In a find that could change the perception of dinosaurs, scientists have unearthed a smaller cousin of Tyrannosaurus rex named Duonychus tsogtbaatari from southern Mongolia. Unusual among therizinosaurs, Duonychus had only two clawed fingers, rather than three. This extraordinary discovery exemplifies the unexpected diversity of therizinosaurs. It is the fifth occurrence of a major theropod dinosaur clade having lost its third digit to evolutionary fate.
While drilling a water pipeline through the formation, workers accidentally uncovered the fossil of Duonychus. This incredible discovery took place in Ömnögovi Province, Mongolia. This incredible dinosaur was a bipedal herbivore. With its enormous claws, it was part of the therizinosaur group that flourished in what is now Asia and North America during the Late Cretaceous Period. A translation of Duonychus tsogtbaatari, meaning “two digits” in Greek, was given in honor of Mongolian paleontologist Khishigjav Tsogtbaatar.
"Therizinosaurs are already some of the most unusual dinosaurs out there, but Duonychus tsogtbaatari takes it to another level," stated Yoshitsugu Kobayashi.
The interpretation of Duonychus improves our evolutionary picture of how theropod hands evolved and adapted to new functions. Its claw’s shape and curvature suggest a “hook-and-pull” motion. Our depiction of this theropod incorporates a prehensile grasp, using its long fingers to pluck out branches and pull food close for munching. This adaptation may have improved its efficiency as a forager and helped it to find food more effectively.
The exquisitely preserved claw of Duonychus includes a keratinous sheath, a rarely seen characteristic in medium- to large-sized theropod dinosaurs. This idiosyncratic anatomy lends credence to the theory. It proposes that the loss of the third finger resulted in improved prehensile function for the grasping ability of the other digits.
"This newly discovered species from Mongolia breaks the mold with just two fingers instead of the typical three, offering a rare glimpse into how theropod hands evolved and adapted," Kobayashi explained.
The peculiar evolutionary trajectory of Duonychus underscores a much more important transition that occurred within theropods. The disappearance of the third finger in this species reflects a surprising and previously undocumented evolutionary trajectory. A trend that has been seen in other theropod groups. The research authors propose that Duonychus originated from ancestors with three fingers. In order to better suit its surroundings, this animal adapted by reducing one finger.
"While more fossils would help confirm this, all signs point to Duonychus representing a true evolutionary shift, not just a fluke," noted Yoshitsugu Kobayashi.
This major find not only reinvents the anatomical story of therizinosaurs, but also theropod evolution generally. Researchers emphasize the importance of continued exploration and excavation to further uncover the diversity and evolutionary history of dinosaurs like Duonychus.
