Choyrodon barsboldi is an iguanodontian ornithopod from the Albian Khuren Dukh Formation of Mongolia. Three partial skeletons preserve skulls and postcranial bones, and bone histology indicates that the animals were still growing. Their anatomy combines a relatively open antorbital region with more derived features of the jaws and nasal opening. This unusual mixture helps distinguish the genus while making adult size and proportions uncertain.
Quick facts
| Scientific name | Choyrodon barsboldi Gates et al., 2018 |
|---|---|
| Group | Ornithopoda; Iguanodontia |
| Age | Albian, Early Cretaceous |
| Formation | Khuren Dukh Formation |
| Locality | Khuren Dukh, Mongolia |
| Holotype | MPC-D 100/801, partial skull and postcranial skeleton |
| Other specimens | MPC-D 100/800 and MPC-D 100/803 |
| Growth | All described specimens interpreted as subadult |
Three partial skeletons from Khuren Dukh
Terry Gates and colleagues named Choyrodon barsboldi in 2018. The holotype, MPC-D 100/801, includes a partial skull and associated body bones. Two additional specimens, MPC-D 100/800 and MPC-D 100/803, preserve overlapping skull and postcranial material. The fossils came from Khuren Dukh in the Gobi Desert. Their formation is assigned to the Albian stage of the Early Cretaceous, approximately 113–100 million years ago, a broad interval rather than a precise date for an individual skeleton.
The name combines Choyr, a nearby town, with a Greek-derived “tooth” ending. The species name honours Mongolian palaeontologist Rinchen Barsbold. Histological examination of sampled bones found tissue and growth patterns consistent with subadult animals. The three specimens therefore document a young growth stage, not the maximum body size of the species.
A skull with both open and specialised features
The describing team diagnosed Choyrodon from a suite of characters in the maxilla, nasal, lacrimal, opisthotic, predentary and surangular. One striking feature is an open antorbital fenestra, an opening in front of the eye that is retained in more primitive ornithopods. At the same time, the lower jaw turns downward toward the front and the narial opening is enlarged, features associated with more derived iguanodont anatomy.
A mix of traits is not a ladder showing that one species was halfway to another. It is a character combination that can be assessed in a phylogenetic analysis. The original paper tested two character matrices. Neither placed Choyrodon as the sister species of, or as more primitive than, Altirhinus kurzanovi, even though both occur in the same formation. Some analyses recovered a close relationship with Eolambia, but the exact position remains sensitive to the limited and incomplete fossil record.
The jaw and cheek teeth indicate a plant-processing apparatus typical of iguanodonts. They support herbivory, but no stomach contents or coprolites identify the actual plants eaten. A distinctive skull is not enough to determine a precise feeding height or a specialised diet.
Growth and the limits of body-size estimates
All three known individuals were interpreted as subadults. This matters when comparing Choyrodon with Altirhinus: a smaller fossil does not automatically represent a smaller adult species, nor does size alone show that one is a juvenile form of the other. Skull proportions change during growth, so the strongest distinctions come from characters expected to remain informative as the animal matured.
No complete adult skeleton is known, and a maximum length cannot be measured directly. The available bones show a medium-sized ornithopod body plan, but mature limb proportions, final body mass and changes in the skull remain open. The fossils also do not establish herd behaviour, nesting, skin colour or locomotor speed.
Early Cretaceous Mongolia
The Khuren Dukh Formation records a diverse terrestrial ecosystem in the Gobi region. The presence of Choyrodon and Altirhinus in the same formation provides a useful comparison of coexisting iguanodonts, but it does not prove that they fed together or occupied identical habitats. Their anatomy and the published analyses place them as distinct animals rather than growth stages of one species.
Choyrodon adds a well-documented skull to the early iguanodont record. Its value lies in the directly observed combination of skull features and in the histological evidence for immaturity. Compare it with other ornithopods in the dinosaur catalogue and with the Mongolian Altirhinus, whose own skull is known from different evidence.
Frequently asked questions
When was Choyrodon described?
Gates and colleagues named Choyrodon barsboldi in 2018 from fossils in the Albian Khuren Dukh Formation of Mongolia.
Were the known Choyrodon adults?
No. Histological evidence indicates that the described animals were subadults, so their size does not establish the adult maximum.
How did Choyrodon differ from Altirhinus?
The taxa have distinct combinations of skull characters. Phylogenetic analyses do not identify Choyrodon as a juvenile Altirhinus or as its immediate sister taxon.
What does Choyrodon’s open antorbital opening mean?
It is one anatomical feature retained in more primitive ornithopods, while other parts of the skull are more derived. The combination is used in taxonomic comparisons.