Therizinosaurus cheloniformis

Therizinosaurus cheloniformis was the largest known member of a highly unusual branch of maniraptoran theropods that shifted away from the predatory body plan of their ancestors. Its directly known remains are striking but incomplete: giant shoulder and arm bones, three-fingered hands with extremely elongated unguals, ribs and referred hind-limb material. No complete skull, vertebral column or articulated whole skeleton has been found. The small head, long neck, deep torso, broad pelvis and feather-like covering in modern reconstructions therefore come mainly from related therizinosaurs. That comparative framework is strong enough to identify a giant bipedal herbivore, but it cannot fill every anatomical gap with equal confidence.

Its fossils occur between approximately 70 and 66 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

Referred forelimb MPC-D 100/15 includes both scapulocoracoids, humeri about 76 centimetres long, forearm bones, wrist elements and much of a right hand. Each hand had three long functional fingers. The largest incomplete bony unguals reach roughly 50–65 centimetres; a keratin sheath extended them in life, but a one-metre total claw is a reconstruction rather than a preserved measurement. These unguals are unusually straight, flattened side-to-side and poor at resisting several heavy-load scenarios in recent models. Referred hind-limb material supports bipedal locomotion and a broad, four-toed foot characteristic of advanced therizinosaurs. The long neck, small beaked head, capacious gut and feathers are plausible family-level inferences, not structures preserved with this species.

• Exceptionally large shoulder girdle and forelimbs
• Three long functional fingers on each hand
• Laterally flattened manual unguals about 50–65 centimetres as preserved
• Deep grooves on the claw bones that anchored keratin sheaths
• Powerful flexor attachment below each ungual joint
• Robust bipedal hind limb referred from the Nemegt Formation
• Broad four-toed foot inferred from referred material and close relatives
• Long neck, small head, broad gut and feathers reconstructed from other therizinosaurs

The Mongolian Paleontological Expedition of the USSR Academy of Sciences found the holotype PIN 551-483 at Nemegt’s Quarry V in 1948. It comprises three incomplete hand unguals, a metacarpal fragment and rib fragments. Evgeny A. Maleev named Therizinosaurus cheloniformis in 1954 but interpreted the animal as an enormous turtle-like aquatic reptile whose claws might cut vegetation. Rinchen Barsbold described giant shoulder, arm and hand material from additional Nemegt localities in 1976 and recognized a theropod. Discoveries of more complete relatives—especially Erlikosaurus, Alxasaurus, Beipiaosaurus and North American forms—eventually established therizinosaurs as herbivorous or omnivorous maniraptorans and supplied the missing body outline.

Discovery credit: Mongolian Paleontological Expedition of the USSR Academy of Sciences.

Naming authors: Evgeny A. Maleev.

Therizinosaurus inhabited the extensive river channels and floodplains of the Nemegt Formation in southern Mongolia. Seasonal floods laid down sandbars and muddy overbank deposits, while plants supported an unusually varied large-herbivore community. Saurolophus, Deinocheirus, gallimimids, pachycephalosaurs and sauropods shared the wider formation; Tarbosaurus was its largest known predator. Therizinosaurus remains come from several Nemegt localities, including Nemegt, Hermiin Tsav and Altan Uul, so the animal occupied a regional landscape rather than one precisely mapped quarry point.

Herbivory is strongly supported by the anatomy and evolutionary distribution of better-known therizinosaurs, although Therizinosaurus itself preserves no skull or teeth. Its long arms and claws inspired feeding, digging and defence hypotheses. A 2014 finite-element study found hook-and-pull use more compatible than digging for most therizinosaurs, whereas a broader 2023 analysis found the extreme Therizinosaurus unguals poorly suited to all tested high-stress tasks and proposed display or intimidation as their principal role. These models test idealized forces on claw bones; they do not show that the living hands never touched vegetation or struck an attacker. No trackway, nest, healed combat wound or group association establishes its speed, temperament, sociality or parental behaviour.

• The name cheloniformis preserves Maleev’s original—and incorrect—idea that the animal was turtle-like.
• The famous claws are the terminal bones inside the living claw; the keratin sheath made them longer but has never been preserved.
• A partial ungual restored by Maleev was already about 52 centimetres long, with an estimated complete bony length of 60–65 centimetres.
• Nearly the whole familiar body silhouette is a carefully constrained composite from relatives because no complete Therizinosaurus skeleton exists.
• Therizinosaurus and Tarbosaurus occur in the same formation, but there is no fossil preserving an encounter between them.

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1. 01
   The Paleobiology Database

   Open source
2. 02
   A New Turtle-Like Reptile from Mongolia

   Evgeny A. Maleev · Priroda 1954(3) · 1954

   Open source
3. 03
   New data on Therizinosaurus (Therizinosauridae, Theropoda)

   Rinchen Barsbold · Paleontology and Biostratigraphy of Mongolia, Joint Soviet-Mongolian Paleontological Expedition Transactions · 1976

   Open source
4. 04
   A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora)

   Lindsay E. Zanno · Journal of Systematic Palaeontology 8(4) · 2010

   Open source
5. 05
   Morphological and functional diversity in therizinosaur claws and the implications for theropod claw evolution

   Stephan Lautenschlager · Proceedings of the Royal Society B 281(1785) · 2014

   Open source
6. 06
   Functional space analyses reveal the function and evolution of the most bizarre theropod manual unguals

   Zichuan Qin, Chun-Chi Liao, Michael J. Benton, Emily J. Rayfield · Communications Biology 6, 181 · 2023

   Open source
7. 07
   Therizinosaurus: Dino Directory

   Natural History Museum, London

   Open source
8. 08
   Therizinosaurus cheloniformis forelimb specimen database entry

   Fukui Prefectural Dinosaur Museum

   Open source
9. 09
   Museum of Evolution Guidebook: Dinosaur claws cabinet

   Museum of Evolution, Polish Academy of Sciences · 2017

   Open source