Duonychus tsogtbaatari

Duonychus tsogtbaatari was a peculiar therizinosaur that lived during the Late Cretaceous period, approximately 90 million years ago, in what is now Mongolia. It was discovered in the Bayanshiree Formation, a fossil-rich deposit preserving a unique assemblage of theropods, ornithopods, and other dinosaurs.

The genus name Duonychus means “two-clawed”, referring to its highly unusual didactyl (two-fingered) hands, a feature previously unknown in therizinosaurs. The species name tsogtbaatari honors Tsogtbaatar Khishigjav, a Mongolian paleontologist who has contributed extensively to the study of Cretaceous vertebrates in Mongolia.

This discovery is particularly important because therizinosaurs are typically known for their three-fingered hands, and Duonychus challenges previous assumptions about their evolutionary trajectory. It also provides critical insights into digit loss in theropod dinosaurs, showing that different groups evolved similar adaptations independently.

Physical Characteristics

Duonychus tsogtbaatari was a medium-sized therizinosaur, estimated to reach 3–4 meters (10–13 ft) in length and weighing approximately 250–300 kg (550–660 lb). While many therizinosaurs are known for their bizarre, pot-bellied appearance, Duonychus exhibits several unique anatomical traits that set it apart from its relatives.

The most distinctive feature of Duonychus tsogtbaatari was its two-fingered hand, making it the only known therizinosaur to exhibit digit III reduction and loss. Instead of the typical three-fingered grasping hands, Duonychus had only two functional, enlarged digits, with a massive claw on digit I.

Additional key features include:

• A deep, robust lower jaw, indicating a strong bite.
• Large neural spines on the vertebrae, suggesting well-developed back muscles.
• Stocky hind limbs, likely supporting a slow-moving, herbivorous lifestyle.
• Relatively small skull compared to body size, a common trait among therizinosaurs.

While many therizinosaurs evolved large bellies for processing plant material, Duonychus appears to be somewhat less extreme in its skeletal proportions, hinting at potential differences in diet or behavior compared to more derived forms like Therizinosaurus.

Diet and Feeding Habits

Like other therizinosaurs, Duonychus tsogtbaatari was a herbivore, despite being a member of the theropod lineage, which is typically carnivorous. It likely fed on:

• Ferns and cycads, which were abundant in its environment.
• Conifer needles and branches, stripped using its beak and large claws.
• Possibly soft fruits or seeds, though direct evidence of this remains unknown.

The function of its two-fingered hands is still debated. Its enlarged claw on digit I may have been used for:

• Pulling down branches to access high-growing vegetation.
• Defensive strikes against predators like Achillobator or Bagaraatan.
• Digging or grasping, though its exact functionality is still uncertain.

While later therizinosaurs developed more extreme adaptations for herbivory, Duonychus represents a transitional stage, showing how these dinosaurs diversified in their feeding strategies.

Habitat and Distribution

Duonychus tsogtbaatari was discovered in the Bayanshiree Formation, a geological unit in Mongolia dating to the Cenomanian–Turonian stages of the Late Cretaceous.

At the time, this region was characterized by:

• Semi-arid floodplains, with seasonal rainfall and water sources.
• Sparse forests, dominated by conifers and ferns.
• A diverse dinosaur fauna, including both predators and herbivores.

Dinosaurs that lived alongside Duonychus included:

• Achillobator, a large dromaeosaurid predator.
• Bagaraatan, a smaller, possibly omnivorous theropod.
• Ornithopods, which likely competed with therizinosaurs for plant resources.

The presence of large predators suggests that Duonychus may have relied on its claws and size for defense, rather than speed or agility.

Behavior and Social Structure

Although no direct evidence of Duonychus tsogtbaatari’s behavior exists, comparisons with other therizinosaurs suggest that it may have:

• Lived a solitary or small-group lifestyle, browsing in forested environments.
• Used its claws for both feeding and defense, an adaptation seen in later therizinosaurs.
• Had slow but powerful movements, relying on its bulky frame for deterrence against predators.

Therizinosaurs are often depicted as docile herbivores, but their massive claws could have been effective defensive weapons, similar to how modern sloths use their claws when threatened.

Discovery and Research

The fossils of Duonychus tsogtbaatari were discovered in the Bayanshiree Formation, Mongolia, and were formally described in 2025 by a team of paleontologists in iScience.

The holotype specimen (MPC-D 102/110) consists of:

• A well-preserved manus (hand), showing the loss of digit III.
• Partial lower jaw, providing insight into its feeding mechanics.
• Vertebral fragments, helping reconstruct its posture and musculature.

The discovery of Duonychus is particularly significant because it challenges traditional views of therizinosaur evolution, showing that digit loss occurred independently in multiple theropod groups, including:

• Tyrannosaurids (which reduced their fingers from three to two).
• Alvarezsaurs (which lost most of their digits, leaving only a single functional claw).
• Ornithomimosaurs (which also show digit reduction in some species).

This makes Duonychus a crucial find for understanding convergent evolution in theropod limb reduction.

Significance and Interesting Facts

• First known therizinosaur with only two functional fingers, making it a major evolutionary outlier.
• Discovered in the Bayanshiree Formation, expanding our knowledge of therizinosaur diversity in Mongolia.
• Provides new evidence of digit reduction in theropods, showing that multiple groups evolved similar limb adaptations independently.
• May have used its large claw for feeding, defense, or display, though its exact function remains uncertain.
• Challenges previous assumptions about therizinosaur evolution, suggesting greater ecological diversity than previously thought.

Conclusion

Duonychus tsogtbaatari was a medium-sized therizinosaur that roamed Late Cretaceous Mongolia, showcasing a highly unusual adaptation: the loss of its third finger, making it the first known therizinosaur with only two functional digits. Its discovery provides critical insights into theropod evolution, particularly in how different dinosaur groups evolved digit reduction independently.

While Duonychus may not have been as massive as Therizinosaurus, its unique hand structure and transitional features make it one of the most important new discoveries in therizinosaur research.