Editing Segnosaurus (section)

===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 [[Gracility|gracile]]. The nearly complete right hemimandible (half of the mandible) is {{convert|379|mm|in|abbr=on}} long from front to back, {{convert|55.5|mm|in|abbr=on}} at the highest point, and {{convert|24.5|mm|in|abbr=on}} at the lowest. The {{Dinogloss|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 {{convert|25.5|mm|in|abbr=on}} on the right hemimandible of the holotype. Proportionally, the toothless part of the dentary is 20% of its tooth row, which is {{convert|150.3|mm|in|abbr=on}} 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 {{Dinogloss|surangular}} bone behind it; by contrast, the hind part of the dentary in ''Erlikosaurus'' gradually approached the surangular in a gentle arc.<ref name="Zanno2016">{{cite journal|last1=Zanno|first1=L. E.|last2=Tsogtbaatar|first2=K.|last3=Chinzorig|first3=T.|last4=Gates|first4=T. A.|author1-link=Lindsay Zanno|year=2016|title=Specializations of the mandibular anatomy and dentition of ''Segnosaurus galbinensis'' (Theropoda: Therizinosauria)|journal=PeerJ|volume=4|at=e1885|doi=10.7717/peerj.1885|pmid=27069815|pmc=4824891 |doi-access=free }}</ref><ref name="Perle1979" />

[[File:Segnosaurus hemimandible.jpg|thumb|left|Right half of the holotype mandible in outer and inner view, with component bones marked by different colors; the {{Dinogloss|dentary bone}} (green) bore the teeth.]]

''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 {{Dinogloss|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 {{Dinogloss|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 {{Dinogloss|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 {{Dinogloss|splenial}}, surangular, {{Dinogloss|angular}}, and {{Dinogloss|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.<ref name="Zanno2016"/> 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.<ref name="Perle1979" />

[[File:Mesial dentary teeth of Segnosaurus.png|thumb|upright|Frontmost dentary teeth, showing folded {{Dinogloss|carinae}} (lf) and accessory {{Dinogloss|denticles}} (ad)]]
''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.<ref name="Zanno2016"/><ref name="Perle1979" />

The dentary teeth were tightly packed, but not pressed closely together, with the tooth crowns approaching each other at mid-length. The {{Dinogloss|denticles}} (serrations) were large and bulbous, diminishing slightly in size towards the tooth tips, with about 5–6 denticles per {{convert|3|mm|in|abbr=on}}. The front {{Dinogloss|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.<ref name="Zanno2016"/>

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'' [[Polyphyodont|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 {{Dinogloss|enamel}} appears to have been broadly irregular and the roots of the teeth were almost circular.<ref name="Zanno2016"/>