinguish it from other dinosaurs:

• Leaf-shaped teeth, indicating a herbivorous diet, which is rare among early theropods.
• A relatively small skull compared to its body size, with a short snout.
• Robust forelimbs with hands bearing only two well-developed, stubby fingers, lacking sharp claws.
• A pelvis where the pubis bone is oriented backwards (opisthopubic condition), similar to that of ornithischian dinosaurs like Stegosaurus or Triceratops, and birds. This is an example of convergent evolution, as Chilesaurus is not closely related to ornithischians.
• An overall body plan combining traits from theropods (like its general build), sauropodomorphs (like some neck vertebrae features), and ornithischians (like its pelvis and teeth).

Paleoenvironment and Diet

Chilesaurus lived in what is now southern Chile during the Tithonian stage of the Late Jurassic period, around 145 million years ago. The fossils were discovered in the Toqui Formation, which represents an ancient coastal or deltaic environment. This area likely experienced volcanic activity, as indicated by associated rocks. The climate would have been temperate to subtropical. Chilesaurus shared its environment with other prehistoric life, including diplodocid sauropod dinosaurs, crocodylomorphs, and various invertebrates.

The diet of Chilesaurus was exclusively herbivorous. This is strongly supported by the shape of its teeth, which were small, spatulate (spoon-shaped), and ideal for stripping leaves or grinding plant material, rather than tearing flesh. It likely fed on low-growing vegetation such as ferns, cycads, and conifers that were present in its habitat. This dietary adaptation is unusual for a theropod of its time, as most known Jurassic theropods were carnivorous. Other herbivorous theropod groups, like the Therizinosauria, appeared much later in the Cretaceous period.

Significance and Ongoing Research

Chilesaurus is a highly significant discovery in paleontology because it dramatically expands our understanding of dinosaur diversity and evolutionary pathways. Its bizarre mix of anatomical traits challenges long-held assumptions about how dinosaur groups are defined and how specific features evolved. It demonstrates that herbivory evolved independently in theropods more times and earlier than previously thought. The “platypus-like” nature of Chilesaurus, combining features from distantly related groups, highlights the complexity of dinosaur evolution and the phenomenon of convergent evolution, where unrelated animals evolve similar traits.

Ongoing research on Chilesaurus continues to focus on several key areas. Scientists are working to refine its exact position within the theropod family tree using more comprehensive anatomical data and advanced analytical methods. Studies are also exploring how its unique combination of features evolved and what this tells us about the developmental plasticity of dinosaurs. Further fossil discoveries in the Toqui Formation and other contemporary sites could reveal more specimens of Chilesaurus or related species, providing more information about their anatomy, biology, and the ecosystem they inhabited. The find has also prompted paleontologists to re-examine other fragmentary dinosaur fossils from around the world that might represent relatives of this unusual herbivorous theropod.