A very long claw found in the Nemegt Basin became a paleontological puzzle for many years. This mysterious claw belonged to a hand of an unknown creature. When first described, it was referred to a turtle, and hence, given the specific name as Therizinosaurus cheloniformis.In 1965, a pairs of complete forelimbs with massive claws were found in Nemegt by the Polish- Mongolian Expedition. It has become clear that the both the massive claws belonged to theropod dinosaur, named as Deinocheirus. Later on, A.Perle of the Mongolian National Ulaan Bataar University found more complete skeletal remains of medium size theropods in Eastern Mongolia. In 1979, he described two new dinosaurs, Segnosaurus and Erlikosaurus.These were similar to Therizinosaurus , and all three taxa apparently belonged to a new group of theropods, that he finally named as Therizinosauria.
In these early days of the therizinosaur record, different opinions on higher taxonomy of therizinosaurs were proposed. Due to the lack of a complete specimen the controversy continued for a long time. In 1993, D. Russell and the author of this book published a paper on more complete skeletal remains of a therizinosauid from Alxa of Inner Mongolia. They named is as Alxasaurus. Since then, Therizinosauria have been accepted to represent bizarre forms of theropods. These theropods had some skeletal features similar to those of herbivorous dinosaurs. More than ten genera have been described, and it is believed now that they were even closely related to basal birds. Their bodies might have been covered with fuzzy feathers found in Beipiaosaurus. Some scholars go even further and claim that therizinosaurs were a kind of flightless birds.
Now days, most paleontologists agrees that therizinosaurs had, although modified, a theropod-type of body. They were probably partly specialized to herbivory. It has been claimed that they used their long forelimbs and large claws to pull down branches to feed on them. Other paleontologists suggest that therizinosaurs may have used their massive claws to disintegrate termite nests. The claws had a horny sheath that might have been over a meter long.