Therizinosauridae (meaning 'scythe lizards') is an extinct family of therizinosauroid dinosaurs whose fossil remains have been found in mostly Late Cretaceous boundary. Even though representative fossils have only been found throughout Asia and North America, the range of Therizinosauridae is believed to have spanned much of the supercontinent of Laurasia based on several footprints and isolated remains in Europe and Africa. As of 2025, the family Therizinosauridae comprises nine definitive genera. Therizinosauridae was named in 1954 by paleontologist Evgeny Maleev after the large, claw-bearing unguals of the type species Therizinosaurus cheloniformis. Therizinosaurids were generally large and very robustly built animals that had a near convergent body plan with the more recent (and also extinct) ground sloths. The largest genera of the group are Therizinosaurus and Segnosaurus, which were about 10 m (33 ft) and 7 m (23 ft) long, respectively. The physiology of therizinosaurids include a broad and rounded belly supported by a wide and robust pelvis with thick hind-limbs composed of very stout, four-toed feet, a strong arm build with enhanced hand flexibility, elongated hand claws, and a highly derived, nearly avian inner-ear. Traits that are also well-known include an elongated neck, a prominent keratinous beak and a prosauropod-like dentition that differs from all theropods. Primitive therizinosaurs (Beipiaosaurus or Jianchangosaurus) are estimated to have had less advanced feathers in comparison to more recent therizinosaurids. The unique and bizarre features of the group have encouraged research into the paleobiology and paleoecology of the family. A fair portion of modern research has concentrated on the feeding-patterns of these dinosaurs, as they are considered to be the best regarded theropod candidates for herbivory. While other theropod groups are fully carnivorous, members of Therizinosauridae diverged and adopted an herbivorous and possibly omnivorous lifestyle. This is even more supported by their unusual morphology. As indicated by their feet morphology and several footprints from Asia, Africa and Europe, they probably were plantigrade walkers, but further examination may be required. Therizinosaurids were oviparous animals that nested in colonies and laid egg clutches. In fact, therizinosaur eggs are particularly common in Late Cretaceous formations, mainly in Asia. The oofamily Dendroolithidae is often attributed to therizinosaur-grade dinosaurs. Some of the first dendroolithid eggs were found on the Bayan Shireh and Nanchao formations. Their relationships were confusing and obscure on the early years of research mainly because of the unusual traits among members. Several alternative classifications were proposed (such as the naming of Segnosauridae in 1979) until more complete specimens and other taxa were described during the 1990s, which confirmed them as theropods. Many of the shared characters within the group also showed that Segnosauridae was a junior synonym of the much earlier family Therizinosauridae. The current phylogenetic consensus is that therizinosaurids evolved from small, bird-like maniraptorans, and thus they fall within the coelurosaurian clade called Maniraptora. Moreover, most of the traits of therizinosaurids (such as the ear structure) were inherited by smaller, agile, carnivorous ancestors. Extensive phylogenetic analyses have concluded that within Maniraptora, therizinosaurs were the first of five major groups to diverge.
History of discovery
The first definitive therizinosaurid discovered was Therizinosaurus. The fossil remains were discovered in 1918 during Mongolian field expedition on the Nemegt Formation of the Gobi Desert. Several claws were unearthed by the team and later described by the Soviet paleontologist Evgeny Maleev in 1954. The claws were unusually large, approximately 1 m (3.3 ft) long if restored, very straight and flattened. Maleev considered that the claws belonged to a giant marine turtle and named the new genus and species Therizinosaurus cheloniformis, also erecting the Therizinosauridae to include the new species. In the 1970s more findings were made including the naming of more taxa. Additional specimens of Therizinosaurus were referred by the Mongolian paleontologist Rinchen Barsbold in 1976. He described a set of partial arms from the Nemegt Formation and given the similarities within the claws, he referred this specimen to the genus. The Mongolian paleontologist Altangerel Perle described and named the new genus Segnosaurus in 1979, based on lower jaws and much of the hindlimbs. He also coined the newer Segnosauridae (now synonym of Therizinosauridae) to contain this species. In the same year, paleontologist Dong Zhiming described the genus Nanshiungosaurus, but wrongly interpreted the remains to have pertained to some kind of dwarf sauropod. In the following year, Barsbold and Perle coined the family Segnosauria (now Therizinosauria) to contain the Segnosauridae and kin. Also, they named and briefly described Erlikosaurus, a new genus smaller than Segnosaurus. Confusingly, Perle redescribed Erlikosaurus treating the taxon as a new genus and species in 1981. Also, Perle described another specimen of Therizinosaurus in 1982, this time a partial hind limb. He referred the hindlimb to the genus based on the similarities with Segnosaurus.
In 2001, the new therizinosaurids Neimongosaurus and Nothronychus (N. mckinleyi) were described and named, and in the following year Erliansaurus was also described and named. Although a new species of Nothronychus (N. graffami) was named in 2009. With this, both species make Nothronychus the most complete therizinosaurid known.
Description
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