Segnosaurus is a genus of therizinosaurid dinosaur that lived in what is now southeastern Mongolia during the Late Cretaceous, about 102–86 million years ago. Multiple incomplete but well-preserved specimens were discovered in the Gobi Desert in the 1970s, and in 1979 the genus and species Segnosaurus galbinensis were named. The generic name Segnosaurus means "slow lizard" and the specific name galbinensis refers to the Galbin region. The known material of this dinosaur includes the lower jaw, neck and tail vertebrae, the pelvis, shoulder girdle, and limb bones. Parts of the specimens have gone missing or become damaged since they were collected. Remains from Japan originally assigned to Allosaurus were reassigned to Segnosaurus sp. (of unspecified species) in 2025.
Segnosaurus was a large-bodied therizinosaur that is estimated to have been about long and to have weighed about . It would have been bipedal, with the trunk of its body tilted upwards. The head was small with a beak at the tip of the jaws, and the neck was long and slender. The lower jaw was down-turned at the front and the teeth were distinct in having additional as well as third cutting edges in some of the hindmost teeth. The forelimbs were robust and had three fingers which bore large claws, and the feet had four toes supporting the foot—apart from therizinosaurs, all theropods had three-toed feet. The front of the pelvis was adapted to support the enlarged belly. The pubic bone was turned backwards, a feature that is only seen in birds and the dinosaurs most closely related to them.
The affinities of Segnosaurus were originally obscure and it received its own theropod family, Segnosauridae, and later when related genera were identified, an infraorder, Segnosauria. Alternative classification schemes were proposed until more complete relatives were described in the 1990s, which confirmed them as theropods. The new fossils also showed Segnosauridae was a junior synonym of the earlier named family Therizinosauridae. Segnosaurus and its relatives are thought to have been slow-moving animals that, as indicated by their unusual features, were mainly herbivorous, whereas most other theropod groups were carnivorous. Therizinosaurs probably used their long forelimbs, long necks, and beaks when browsing, and large guts for processing food. Segnosaurus is known from the Bayan Shireh Formation, where it lived alongside the fellow therizinosaurs Erlikosaurus and Enigmosaurus; these related genera were probably niche partitioned.
History of discovery
In 1973, a joint Soviet–Mongolian expedition investigating the Bayan Shireh Formation at the Amtgay locality in the Gobi Desert of southeastern Mongolia discovered fossils that included the partial skeleton of an unknown dinosaur. Through 1974 and 1975, more remains were uncovered at the Amtgay and Khara-Khutul localities; though the skeletons were incomplete, the recovered bones were well-preserved. Other localities listed in the literature include Bayshin-Tsav and Urilbe-Khuduk. These fossils were scientifically described in 1979 by the paleontologist Altangerel Perle, who named the new genus and species Segnosaurus galbiensis. The generic name is derived from the Latin word segnis ("slow") and the Ancient Greek sauros ("lizard"). The specific name refers to the Galbin region of the Gobi Desert.
The holotype specimen from the Amtgay locality is housed at the Mongolian Academy of Sciences under the specimen number IGM 100/80 (Mongolian Institute of Geology, formerly GIN). It includes the (lower jaws), an incomplete , a complete and (lower arm bones), of the fingers, a forelimb (claw bone), an almost-complete , an incomplete right , six , ten from the front of the tail, fifteen from the hindmost part of the tail, the first , and fragments of the dorsal ribs. Two more specimens were designated as paratype specimens; specimen IGM 100/82 from the Khara Khutul locality includes a femur, and (leg bones), and , five toe phalanges including a foot ungual, rib fragments, complete , the upper portion of an , and the lower portion of a . Specimen IGM 100/83 includes a left (shoulder girdle), a radius, an ulna, forelimb unguals, and a fragment of a (neck) vertebra. In 1980, Perle and the paleontologist Rinchen Barsbold assigned another specimen to Segnosaurus; IGM 100/81 from the Amtgay locality included a left tibia and fibula.
In 1983, Barsbold listed additional specimens GIN 100/87 and 100/88. In 2010, however, the paleontologist Lindsay E. Zanno suggested these may refer to paratypes IGM 100/82 and IGM 100/83 (which had already been listed in 1979) because the Russian-to-English translation of Barsbold's article has several typographical errors in regard to specimen numbers. Zanno also noted that by the time of her study, there were numerous problems with the Segnosaurus IGM specimens, including damage caused since collection, the disappearance of elements of the holotype, incorrect identification of assigned elements, and more than one individual bearing the same specimen number. Holotype elements Zanno was able to access in 2010 included a severely damaged ilium, a sacrum missing the left sacral ribs with damage so it could not conjoin well with the rest of the ilium, and a pubic bone and ischium missing their upper portions. More bones bearing the specimen number IGM 100/82 were located but were not mentioned in Perle's description, while the whereabouts of some paratype elements was unknown. In a 2016 re-description of the holotype mandible, which had been little studied since 1979, Zanno and colleagues reported the majority of the tooth crowns had been damaged after collection, and most of them were missing their tips. Of the two (halves of the lower jaw), the right is nearly complete; only the hindmost part and the upper front of its mandibular symphysis (the area where the halves of the mandible meet) was missing. The left hemimandible is fragmented and preserves the front part with some displacement of bone due to crushing.
In 1991, the paleontologist Minoru Tamura and colleagues reported vertebrae, limb bones, and teeth from the Late Cretaceous Jobu Formation of Kumamoto, Japan, which they assigned to Allosaurus sp. (of unspecified species), which would make it the youngest representative of that genus. The paleontologists André Burigo and Octávio Mateus instead found the femur similar to that of to Segnosaurus in 2025, and noted that other therizinosaur remains were known from the site, and they therefore reassigned these remains to Segnosaurus sp.
Description
Segnosaurus was a large-bodied therizinosaur that is estimated to have been about long and to have weighed about . Campione & Evans in 2020, however, calculated its body mass at . Segnosaurus is incompletely known, but as a therizinosaurid, it would have been bipedal and robustly built with the trunk of the body tilted upwards compared to other theropods. The head would have been small with a (horny beak) at the tip of the jaws, and a long, slender neck. The fingers were not particularly long, but bore large claws. The front of the pelvis was adapted to support the enlarged belly. Therizinosaurs are known to have had simple, primitive feathers as evidenced by fossils of the basal (or "primitive") genera Beipiaosaurus—the second-known non-bird dinosaur preserved with such integuments after Sinosauropteryx—and Jianchangosaurus. Since most therizinosaurs are incompletely known, it is uncertain how many of the anatomical features that are used to distinguish Segnosaurus are widespread among the group; many genera cannot be directly compared because the equivalent bones are not preserved.
Mandible and lower dentition The mandible of Segnosaurus was low and elongated, yet relatively robust and shapeless compared to that of Erlikosaurus, which was more gracile. The nearly complete right hemimandible (half of the mandible) is long from front to back, at the highest point, and at the lowest. The , the tooth-bearing bone forming most of the mandible's front part, was complex in shape compared to those of early therizinosaurs. The tooth-bearing part was almost rectangular and sloped downwards in side view with a pronounced arc throughout the upper length of the front end—more extreme than what is known in other therizinosaurs. The front-most part of the dentary was strongly deflected downwards at about a 30-degree angle, a unique feature for this genus. When each hemimandible is articulated with the other, they form a broadly U-shaped, toothless mandibular symphysis that projects upwards towards the front as in Erlikosaurus and Neimongosaurus. The expansive, toothless front region of the dentary spans on the right hemimandible of the holotype. Proportionally, the toothless part of the dentary is 20% of its tooth row, which is long. By comparison, the toothless region of Erlikosaurus was about 12% of the tooth row's length and was almost absent in Jianchangosaurus. The height of the dentary diminished towards the hindmost extend of the tooth row, whereafter it sharply fanned out to contact the bone behind it; by contrast, the hind part of the dentary in Erlikosaurus gradually approached the surangular in a gentle arc.
Segnosaurus was distinct among therizinosaurs in that the hindmost part of the dentary was toothless. The teeth were restricted to the front two-thirds of the dentary, which bore 24 (tooth sockets) in a manner similar to Jianchangosaurus but different from Erlikosaurus, in which nearly the entire dentary was toothed, bearing 31 alveoli. The tooth row of Segnosaurus was inset and demarcated by a shelf on the outer side as it was in all derived (or "advanced") therizinosaurs. Unlike in other related taxa, the shelf was restricted to the hind part of the dentary and the raised rim that defined it was not as pronounced. Segnosaurus was unique in having a low ridge rising between the fifth and fourteenth alveoli that divided the dentary into two almost-equally sized front and hind parts. Just above this ridge, the dentary was pierced by a row of as in Jianchangosaurus and Alxasaurus, which became less regular by the region around the mandibular symphysis, where the two halves of the mandible met at the front. This row was instead directly in line with and on the side of the ridge in Erlikosaurus. The that ran along the inner side of the mandible, was placed further down than in Erlikosaurus and had a consistent depth until the thirteenth tooth position, whereafter it widened. The lower jaw elements behind the dentary (the , surangular, , and bones) were distinct from those of other therizinosaurs, being gracile and linear, and contributing to the hind part of the hemimandible being elongate and almost rectangular. The surangular was long and sword-shaped, the angular was wing-like in shape, the prearticular was narrow and curved, and the splenial was thin and triangular in outline. The external mandibular fenestra, an opening at the outer side of the mandible, was larger than that of Erlikosaurus because the surangular was shallow from top to bottom.