Scientists have named a unique, two-fingered therizinosaur species.  The fossils of Duonychus tsogtbaatari come from the Upper Cretaceous Bayanshiree Formation of the Gobi Desert (Mongolia).  This is the first time that a reduction of the digits on the manus has been recorded within the Therizinosauridae family.  All other therizinosaurs possessed three fingers on each hand.  Moreover, each finger ended in a curved claw.  Some of the claws of therizinosaurs were huge.  For example, one of the largest, Therizinosaurus cheloniformis had finger claws over a metre in length.

A life reconstruction of Duonychus tsogtbaatari. This dinosaur had two fingers on each hand. Although the fossils from which this new species has been described represent a subadult. Scientists have calculated that It weighed approximately 260 kilograms and was around three metres in length. Picture credit: Tim Bollinger.

Picture credit: Tim Bollinger

Duonychus tsogtbaatari

Duonychus (pronounced due-oh-ny-kus) translates from the Greek as “two claws”.  The species name honours the Mongolian palaeontologist Khishigjav Tsogtbaatar.  The discovery of this therizinosaur is doubly unique.  One of the sheaths on a fossil claw (digit I) has been preserved.  The sheath made from the protein keratin (same as our hair and fingernails), reveals that the claw was much longer than the underlying bone.  It was forty percent longer than the claw’s bony core.  It is the largest claw of its kind found fully preserved in this way.  The total claw length has been estimated at around thirty centimetres.

Duonychus tsogtbaatari was not carnivorous.  These long claws were probably not used to attack other dinosaurs.  Although, they could have had a defensive function or have been used in intraspecific combat.  The researchers studying the fossil material believe the long, curved claws and the ability of Duonychus to flex them would have helped this animal to grasp vegetation.

Duonychus tsogtbaatari data card. Picture credit: Tim Bollinger.

Picture credit: Tim Bollinger

Digit Reduction in Theropod Dinosaurs

The most famous two-fingered theropod dinosaur is Tyrannosaurus rex.  Duonychus is not closely related to T. rex.  However, both these dinosaurs evolved from ancestors that had three digits on each hand.  So, why did some theropod dinosaurs lose digits?  It is likely that the outer fingers of the hand became less useful and so they were gradually reduced over the time.  An evolutionary pathway led to the outer fingers becoming redundant, then vestigial, before finally disappearing altogether. Alternatively, digits I and II became increasingly dominant and thus enlarged.  This led to the eventual reduction and elimination of digit III.

Intriguingly, the hand claws of Duonychus were highly curved.  The researchers conclude that the extremely curved claws were adaptations to help this animal to “hook” branches and pull them within easy reach.  In essence, the reduction of digits was an evolutionary adaptation towards more efficient feeding.

Potential Niche Partitioning?

The sediments of the Bayanshiree Formation preserve a diverse dinosaur biota.  For example, there were at least three other therizinosaurs that were likely coeval with Duonychus.  All the other Bayanshiree Formation therizinosaurs described to date had three fingers on each hand.  With so many therizinosaurs in the same environment, it is possible that Duonychus evolved reduced digits to permit it to feed on different types of vegetation.  The reduced fingers could represent evidence of niche partitioning within the Therizinosauridae associated with these strata.

The four therizinosaurs described from the Bayanshiree Formation to date:

• Segnosaurus (S. galbinensis) named in 1979. One of the largest therizinosaurs known with a length of around seven metres.
• Erlikosaurus (E. andrewsi) named in 1980.  Measuring around three and half metres in length it had a robust beak suggesting a diet of mainly vegetation.
• Enigmosaurus (E. mongoliensis) named in 1983.  Some scientists consider Enigmosaurus a junior synonym of Erlikosaurus.  Although, the discovery of Duonychus lends support to the idea that several different types of therizinosaur were present.
• Duonychus (D. tsogtbaatari) named in 2025.  The first didactyl therizinosaur, possessing only two manual digits. In addition, the keratinous sheath on digit I provides a rare insight into theropod claw structure.

It is also worth noting that several other potential therizinosaur fossil specimens have been excavated from the Bayanshiree Formation.  These have yet to be formally named and described.

Our thanks to Tim Bollinger for permission to use his Duonychus illustrations in the compilation of this article.

Take a look at Tim Bollinger’s work under the moniker UnexpectedDinoLesson:

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The scientific paper: “Didactyl therizinosaur with a preserved keratinous claw from the Late Cretaceous of Mongolia” by Yoshitsugu Kobayashi, Darla K. Zelenitsky, Anthony R. Fiorillo and Tsogtbaatar Chinzorig published in the journey iScience.