| Therizinosaurus Temporal range: Late Cretaceous (Maastrichtian),
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| Mounted forelimbs of specimen MPC-D 100/15 at Nagoya City Science Museum | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Therizinosauroidea |
| Family: | †Therizinosauridae |
| Genus: | †Therizinosaurus Maleev, 1954 |
| Type species | |
| †Therizinosaurus cheloniformis Maleev, 1954
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Therizinosaurus (/ˌθɛrəˌzɪnoʊˈsɔːrəs/ ; meaning 'scythe lizard') is a genus of very large therizinosaurid that lived in Asia during the Late Cretaceous period in what is now the Nemegt Formation around 70 million years ago. It contains a single species, Therizinosaurus cheloniformis. The first remains of Therizinosaurus were found in 1948 by a Mongolian field expedition at the Gobi Desert and later described by Evgeny Maleev in 1954. The genus is only known from a few bones, including gigantic manual unguals (claw bones), from which it gets its name, and additional findings comprising fore and hindlimb elements that were discovered from the 1960s through the 1980s.
Therizinosaurus was a colossal therizinosaurid that could grow up to 9–10 m (30–33 ft) long and 4–5 m (13–16 ft) tall, and weigh possibly over 5 t (5.5 short tons). Like other therizinosaurids, it would have been a slow-moving, long-necked, high browser equipped with a rhamphotheca (horny beak) and a wide torso for food processing. Its forelimbs were particularly robust and had three fingers that bore unguals which, unlike other relatives, were very stiffened, elongated, and only had significant curvatures at the tips. Therizinosaurus had the longest known manual unguals of any land animal, reaching above 50 cm (20 in) in length. Its hindlimbs ended in four functionally weight-bearing toes differing from other theropod groups in which the first toe was reduced to a dewclaw and also resembling the very distantly related sauropodomorphs.
It was one of the last and the largest representative of its unique group, the Therizinosauria (formerly known as Segnosauria; the segnosaurs). During and after its original description in 1954, Therizinosaurus had rather complex relationships due to the lack of complete specimens and relatives at the time. Maleev thought the remains of Therizinosaurus to belong to a large turtle-like reptile, and also named a separate family for the genus: Therizinosauridae. Later on, with the discovery of more complete relatives, Therizinosaurus and kin were thought to represent some kind of Late Cretaceous sauropodomorphs or transitional ornithischians, even though at some point it was suggested that it may have been a theropod. After years of taxonomic debate, nevertheless, they are now placed in one of the major dinosaur clades, Theropoda, specifically as maniraptorans. Therizinosaurus is widely recovered within Therizinosauridae by most analyses.
The unusual arms and body anatomy (extrapolated after relatives) of Therizinosaurus have been cited as an example of convergent evolution with chalicotheriines and other primarily herbivorous mammals, suggesting similar feeding habits. The elongated hand claws of Therizinosaurus were more useful when pulling vegetation within reach rather than being used for active attack or defense because of their fragility, however, they may have had some role for intimidation. Its arms also were particularly resistant to stress, which suggests a robust use of these limbs. Therizinosaurus was a very tall animal, likely having a reduced competition over the foliage in its habitat and outmatching predators like tyrannosaurid Tarbosaurus.