Various theories were proposed to explain the ancestry of the "segnosaurids", with some scientists suggesting they were descendants of the sauropodomorphs or even that they were neither saurischians nor ornithischians. However, new, well-preserved finds, such as "Alxasaurus" in 1993 and "Beipiaosaurus" in 1999, provided details about the bird-like pelvis, feet, and skulls of primitive members and helped confirm that segnosaurids belonged to the same group of theropod dinosaurs as "Therizinosaurus" (and were therefore renamed therizinosaurids), and that therizinosaurs were, more specifically, advanced, herbivorous maniraptoran theropods. For maniraptoran standards, "Therizinosaurus" obtained enormous sizes, estimated to have reached in length with a ponderous weight from , or possibly over 5 tonnes. These dimensions make "Therizinosaurus" the largest therizinosaurs known and the largest known maniraptorans. Along with the contemporaneous "Deinocheirus", these are the largest known maniraptoriforms. Though the fossil remains of "Therizinosaurus" are incomplete, inferences can be made about their physical characteristics based on related therizinosaurids. Like other members of their family. "Therizinosaurus" probably had small skulls bearing a keratinous beak atop long necks, with bipedal gaits and heavy, deep, broad bodies for foliage processing (as evidenced by the wide pelvis of other therizinosaurids) with the addition of sparse feathering. Their forelimbs may have reached lengths of up to or even in the largest known specimen. Their hindlimbs ended in four weight-bearing toes, unlike other theropod groups, in which the first toe was reduced to a dewclaw. In 2010, Senter and James used hindlimb length equations to predict the total length of the hindlimbs in "Therizinosaurus" and "Deinocheirus". They concluded that an average individual of "Therizinosaurus" may have had approximately long legs. "Therizinosaurus" can be distinguished from other members in having very straight manual unguals that are transversely compressed (flattened) and hypertrophied, a hypertrophied deltopectoral crest on the humerus, the length of metacarpal I is larger than 2/3 the length of metacarpal III and the metacarpal I has an enlarged medial crest that connects the medial distal condyle and proximal medial lobe.