The first fossil remains of Therizinosaurus were discovered in 1948 by the Mongolian Paleontological Expedition of the USSR Academy of Sciences at the Nemegt Formation of the Gobi Desert, southwestern Mongolia.
The expedition unearthed enormous manual unguals (claws) from the stratotype locality, Nemegt.
This unusual specimen was labelled under the number of PIN 551-483 and consisted of three partial manual unguals, a metacarpal fragment and several rib fragments.
Later on, the fossils were named and described by the Russian paleontologist Evgeny Maleev in 1954, who thought they belonged to a large, 4.5 m (15 ft) long marine turtle that used the giant claws to harvest seaweed.
This specimen became the holotype for the new genus and species Therizinosaurus cheloniformis.
The generic name, Therizinosaurus, is derived from the Greek θερίζω (therízo, meaning scythe, reap or cut) and σαῦρος (sauros, meaning lizard), in reference to the enormous manual claws and the specific name, cheloniformis, is taken from the Greek χελώνη (chelóni, meaning turtle) and the Latin formis, since the remains were thought to belong to a turtle-like reptile.
Maleev only depicted a fragmented rib and the most complete claw and noted some inconsistencies regarding the affinities of this new taxon.
Though it was not fully understood what general kind of creature these claws belonged, in 1970, the Russian paleontologist Anatoly K. Rozhdestvensky was one of the first authors in suggest that Therizinosaurus was a theropod dinosaur and not a turtle.
Contrary to Maleev, he illustrated the three holotypic manual unguals and reidentified the metacarpal fragment as a metatarsal.
The unusual shape of both metatarsal and ribs fragments led Rozhdestvensky to classify them as sauropod remains.

Further expeditions in the Nemegt Formation unearthed more fossils of this particular genus.
In 1968, prior to Rozhdestvensky statements, the upper portion of a manual ungual was found in the Altan Uul locality and labelled as IGM 100/17.
Another fragmented ungual was discovered at the Hermiin Tsav locality in 1972; this specimen, IGM 100/16, was found preserving its lower portion.
During the year 1973, the fairly more complete specimen IGM 100/15 was also recovered from Nemegt, and consisted of the left and right arms compromising both scapulocoracoids (left is fragmented though), both humeri, right ulna with radius and left ulna, two right carpals, the right metacarpus including a complete digit Il, and some ribs with gastralia.
All of these specimens were first described by the Mongolian paleontologist Rinchen Barsbold in 1976.
In this new monograph, he observed that the rib fragments in IGM 100/15 are more slender than the ones of the holotype, and identified IGM 100/16 and 100/17 as pertaining to the first and third digit, respectively.
During the same year and also from Hermiin Tsav, the specimen IGM 100/45 was discovered by the Joint Soviet-Mongolian Paleontological Expedition.
Unlike the previous findings, this specimen is represented by a hindlimb composed of a very fragmented femur with tibia, astragalus, calcaneum, a lower tarsal, a tetradacyl pes compromising four partial metatarsals, nearly complete digits I, II and IV (although II and IV are missing the unguals) and the presumed second phalanx from the digit III.
These newer remains were described by the also Mongolian paleontologist Altangerel Perle in 1982.
He referred the specimen to Therizinosaurus based on the striking resemblance to Segnosaurus, another therizinosaurid genus also known from limb elements.
The first fossil remains of Therizinosaurus were discovered in 1948 by the Mongolian Paleontological Expedition of the USSR Academy of Sciences at the Nemegt Formation of the Gobi Desert, southwestern Mongolia. The expedition unearthed enormous manual unguals (claws) from the stratotype locality, Nemegt. This unusual specimen was labelled under the number of PIN 551-483 and consisted of three partial manual unguals, a metacarpal fragment and several rib fragments. Later on, the fossils were named and described by the Russian paleontologist Evgeny Maleev in 1954, who thought they belonged to a large, 4.5 m (15 ft) long marine turtle that used the giant claws to harvest seaweed. This specimen became the holotype for the new genus and species Therizinosaurus cheloniformis. The generic name, Therizinosaurus, is derived from the Greek θερίζω (therízo, meaning scythe, reap or cut) and σαῦρος (sauros, meaning lizard), in reference to the enormous manual claws and the specific name, cheloniformis, is taken from the Greek χελώνη (chelóni, meaning turtle) and the Latin formis, since the remains were thought to belong to a turtle-like reptile. Maleev only depicted a fragmented rib and the most complete claw and noted some inconsistencies regarding the affinities of this new taxon. Though it was not fully understood what general kind of creature these claws belonged, in 1970, the Russian paleontologist Anatoly K. Rozhdestvensky was one of the first authors in suggest that Therizinosaurus was a theropod dinosaur and not a turtle. Contrary to Maleev, he illustrated the three holotypic manual unguals and reidentified the metacarpal fragment as a metatarsal. The unusual shape of both metatarsal and ribs fragments led Rozhdestvensky to classify them as sauropod remains.
Further expeditions in the Nemegt Formation unearthed more fossils of this particular genus. In 1968, prior to Rozhdestvensky statements, the upper portion of a manual ungual was found in the Altan Uul locality and labelled as IGM 100/17. Another fragmented ungual was discovered at the Hermiin Tsav locality in 1972; this specimen, IGM 100/16, was found preserving its lower portion. During the year 1973, the fairly more complete specimen IGM 100/15 was also recovered from Nemegt, and consisted of the left and right arms compromising both scapulocoracoids (left is fragmented though), both humeri, right ulna with radius and left ulna, two right carpals, the right metacarpus including a complete digit Il, and some ribs with gastralia. All of these specimens were first described by the Mongolian paleontologist Rinchen Barsbold in 1976. In this new monograph, he observed that the rib fragments in IGM 100/15 are more slender than the ones of the holotype, and identified IGM 100/16 and 100/17 as pertaining to the first and third digit, respectively. During the same year and also from Hermiin Tsav, the specimen IGM 100/45 was discovered by the Joint Soviet-Mongolian Paleontological Expedition. Unlike the previous findings, this specimen is represented by a hindlimb composed of a very fragmented femur with tibia, astragalus, calcaneum, a lower tarsal, a tetradacyl pes compromising four partial metatarsals, nearly complete digits I, II and IV (although II and IV are missing the unguals) and the presumed second phalanx from the digit III. These newer remains were described by the also Mongolian paleontologist Altangerel Perle in 1982. He referred the specimen to Therizinosaurus based on the striking resemblance to Segnosaurus, another therizinosaurid genus also known from limb elements.
Therizinosaurus