Therizinosaurs (ⓘ; once called segnosaurs) are an extinct group of large herbivorous theropod dinosaurs whose fossils have been mainly discovered from Cretaceous deposits in Asia and North America.

He distinguished Segnosauridae from the theropod families Deinocheiridae and Therizinosauridae (then only known from the genera Deinocheirus and Therizinosaurus, both mainly represented by large forelimbs found in Mongolia) by features of their humeri and hand claws.

[5] Later in 1979, Barsbold and Perle found the pelvic features of segnosaurids and dromaeosaurids so different from those of "true" theropods that they should be separated into three taxa of the same rank, possibly at the level of infraorder within Saurischia (one of the two main divisions of dinosaurs, the other being Ornithischia).

Combined, the specimens provided relatively complete data on this group; they were united by their opisthopubic pelvis, slender mandible, and the toothless front of their jaws.

Since the Erlikosaurus specimen lacked a pelvis, the authors were unsure if that of the undetermined segnosaurian could belong to it, in which case they would consider it part of a separate family.

Segnosaurus and Therizinosaurus were particularly similar, leading Perle to suggest they belonged in a family to the exclusion of Deinocheiridae (today, Deinocheirus is recognized as an ornithomimosaur).

[11] In the same year, Barsbold stated that the segnosaurian pelvis deviated strongly from the theropod norm, and found the configuration of their ilia generally similar to those of sauropods.

[12] Paleontologist Gregory S. Paul concluded in 1984 that segnosaurs did not possess any theropodan features, but were instead derived, late-surviving Cretaceous prosauropods with adaptations similar to those of ornithischians.

[17] In a 1990 review article, Barsbold and paleontologist Teresa Maryańska found Segnosauria to be a rare and aberrant group of saurischians, in an unresolved position among sauropodomorphs and theropods, probably closer to the former.

Russell and Dong therefore proposed that Segnosauridae was a junior synonym of Therizinosauridae (since the latter name was older), with Alxasaurus being the most completely known representative so far, providing a better understanding of the group.

They also named the new higher taxonomic rank Therizinosauroidea to contain Alxasaurus and Therizinosauridae (since the new genus was somewhat different from its relatives), which they placed in the group Tetanurae within Theropoda.

[19][20] The synonymy of Segnosauridae with Therizinosauridae was accepted by Perle himself and co-authors of a redescription of the holotype skull of Erlikosaurus in 1994, and they considered therizinosaurs maniraptoran theropods, the group that also includes modern birds (since they did find Maniraptora to be valid through their analysis).

[23] In 1999, paleontologist Xing Xu and colleagues described a small, basal therizinosauroid from China, Beipiaosaurus, which confirmed that the group belonged among the coelurosaurian theropods, and that similarities with prosauropods had evolved independently.

Therizinosaurs were not considered as rare or aberrant anymore, but more diverse than previously thought (including in size), and their classification as maniraptoran theropods was generally accepted.

[32] Barsbold and Maryanska in 1990 considered C. zheziangensis as a tentative segnosaur (later known as therizinosaurs) based on its relatively short and robust pedal phalanges and enlarged, strongly curved unguals, mostly similar to Segnosaurus.

[36] In 1997 Dong Zhiming and You Hailu named and described a supposed second species of Nanshiungosaurus, N. bohlini, based on specimen IVPP V 11116 found in 1992 at Early Cretaceous strata from the Zhonggou Formation, Xinminbao Group.

However, they noted that a direct comparison between specimens is difficult to near impossible because there is no overlapping material (besides dorsal vertebrae) and the holotype of N. bohlini is apparently lost.

[35] In 1998 Zhao Xijin and Xu Xing briefly discovered a partial lower jaw with teeth (IVPP V11579) from the Early Jurassic-aged Lufeng Formation in Yunnan, and concluded that this specimen represented the oldest known record of a coelurosaurian theropod.

[41] In 2012 the Mongolian Academy of Sciences recovered a partial theropod specimen from the Bayan Shireh Formation at the Urlibe Khudak (also Ulribe Khuduk) locality.

The specimen was in a 2015 abstract by Yoshitsugu Kobayashi and team briefly described and identified as a new therizinosaur taxon distinct from the concurring Enigmosaurus, Erlikosaurus and Segnosaurus.

The unusual range of motion in therizinosaur forelimbs, which allowed them to reach forward to a degree other theropods could not achieve, also supports the idea that they were mainly herbivorous.

Therizinosaurs may have used their long reach and strongly curved claws to grasp and shear leafy branches, in a manner similar to large mammals that lived later on, such as chalicotheres, ground sloths, great apes, and giant pandas.

[31] Falcarius Beipiaosaurus Alxasaurus Erliansaurus Neimongosaurus Enigmosaurus Suzhousaurus Nothronychus Nanshiungosaurus Segnosaurus Erlikosaurus Therizinosaurus Below is the recently performed phylogenetic analysis performed by Hartman et al. 2019 using the data provided by Zanno in 2010:[36] Falcarius Jianchangosaurus Beipiaosaurus "Chilantaisaurus" zheziangensis Enigmosaurus Alxasaurus Suzhousaurus Neimongosaurus Therizinosaurus Erliansaurus Nanchao embryos Nanshiungosaurus Segnosaurus

Nothronychus mckinleyi Erlikosaurus "Nanshiungosaurus" bohlini Nothronychus graffami CT scans published by Stephan Lautenschlager et al. 2012 focused on the skull and brain cavity of Erlikosaurus, revealing it to have a large forebrain, and suggesting it had well developed senses of balance, hearing and smell, all of which would have been useful in evading predators, finding food, or in performing complex social behavior.

These senses were also well-developed in earlier coelurosaurs and other theropods, indicating that therizinosaurs may have inherited many of these features from their carnivorous ancestors and used them for their specialized dietary purposes.

[55] Dinosaur eggs with embryos of the Dendroolithidae type from the Nanchao Formation were identified as belonging to therizinosaurs based on anatomical features, and described by paleontologist Martin Kundrát and colleagues in 2007.