A very odd new dinosaur has been described from Mongolia, adding to our knowledge of dinosaur diversity and anatomy.
Dubbed Duonychus tsogtbaatari (‘Tsogtbaatar’s two-claw’) by Yoshitsugu Kobayashi and colleagues, the new species is from the Upper Cretaceous Bayanshiree Formation of Mongolia’s Gobi Desert and is between 96 and 90 million years old.
Its species name honours palaeontologist Khishigjav Tsogtbaatar, former director of the Institute of Paleontology at the Mongolian Academy of Sciences in Mongolia’s capital Ulaanbaatar, the institution that houses the fossil today.
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The sole specimen is incomplete but well preserved, consisting of the forelimbs and hips, part of the shoulder girdle and vertebrae from the back, hip region and tail base. Duonychus is a therizinosaur, a mostly Asian, mostly Cretaceous group.
Therizinosaurs are bizarre. They are theropods (the group that includes all predatory dinosaurs in addition to birds), and part of the same subgroup – the maniraptorans – as Velociraptor, birds, and the omnivorous and herbivorous oviraptors. They were bipedal, long-necked, short-tailed, and had deep, wide hips and clawed, feathered forelimbs.
The biggest therizinosaurs were giants that weighed several tonnes. Duonychus was mid-sized. The sole known specimen wasn’t mature at death but close to adult size, so this was likely around 260 kg (575 lbs) and 3 m (10 ft).
Its skull is unknown but was presumably like that of other therizinosaurs: long, narrow and with small, leaf-shaped teeth suited for eating leaves.
It's the hands that are of special interest. Duonychus has just two fingers, the third being absent apart from a reduced metacarpal (one of the bones that forms the palm). This means that therizinosaurs have joined the select club of theropod groups that contain two-fingered species. Tyrannosaurids famously include two-fingered species, but other groups do too, including oviraptors.
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Why theropods were so good at evolving ‘reduced’ hands is a good question. The usual answer is that the outer fingers became less useful and so reduced over time.
An alternative possibility is that fingers 1 and 2 became enlarged due to a specialised function, and that this enlargement made the third finger redundant.
Duonychus’s hand claws are extremely curved. By comparing their curvature with that of other animals, Kobayashi and colleagues concluded that they were suited for hooking onto vegetation and pulling it within easier reach.
One of the claws preserves the horny keratin covering. This shows that the claw in the live animal was both 40% longer than the claw’s bony core and markedly more curved.
A final point of interest is that Duonychus comes from a fauna that has already yielded three other therizinosaur species. Presumably, these fed in different ways on different resources and hence avoided competition.
Find out more about the study: Didactyl therizinosaur with a preserved keratinous claw from the Late Cretaceous of Mongolia
Main image: Duonychus reconstruction. Credit: Masato Hattori
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