Therizinosaurus: Mongolia’s giant with enormous claws

Therizinosaurus cheloniformis was a giant theropod from Late Cretaceous Mongolia, probably living about 70–68 million years ago. It is famous for the longest known bony claw phalanges of any land animal. Its fossil record, however, is remarkably incomplete: large parts of the forelimbs are known, while the skull, backbone, pelvis and most of the body have not been found.

The animal’s usual long-necked, deep-bodied outline is reconstructed from better-known relatives such as Erlikosaurus and Segnosaurus. That comparison is useful, but a museum mount is a composite model, not a complete skeleton of Therizinosaurus. The giant claws are direct evidence; their exact soft-tissue covering and function are not.

Quick facts

Name and mistaken first interpretation

The genus name combines Greek roots for “scythe” and “lizard”, often rendered as “scythe lizard”. The species name cheloniformis means “turtle-shaped” and preserves the first, mistaken reading of the fossil. The enormous claws initially looked like parts of a large aquatic turtle rather than a dinosaur hand.

The holotype, PIN 551-483, was found in 1948 near the Nemegt locality during a Soviet–Mongolian expedition. Evgeny Maleev described it in 1954. It includes three huge hand-claw phalanges, two incomplete, a fragment of metacarpal and several bones then thought to be ribs. Maleev proposed a giant marine or semi-aquatic turtle that might have used its limbs to swim and cut aquatic plants.

Later study showed that the supposed ribs were not part of Therizinosaurus and probably came from a sauropodomorph. In 1976 Rinchen Barsbold described specimens IGM 100/15, IGM 100/16 and IGM 100/17. These included parts of the shoulder girdle, both upper arm bones, forearm and wrist elements, metacarpals, finger bones and additional claws. The new material established that the animal was an unusual theropod, not a turtle.

In 1982 Altangerel Perle referred a hind limb, IGM 100/45, from Khärmen Tsav. It includes fragments of the femur and tibia, ankle bones, a short broad metatarsus and toes. Its anatomy fits a derived therizinosaur and it was found in the same formation, but it is not articulated with the diagnostic hand. Its referral is reasonable, yet remains testable rather than certain.

Classification and species

Therizinosaurus belongs to Theropoda, Coelurosauria and Maniraptora, within Therizinosauria and Therizinosauridae. This placement explains why it is related to feathered maniraptorans even though the group evolved a largely plant-based feeding style. The wider dinosaur classification guide describes how shared anatomical features support these relationships; the dinosaur catalogue places the genus among other named theropods.

Therizinosaurus is the type genus of Therizinosauridae, but the branching order among its closest relatives is less secure. Some analyses recover it near Suzhousaurus, Paralitherizinosaurus and unnamed material from the Bissekty Formation. Many of the bones needed for direct comparison are missing from Therizinosaurus, so its precise position within the family changes with the taxa and characters included.

Only one species is recognised: Therizinosaurus cheloniformis. The main taxonomic question concerns which isolated bones belong to the genus. Its diagnostic claws and forelimb have distinctive proportions, but IGM 100/45 does not overlap the holotype in a diagnostic bone. Reviews generally regard the hind limb as a plausible referral, while avoiding its use as a defining feature of the name.

What the fossils directly show

The best-supported anatomy comes from the forelimb. IGM 100/15 preserves shoulder elements, humeri, parts of the forearm and a right hand. Together with the isolated claws, this material shows exceptionally long, powerful arms. A reconstructed forelimb reaches about 2.4 metres, although the component bones are not all equally complete or associated in the same way.

No skull has been found. The teeth, jaws, neck vertebrae, most of the torso, pelvis and secure hind limbs are also missing. The small head, long neck, broad torso and backward-facing pubis shown in many illustrations are inferred from relatives such as Erlikosaurus, Segnosaurus and Nothronychus. These features fit therizinosaur anatomy, but they are not observations from the name-bearing Therizinosaurus fossils.

Skin and feathers have not been preserved with this genus. Filament-like feathers are known in the earlier therizinosaur Beipiaosaurus, confirming that integument occurred within the group. Feathering in Therizinosaurus is plausible by relationship, but the density and distribution on a multi-tonne adult are unknown. No colour pattern can be recovered.

Size and forelimb anatomy

Common reconstructions place the animal at about 8–10 metres long and roughly 3–5 tonnes. Higher estimates exist, but they depend on scaling from different relatives and assumptions about torso depth. Without its backbone, pelvis and a securely associated femur, precise body measurements would imply evidence that is not available.

The upper arm bones were massive and bore a large deltopectoral crest for shoulder muscles. The hand had three digits. Its metacarpals had unusual proportions, while the claw phalanges were long, narrow from side to side and almost straight through most of their length. Their strongest curve occurred near the tip. These are the clearest specialisations documented directly.

If IGM 100/45 belongs to the genus, the foot was short, broad and supported by four toes. That arrangement differs from most theropods, in which the first toe was raised above the ground. It would suit a heavy biped better than a specialised runner, but it cannot tell us the animal’s top speed. The qualification matters because the hind limb is not part of the holotype.

How long were the claws, and what did they do?

The largest preserved bony claw phalanx is a little more than half a metre long. One incomplete element was reconstructed at around 60–65 centimetres. In life, a keratin sheath would have extended beyond the bone, but no sheath is preserved for Therizinosaurus. Popular claims of an exactly one-metre claw therefore describe a reconstruction, not a measured fossil.

Function remains debated. Computer-based studies in 2014 and 2022 found that the elongated claws of derived therizinosaurs were poorly suited to forceful digging. One proposed role is hook-like use for drawing branches closer or holding vegetation. A 2023 analysis argued that the extreme claws of Therizinosaurus show no clear mechanical specialisation and may have grown chiefly as display structures along with the animal’s size.

Those interpretations need not be mutually exclusive. A claw can have more than one use, and biomechanical models test a limited set of loads. Plant handling, display and extra defence remain possible, but there are no direct marks of blows, wear patterns or observations of behaviour. The image of claws as swords for disembowelling prey conflicts with both their mechanics and the likely feeding ecology of therizinosaurs.

Diet, habitat and life history

The skull and teeth of Therizinosaurus are unknown, so diet is inferred from better-preserved members of its group. These relatives had small heads, a beak-like front to the jaws, leaf-shaped teeth and a broad torso consistent with a large digestive tract. Therizinosaurus probably browsed leaves and young shoots at several heights. Eating small animals cannot be excluded, but there is no evidence that it hunted large prey.

The Nemegt Formation belongs to the Cretaceous Period and records major rivers, floodplains, lakes and wet ground. Its environment was more humid than older desert deposits in Mongolia. Tarbosaurus, Deinocheirus, Gallimimus, Saurolophus, sauropods, turtles, crocodylomorphs and freshwater animals lived in the broader ecosystem. Therizinosaurus may have fed in riverside woodland, but no fossil ties it to a single plant community.

No direct evidence establishes herds, solitary habits, mating displays, nests or parental care. Calls, colour and movement are also unknown. Even the suggested display function of the claws is a functional hypothesis, not proof of a particular courtship ritual.

Common errors and what remains unknown

The first major error was the giant-turtle interpretation. It arose from isolated claws and misidentified bones, and was overturned when forelimb material showed a theropod. Therizinosaurus was not a marine reptile and did not paddle with its hands.

The opposite exaggeration presents it as an active predator that tore apart large dinosaurs. Theropods include more than meat-eaters, and the better-preserved therizinosaurs show adaptations for plant feeding. Hunting is not documented for Therizinosaurus. Its skull and teeth are absent, so a strict diet label still needs care.

Its head, neck, torso and pelvis are not fully known. Those features on reconstructions come from relatives. The one-metre claw is not a direct measurement, because the keratin sheath is missing. Thick body-wide feathers are also not demonstrated, even though some feathering is plausible. A cautious profile separates the dramatic preserved hands from the much less certain body around them.

Therizinosaurus was a very large Late Cretaceous theropod from Mongolia and the most extreme example of its unusual plant-oriented lineage. Its forelimbs and claws are informative, while much of its familiar silhouette is comparative reconstruction. The strongest interpretation is a heavy biped whose oversized claws may have helped with feeding, display or defence, with their exact role unresolved.