Segnosaurus

***Segnosaurus*** (meaning "slow lizard") is a genus of therizinosaurid 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 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.

*Segnosaurus* was a large-bodied therizinosaur that is estimated to have been about 6–7 m (20–23 ft) long and to have weighed about 1.3 t (1.4 short tons). 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 denticles 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.

## Description

*Segnosaurus* was a large-bodied therizinosaur that is estimated to have been about 6–7 m (20–23 ft) long and to have weighed about 1.3 t (1.4 short tons). Campione & Evans in 2020, however, calculated its body mass at 4.17 metric tons (4.60 short tons). *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 rhamphotheca (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 379 mm (14.9 in) long from front to back, 55.5 mm (2.19 in) at the highest point, and 24.5 mm (0.96 in) at the lowest. The dentary bone, 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 25.5 mm (1.00 in) on the right hemimandible of the holotype. Proportionally, the toothless part of the dentary is 20% of its tooth row, which is 150.3 mm (5.92 in) 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 surangular 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 alveoli (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 foramina 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 Meckelian groove 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 splenial, surangular, angular, and prearticular 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.

*Segnosaurus* had the fewest teeth in the dentary; 24 in each half determined from the number of sockets, as well as the largest teeth known among therizinosaurs. The dentary teeth were foliodont (leaf-shaped) and bore enlarged, relatively tall, sideways compressed crowns with a slight recurvature at the upper margin of the tips. By comparison, the teeth of *Erlikosaurus* were smaller, symmetrical, and simpler. The bases of the crowns increased slightly in size hindwards across the tooth row, which reflected a decrease in sideways compression. The front surfaces of the crowns and outward-facing sides were convex while the inward-facing sides were concave. A thickened ridge ran along the longitude of the inward-facing side near the upper half of the crown, which was flanked by weak grooves near the front and back edges of the teeth, reaching almost to the cervix (neck; the transition between the crown and root) of the teeth. In general, the 18 front-most teeth were relatively homodont (of the same type), though the crown of the second tooth was relatively shorter and more tapered; this may also have been true for the first tooth, but it was not preserved. The teeth further back in the row also decreased in relative height hindwards. By comparison, the front four to five dentary teeth of *Erlikosaurus* were conidont (cone-shaped) with a gradual transition to foliodont teeth.

The dentary teeth were tightly packed, but not pressed closely together, with the tooth crowns approaching each other at mid-length. The denticles (serrations) were large and bulbous, diminishing slightly in size towards the tooth tips, with about 5–6 denticles per 3 mm (0.12 in). The front carinae (cutting edges) folded upwards to overlap the inner surface of the crowns on the third to eighteenth teeth, but such folds were absent on the second and probably first crowns. The denticles were roughly perpendicular with the tip of the tooth crowns but parallel to the crown height on the front side fold and triangular facet on the hind side. There was a series of accessory denticles (in addition to those on the carinae) that projected from the front surfaces of the carinal folds, which made the front edges of the crowns more broadly roughened. The carinae of the hind edges were also very modified, and bifurcated (split in two) near the cervix, where they formed a flattened triangular, raised facet, which projected from the tooth crown and contacted or approached the folded carinae on the front edge of the crowns behind them (this arrangement is present in teeth 2–12). Such split carinae are known from other tetanuran theropods, where they are considered abnormalities caused by trauma, aberrant tooth replacement, or genetic factors. Though the condition in *Segnosaurus* was similar, it was uniformly expressed across the teeth of both dentaries, and does not appear to have been an abnormality, but served to roughen the contacts between tooth bases.

The 22nd and 23rd dentary teeth of *Segnosaurus* were significantly smaller than the rest, almost conidont, and had an additional third carina with denticles on their inner sides. Most of the other hindmost tooth crowns are damaged so their complete features are unknown. The additional carina on tooth 23 appears to have been fully denticulated while the denticles were restricted to the basal side of the crown in tooth 22. *Segnosaurus* was unique among all known theropods in possessing triple carinae. The 14th alveolus on the right dentary of the holotype is walled over by seemingly pathological (due to injury or disease) bone growth but the teeth in that part of the dentary are damaged so it is not possible to determine how the teeth were affected by this. The teeth in the same area of the left dentary bear triple carinae, though this dentary has no external indications of pathology that could have led to this condition, thus it cannot be concluded nor ruled out that this feature is the result of a pathology. *Segnosaurus* replaced its teeth in waves running from back to front of the jaws, that encompassed two to three erupting crowns. Some of the fully erupted teeth have wear on the carinae of their hind sides, unlike what is seen in other therizinosaurs. The texture of the enamel appears to have been broadly irregular and the roots of the teeth were almost circular.

### Postcranial skeleton

The scapula (shoulder blade) of *Segnosaurus* was straight and flat at the upper end, and was fused to the coracoid bone, forming the scapulocoracoid. The coracoid was very wide, rectangular in outline and thick at the middle. The massive humerus was 560 mm (22 in) in length; it had an almost-cylindrical shaft and well-defined condyles for articulation with the radius and ulna of the lower arm. The deltopectoral crest, where the deltoid muscle was attached to the upper front of the humerus, was well-developed. The humerus was distinct from those of other therizinosaurs, being straight rather than sigmoid shaped and not expanded or deflected forwards at its upper end. The humerus was also not expanded at the middle, and the entepicondyle was not well-developed. The lack of these features was more similar to ornithomimosaurs and troodontids than to other therizinosaurs. The radius was also massive—about 60 percent of the humerus—with a straight shaft. The ulna was thicker than the radius and slightly longer—about 70 percent of the humerus—and slightly twisted along its middle axis. The hand was tridactyl (three-fingered). The phalanx bones of the fingers were flattened from top to bottom and the articular depressions on their sides were not very developed. The first phalanx of the first finger was long and thin while the first and second phalanxes of the second finger were short. The ungual of the third finger was somewhat longer than the second phalanx and quite flat from top to bottom, which may have been a unique feature of *Segnosaurus*. This ungual was sharpy curved, very pointed, and compressed from side to side. The lower tubercle, where the flexor tendons attached to the ungual, was thick and robust.

The pelvis of *Segnosaurus* was robust and had sharply sideways-directed lobes at the front. The pelvis was shortened at the front, a feature found among bird-like theropods but uncommon among theropods as a whole. The pubic bone was directed backwards and down in parallel with the ischium; this backwards orientation of the pubic bone is known as the opisthopubic condition. This feature is only known from birds and their closest coelurosaurian relatives while other theropod dinosaurs had forwards-directed pubic bones. The pubic bone was elongated, flattened sideways, and had an ellipsoid projection or "boot" at the front of its lower end. The pelvis was distinct from those of other therizinosaurs in that the upper margin of the ilium had a pronounced overhang on the lower side and that the hindwards projecting process of the ischium was extensive, almost 50 percent of the front-to-back length of the obturator process. Some features of the pelvis were similar to that of *Nothronychus*, particularly the ischia, but it is uncertain whether these similarities were due to them having a common ancestor to the exclusion of other derived therizinosaurids, or because they retained basal features since lost in other relatives. The ischium of *Segnosaurus* was distinct from that of *Nothronychus* in that it had an almost-rectangular obturator process and an almost-circular obturator foramen. The pelvis was distinct from that of *Enigmosaurus* by its deep obturator process not fusing with its counterpart at the middle, by its unfused pubic boot, and because the lower part of the pubic shaft was wide from front to back. *Segnosaurus* was distinct from both *Nothronychus* and *Enigmosaurus* in having a deep brevis fossa (a groove where the caudofemoralis brevis muscle of the tail originated) and because its pubic boot lacked a well developed hindwards projection.

The femur was straight with an oval cross-section and was 840 mm (33 in) in length. The head of the femur was placed on a long "neck" and the lower condyles were well-defined. The tibia was straight, slightly shorter than the femur, and twisted along its axis. The fibula was long and narrowed towards its lower end. The metatarsus of the foot was short, massive, and consisted of five bones—four of which functioned as support elements and terminated in four toes. Functionally tetradactyl (four-toed) feet were unique to derived therizinosaurs; basal therizinosaurs and all other theropods had tridactyl feet in which the first toe was short and did not reach the ground. Externally, the metatarsus was similar to, though proportionally larger than, those of prosauropods, an early evolutionary grade of sauropodomorphs. The epiphyses on the upper metatarsals were hypertrophied (enlarged), a distinctive feature of the genus. The first toe was shorter than the others but was of equal functional importance; the second and third toes were equally long while the fourth was thinnest. The toe ungual was robust, sharply curved, flat at the side, and more pointed than those of prosauropods. The lower tubercle where the flexor ligaments attached was robust. While the lack of strong compression of the toe unguals distinguished *Segnosaurus* from *Erlikosaurus* from the same formation, the lack of compression was common among therizinosaurs and therefore not unique to *Segnosaurus*. The cervical vertebrae were platycoelous and had large, massive centra (bodies) and low neural arches. The sacrum consisted of six, firmly fused vertebrae; the centra of these vertebrae were broadened and relatively elongated, and each centrum was slightly longer than their width. The neural spines here were not very long but surpassed the level of the ilia. The caudal (tail) vertebrae closest to the body were massive, high, and somewhat compressed from side to side. The neural arch was low with a small neural canal. The caudal vertebrae closer to the tip of the tail were platycoelous and had short, massive centra. The transverse processes of the caudal vertebrae and the ribs were robust and elongated.