This animal was part of a group of dinosaurs called theropods, making it a distant relative of ferocious meat-eaters like Tyrannosaurus and Velociraptor. But unlike other theropods, Therizinosaurus was a vegetarian, a quality which also accounts for its strange appearance.
Therizinosaurus roamed the conifer forests on the edges of the desert, using its long neck to stretch for leaves growing on tall trees. It had a large stomach in order to digest all this tough vegetable matter, giving its belly a characteristic swollen appearance.
Intriguingly, scientists think its huge claws were an adaptation to a vegetarian way of life, rather than deadly weapons. Therizinosaurus may have used its huge claws to pull tree branches towards its mouth in order to feed on the leaves.
But its claws might also have been used for mating displays, and some scientists think they could have been used for defence against attack by predators.
The Mongolian desert in the Late Cretaceous was home to fearsome predators. Perhaps the most terrifying of these was Tarbosaurus, an Asian relative of Tyrannosaurus that stood a lofty five metres tall and weighed a whopping five tonnes.
Fast and deadly
Tarbosaurus prowled the broken woodland and desert fringes, pursuing lone or vulnerable dinosaurs, or lying in wait to pounce on them. Though it was heavy, Tarbosaurus' powerful leg muscles made it a good sprinter.
However, experts believe that it probably didn't try to attack Therizinosaurus, as the giant-clawed herbivore was probably too big, even for an awesome carnivore like Tarbosaurus.
A common sight around this desert habitat was Protoceratops, a stout, lumpy dinosaur about the size of a pig. Herds of these creatures were to be seen everywhere. In fact, they were so abundant that experts have dubbed them 'the sheep of the Cretaceous.'
Protoceratops was almost certainly at the bottom of the food chain and would undoubtedly have been preyed upon by almost every carnivore that stalked the Mongolian desert, including Tarbosaurus and a vicious little creature called Velociraptor.
We know this dinosaur hunted Protoceratops because an extraordinary fossil found in Mongolia in 1971 shows a Velociraptor locked in combat with a Protoceratops. It is thought that the dinosaurs were smothered in a sand storm while in mid-combat.
At two metres in length and weighing in at just 20 kilogrammes, Velociraptor was small in comparison with Tarbosaurus, but it had a deadly array of weaponry that it used to inflict lethal damage on its prey.
The most lethal of all these weapons was a set of razor-sharp claws on Velociraptor's hands and feet, including an enlarged second toe claw that was used to rip at the flesh of unsuspecting animals.
Small animals like Protoceratops had more to fear from packs of Velociraptor than from big predators like Tarbosaurus.
This article was written to accompany the Walking with Dinosaurs special The Giant Claw.