Therizinosauridae (meaning 'scythe lizards') Translated paper. 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 and 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 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. Translated paper 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. Translated paper 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. Translated paper 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. Translated paper
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
Therizinosaurids were quite advanced and robustly-built animals that reached multi-ton dimensions, making them among the largest maniraptorans. The smallest known therizinosaurids are Erliansaurus, Erlikosaurus and Neimongosaurus which were around long, furthermore the holotype skull of Erlikosaurus is about . Segnosaurus and the possible therizinosaurid Suzhousaurus were about the same size, in length and weighing . Slightly smaller members are represented by Nanshiungosaurus and Nothronychus, estimated at long and in weight, respectively. Genus List for Holtz 2012 Weight Information The related "Nanshiungosaurus" bohlini was similar in size (about long), however, its assignment to Nanshiungosaurus is unlikely and it may or not represent another species or specimen of the better known Suzhousaurus. Therizinosaurus were the largest representatives, reaching the top dimensions of the group, they grew up to long weighing over . These dimensions that make the genus among the largest-known theropods. The exponential sizes obtained across the Therizinosauridae appear to have been triggered by the bulk and specialized diet within the group and early members of the Therizinosauria.
Skin impressions from Beipiaosaurus and Jianchangosaurus indicate that primitive therizinosaurs were covered with a coat of down-like, sparse feathers similar to those seen in the compsognathid Sinosauropteryx, as well as longer, simpler, quill-like feathers that may have been used in display. However, while the earliest known example of primitive, stage-I feathers are found among early therizinosaurs, more advanced members are thought to have possessed more developed, avian-like feathers. Although typically associated with flight, the feathers which covered these animals were not used for that purpose. Instead, they assisted in either metabolism or display.
Skull
Therizinosaurids had more specialized skulls compared to primitive members such as Falcarius, which had a generalist, beak-less snout. They had relatively elongated snouts and the tip was covered with a rhamphotheca (keratinous beak) used during feeding. Although the extension of the beak on the dorsal surface of the snout is unknown, it is estimated that it slightly overlapped the nasal cavity as in some modern-day birds. The dentary (lower jaw) also developed a rhamphotheca, seen on the edentulous (without teeth) anterior-most end. This extension was supplied by neurovascular foramina (small pits) found on the lateral surfaces. The known specimens of the therizinosaurids Erlikosaurus, Neimongosaurus and Segnosaurus preserve numerous neurovascular foramina (more notorious on Erlikosaurus), indicating that a well-developed beak was present in life. Both maxilla and premaxilla were toothed and some species of therizinosaurids had specialized, recurved dentaries such as Segnosaurus and possibly Neimongosaurus. Braincases are known from three therizinosaurids: Erlikosaurus, Neimongosaurus and N. mckinleyi. Unfortunately, the braincase of Neimongosaurus has not been described nor illustrated. The braincases are directed to the bottom, co-ossified with well-developed sideways oriented paroccipital projections, highly pneumatized and had a prominent central foramen.
The dentition is another featured trait in therizinosaurids which differs from all theropod groups. At least two different tooth morphologies are observed among therizinosaurids; the first is represented by relatively homodont, oval to lanceolate-shaped teeth with moderate coarse denticles (serrations) on the crowns (upper exposed part). This type of dentition is better represented by the complete, three-dimensional holotype skull of Erlikosaurus which features the mentioned characters. Two isolated teeth are known from Nothronychus and they are lanceolate-shaped, symmetrical, have moderate denticles, and strongly resemble those of Erlikosaurus. Furthermore, they seem to derive from the dentary based on comparisons with the latter. The preserved holotype dentary tip of Neimongosaurus preserves an erupted tooth that is lanceolate-shaped with small coarse serrations, falling within this type of dentition. Another type of dental morphology is the one seen on the highly specialized Segnosaurus. In this taxon, the teeth are very heterodont, leaf-shaped with relatively less denticles that are prominently developed being bigger than in the previous therizinosaurids. These denticles are composed of numerous folded carinae (cutting edges) with denticulated front edges, creating a roughened and shredding surface near the base of the tooth crowns. Most therizinosaurids appear to have had a low tooth replacement rate or not able to lose them at all.
Postcranial skeleton
In therizinosaurids, the shoulder girdle was robustly specialized than in other maniraptorans and less bird-like. The scapula (shoulder blade) was very straight and flattened at the upper end being fused to the coracoid bone, forming the scapulocoracoid. Near the region of the scapulocoracoid suture, on the coracoid, a large foramen is present. The coracoid is a wide and broad element that is slightly convex and thickened near of the scapulocoracoid suture. The humerus (upper arm) was exceptionally robust and flexible with wide lower ends as seen on the humeri of the therizinosaurids Nothronychus and Therizinosaurus. Not only that but the biceps muscle was prominently well-developed in Therizinosaurus. In Segnosaurus, the deltopectoral crest (deltoid muscle attachment) was strongly built. The antebrachium was relatively straight in most members, particularly on Therizinosaurus. The reduced carpal block on therizinosaurids enabled an enhanced hand flexion. Manual phalanges are relatively standard in shape among maniraptorans with a formula of digits I, II and III.