Therizinosaurus (/ˌθɛrəˌzɪnoʊˈsɔːrəs/ ⓘ; meaning 'scythe lizard') is a genus of very large therizinosaurid dinosaurs that lived in Asia during the Late Cretaceous period in what is now the Nemegt Formation around 70 million years ago.

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.

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.

In 1948, several Mongolian Paleontological expeditions organized by the USSR Academy of Sciences were conducted in the Nemegt Formation of the Gobi Desert, Southwestern Mongolia, with the main objective of new fossils findings.

Since little was known of Therizinosaurus at the time of the original description, Maleev thought PIN 551-483 belonged to a large, 4.5 m (15 ft) long turtle-like reptile that relied on its giant hand claws to harvest seaweed.

[1] Though it was not fully understood to what general kind of animal these fossils belonged, in 1970, the Russian paleontologist Anatoly K. Rozhdestvensky was one of the first authors to suggest that Therizinosaurus was a theropod and not a turtle.

[2] These theropodan affinities were also followed by the Polish paleontologist Halszka Osmólska and co-author Ewa Roniewicz in 1970 during their naming and description of Deinocheirus—another large and enigmatic theropod from the formation that was initially known from partial arms.

In 1968 prior to Rozhdestvensky, Osmólska and Roniewicz statements, the upper portion of a manual ungual was found in the Altan Uul locality and labeled as MPC-D 100/17 (formerly IGM or GIN).

In 1972, another fragmented ungual (specimen MPC-D 100/16) was discovered at the Upper White Beds of the Hermiin Tsav locality, only preserving its lower portion.

[6] In 1990, Barsbold and Teresa Maryanska agreed with Perle in that the hindlimb material from Hermiin Tsav he described in 1982 was therizinosaurian (then called segnosaurians) given that the metatarsus was stocky and the astragalus had a laterally arched ascending process (bony extension), but cast doubt with his referral of it to Therizinosaurus and the segnosaurian identity for this taxon since it was only known from the pectoral girdle and other forelimb elements, making direct comparisons between specimens impossible.

[7] In 2010 however, the North American paleontologist Lindsay E. Zanno in her large taxonomic reevaluation of Therizinosauria considered the referral of MPC-D 100/45 to Therizinosaurus to be likely based on the rationale that it was collected in the same stratigraphic context (Nemegt Formation) as the holotype, and shared the robust and four-toed morphology of other therizinosaurids such as Segnosaurus.

Like other members of its family, Therizinosaurus had a proportionally small skull bearing a rhamphotheca (horny beak) atop its long neck; bipedal gaits; a large belly for foliage processing; and sparse feathering.

Other traits that were likely present in Therizinosaurus include a heavily pneumatized (air-filled) vertebral column and a robustly-built, ophistopubic (backwards oriented) pelvis.

[16] The scapula measured 67 cm (26 in) long with a stocky and flattened dorsal blade, wide acromial process (bony extension) and a very widened ventral surface.

Near the anterior edge of the scapular widening and near the scapulocoracoid suture (bone joint), a foramen was located; it likely functioned as a channel for blood vessels and nerves.

The coracoid measured 36 cm (14 in) in length, it had a broad and convex lateral surface that formed a slightly inclined concavity near of the scapulocoracoid suture.

[6][15] Maleev originally classified Therizinosaurus as a giant marine turtle and the genus was assigned by him to a separate family, Therizinosauridae given how enigmatic the specimen was.

[20] With the discovery of the referred hindlimb to Therizinosaurus in 1982 by Perle, he concluded that Segnosaurus was very similar to the latter based on the morphology and they possibly belonged to a single, if not the same, group.

[26] However, with the unexpected discovery and description of Alxasaurus in 1993, the widely accepted sauropodomorph affinities of segnosaurs were questioned by paleontologists Dale Russell and Dong Zhiming.

They considered these adaptations to represent an example of convergent evolution—a condition where organisms evolve similar traits without necessarily being related—between extinct mammal and dinosaur genera.

[33] Anthony R. Fiorillo and colleagues in 2018 suggested that Therizinosaurus had a reduced bite force that may have been useful for cropping vegetation or foraging, based on relative therizinosaurids such as Erlikosaurus and Segnosaurus.

[2] Barsbold in 1976 suggested that the unusual claws of Therizinosaurus may have been employed to impale or dig up loose terrain, however, he pointed out their notorious fragility upon impact.

He clarified that there is no evidence that the long claws of Therizinosaurus would have been used in active defense or attack, however, it is possible that these appendages could have had some role when facing a threat, such as intimidation.

This increased ability to withstand stress supports the idea that Therizinosaurus and other therizinosaurians used their arms in a robust fashion that generated significant forces.

Although this formation has never been dated radiometrically because of the discontinuity of exposures and absence of datable volcanic rock facies, the vertebrate fossil assemblage suggests an early Maastrichtian stage possibly about 70 million years ago.

[43] The paleofauna of the Nemegt Formation was diverse and rich, composed of other dinosaurs such as the alvarezsaurs Mononykus and Nemegtonykus; deinonychosaurs Adasaurus, and Zanabazar; ornithomimosaurs Anserimimus and Gallimimus; oviraptorosaurs Avimimus, Gigantoraptor, Rinchenia and Elmisaurus; tyrannosaurids Alectrosaurus, Alioramus and possibly Bagaraatan; ankylosaurids Saichania and Tarchia; and pachycephalosaurids Homalocephale and Prenocephale.

The Nemegt megafauna included the ornithomimosaur Deinocheirus; hadrosaurids Barsboldia and Saurolophus; titanosaurs Nemegtosaurus and Opisthocoelicaudia; and the apex predator Tarbosaurus.

[44][42][45][46] Additional paleofauna includes birds like Judinornis or Teviornis; abundant freshwater ostracods at numerous localities; fish; terrestrial and aquatic turtles such as Mongolochelys and Nemegtemys; and the crocodylomorph Paralligator.