Description and Classification

Choyrodon was a plant-eating dinosaur that likely walked on both two and four legs depending on its activity. It was a medium-sized hadrosauroid, smaller than its later relatives like Edmontosaurus or Parasaurolophus. Adult Choyrodon probably reached lengths of around 6 to 8 meters (20 to 26 feet) and weighed a few tons. Like other ornithopods, it had a horny beak at the front of its mouth for cropping plants and complex dental batteries in its cheeks, containing many tightly packed teeth for grinding tough vegetation. Its skull showed some features that are seen in more advanced hadrosaurs, but overall, Choyrodon represents an earlier stage in their evolution.

Choyrodon is classified as a basal (early or primitive) hadrosauroid. This means it is an early member of the super-family Hadrosauroidea, the group that eventually gave rise to the true “duck-billed” dinosaurs (family Hadrosauridae). It is considered to be more advanced than very early ornithopods like Iguanodon but less specialized than the famous hadrosaurids of the Late Cretaceous, such as Maiasaura. The discovery of Choyrodon helps paleontologists understand the evolutionary steps that led to the great diversity of hadrosaurs.

Distinguishing Features

Choyrodon had several unique anatomical features that help paleontologists identify it and distinguish it from other dinosaurs, including related hadrosauroids:

• A distinct notch on the upper, rear part of its maxilla (the main tooth-bearing bone of the upper jaw).
• A particular combination of characteristics on its nasal bone (forming the snout) and the bones that surround its eye socket.
• Its teeth, while generally typical for an early hadrosauroid in having a primary ridge, showed specific details in their enamel and wear patterns that contribute to understanding the evolution of sophisticated dental batteries.
• The presence of a small opening, called an antorbital fenestra, in the skull in front of the eye socket. This opening is closed or absent in many more advanced hadrosauroids and hadrosaurids.
• Relatively large openings for nerves and blood vessels on the predentary bone (the scoop-shaped bone at the tip of the lower jaw that supported the beak).

Paleoenvironment and Diet

Choyrodon lived in what is now Mongolia during the Early Cretaceous period. The fossils were found in the Khuren Dukh Formation, which dates to the Aptian-Albian ages. The environment at that time was likely a system of floodplains, rivers, and lakes, with a climate that varied from semi-arid to temperate. This landscape would have supported a variety of plant life, including ferns, cycads, conifers, and early flowering plants. These plants would have formed the diet of Choyrodon.

As a herbivore, Choyrodon used its beak to snip off vegetation and its powerful dental batteries to chew and break down tough plant material. Other dinosaurs that lived in the same region around the same time or in similar Early Cretaceous Asian environments include other types of ornithopods, armored ankylosaurs like Shamosaurus, and various predatory theropod dinosaurs. The discovery of multiple Choyrodon individuals, including juveniles, suggests they may have lived in groups or herds, a behavior often attributed to hadrosauroids for protection and foraging.

Significance and Ongoing Research

Choyrodon is a significant discovery because it provides valuable information about the early evolution and diversification of hadrosauroid dinosaurs, a group that became extremely successful and widespread by the Late Cretaceous. The fossils of Choyrodon are relatively complete and include individuals of different ages (ontogenetic stages), allowing scientists to study how these dinosaurs grew and changed throughout their lives. As a basal hadrosauroid from Asia, Choyrodon helps to illustrate the geographic origins and early evolutionary relationships within this important dinosaur lineage.

Ongoing research on Choyrodon and its relatives continues to refine its exact position within the hadrosauroid family tree. Further anatomical studies, comparisons with other early hadrosauroids like Equijubus and Probactrosaurus, and analysis of the microscopic structure of its bones and teeth can reveal more about its biology and lifestyle. Continued exploration of the Khuren Dukh Formation and other Early Cretaceous sites in Asia may uncover more fossils, further enriching our understanding of Choyrodon and the world it inhabited, and how these early forms led to the later, more specialized hadrosaurids.