Our understanding of the dinosaur world is very incomplete, and many dinosaurs are known only from fragments of their skeletons. For example, in 1948, enormous curved claws were discovered in 70-million-year-old, Late Cretaceous deposits in Mongolia. These were the first remains of this animal to be found.

When the animal was described in 1954, it was thought to be a previously unknown form of giant turtle. Accordingly, its species name, cheloniformis, means “turtle-like”. The name of its genus, Therizinosaurus, refers to the scythe-like shape of the giant claws: it is derived from the Greek word therizo, meaning "to reap or mow".

Subsequent discoveries of more Therizinosaurusbones, including articulated fore- and hindlimbs, allowed scientists to classify Therizinosaurusas a theropod dinosaur.

Therizinosaurus cheloniformis is estimated to have been about 11 m (36 ft.) long, with a weight close to 2.7 t (3 tn.). The forelimbs were about 3 m (10 ft.) long. One of the huge claws measured 70 cm (27 in.) along its outer curve.

Discoveries of more-complete skeletal and skull elements of a related dinosaur, Alxasaurus, in Mongolia in the late 1980s, have allowed palaeontologists to reconstruct the skeletal and life-like appearance of Therizinosaurus.The sauropod-like skull and the structure of the jaws and teeth suggest a herbivorous diet, which was unusual amongst the generally carnivorous theropods. With its long neck and flexible forelimbs, Therizinosaurus may have foraged amongst the high vegetation.