Segnosaurus

Segnosaurus Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Diagram showing known remains
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Therizinosauridae
Genus:	†Segnosaurus Perle, 1979
Species:	†S. galbinensis
Binomial name
†Segnosaurus galbinensis Perle, 1979

Segnosaurus is a

In 1973, a joint

Segnosaurus was a large-bodied

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.^[6][1]

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.^[6] 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.^[1]

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.^[6][1]

The dentary teeth were tightly packed, but not pressed closely together, with the tooth crowns approaching each other at mid-length. The

The

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.

In 1980, Barsbold and Perle named the new theropod infraorder Segnosauria, containing only Segnosauridae. In the same article, they named the new genus Erlikosaurus (known from a well-preserved skull and partial skeleton)—which they tentatively considered a segnosaurid—and reported a partial pelvis of an undetermined segnosaurian, both from the same formation as Segnosaurus. The specimens provided relatively complete data on this group; they were united by their opisthopubic pelvis, slender mandible, and the toothless front of their jaws. Barsbold and Perle stated that, though some of their features resembled those of ornithischians and sauropods, these similarities were superficial and distinct when examined in detail. While they were essentially different from other theropods—perhaps due to diverging from them relatively early—and warranted a new infraorder, they did show similarities with the theropods. Because the Erlikosaurus specimen lacked a pelvis, the authors were unsure that the undetermined segnosaurian could belong to the same genus, in which case they would consider it part of a separate family.[5] Though Erlikosaurus was difficult to compare directly to Segnosaurus because its remains were incomplete, Perle stated in 1981 there was no justification for separating it into another family.^[17]

theropods

.

In 1982, Perle reported the discovery of hindlimb fragments similar to those of Segnosaurus and assigned them to Therizinosaurus, whose forelimbs had been found in almost the same location. He concluded that the Therizinosauridae, Deinocheiridae, and Segnosauridae, which all had enlarged forelimbs, represented the same taxonomic group. Segnosaurus and Therizinosaurus were particularly similar, leading Perle to suggest they belonged in a family to the exclusion of Deinocheiridae (today, Deinocheirus is recognized as an

sauropods.^[20]

prosauropod

-like, quadrupedal Erlikosaurus. "Segnosaurs" were often depicted this way until they were definitively identified as theropods.

Robert Bakker had created in 1985 to contain all plant-eating dinosaurs.^[23] In a 1986 study of the inter-relationships of saurischian dinosaurs, Jacques Gauthier concluded segnosaurs were prosauropods. While he conceded they had similarities with ornithischians and theropods, he proposed these features had evolved independently.^[24] In a 1989 conference abstract about sauropodomorph inter-relationships, Paul Sereno also considered segnosaurs to be prosauropods, based on skull features.^[25]

Royal Tyrrell Museum

, the completeness of which confirmed therizinosaurs as theropods

In a 1990

Therizinosauroidea to contain Alxasaurus and Therizinosauridae because the new genus was somewhat different from its relatives. They concluded that therizinosaurs were tetanuran theropods, most closely related to ornithomimids, troodontids, and oviraptorids, which they placed together in the group Oviraptorosauria (because they found Maniraptora—the conventional grouping of these—invalid, and the higher-level taxonomy of theropods was in flux).^[27][28]

feather-like structures, Paleozoological Museum of China

Perle and his co-authors of a 1994 redescription of Erlikosaurus's skull accepted the synonymy of Segnosauridae with Therizinosauridae and they considered therizinosaurs to have been maniraptoran theropods, the group that also includes modern birds (because they did find Maniraptora to be valid through their analysis). They also discussed the alternative previous hypotheses for therizinosaur affinities and demonstrated faults with them.

phylogenetic analysis of coelurosaurian theropods.^[31] Russell coined the name Therizinosauria for the wider group in 1997.^[28] In 1999, Xing Xu and colleagues described Beipiaosaurus, a small, basal therizinosaur from China, which confirmed the group belonged among the coelurosaurian theropods, and that similarities with prosauropods had evolved independently. They published the first cladogram showing the evolutionary relationships of Therizinosauria and demonstrated Beipiaosaurus retained features of more basal theropods and coelurosaurs, which linked them with therizinosaurs. The preservation of feather-like structures in Beipiaosaurus also suggested this feature was more widely distributed among theropods than previously thought.^[10]

By the early 21st century, many more therizinosaur taxa had been discovered—including some outside Asia—the first being Nothronychus from North America in 2001. Basal taxa that helped illuminate the early evolution of the group, such as Falcarius in 2005, had also been discovered. Therizinosaurs were no longer considered as rare or aberrant but more diverse in features—including size—than previously thought and their classification as maniraptoran theropods was generally accepted.[32]^[33][34] The placement of Therizinosauria within Maniraptora continued to be unclear; in 2017, Alan H. Turner and colleagues found them to group with oviraptorosaurs while in 2009 Zanno and colleagues found them to be the most basal clade within Maniraptora, bracketed by Ornithomimosauria and Alvarezsauridae.^[35][36] Despite the additional fossil material, the interrelations within the group were also still uncertain by 2010, when Zanno conducted the most detailed phylogenetic analysis of the Therizinosauria to that point. She cited the inaccessibility, damage, potential loss of holotype specimens, scarcity of cranial remains, and fragmentary specimens with few overlapping elements as the most significant obstacles to resolving the evolutionary relationships within the group. The position of Segnosaurus and those of some other Asian therizinosaurids was affected by these factors; Zanno stated more well-preserved specimens and the rediscovery of missing elements would be necessary. Zanno also revised Therizinosauroidea to exclude Falcarius and retained it in the wider clade Therizinosauria, which became the senior synonym of Segnosauria.^[4] By 2015, Segnosaurus remained one of the best known therizinosaurs, according to Christophe Hendrickx and colleagues.^[11]

The following cladogram shows the relationships within Therizinosauria according to a 2013 study by Hanyong Pu and colleagues, which was based on Zanno's 2010 analysis, with the addition of the basal genus Jianchangosaurus:^[37]

Therizinosauria	Falcarius unnamed Jianchangosaurus Therizinosauroidea Beipiaosaurus unnamed Alxasaurus Therizinosauridae Erliansaurus Nanshiungosaurus Neimongosaurus Segnosaurus Erlikosaurus Suzhousaurus Enigmosaurus Therizinosaurus Nothronychus

The basalmost definite therizinosaur is Falcarius from the

faunal interchange between North America and Asia via a late-Early Cretaceous land bridge before that (during the Aptian/Albian), which is also seen in some other dinosaur groups.^[4]

Paleobiology

In 1979 and 1981, Barsbold and Perle said the short, massive metatarsus and unusually large, splayed toes indicated that Segnosaurus and its relatives were not adapted for rapid locomotion, perhaps because it was not required by their lifestyle; Barsbold and Perle suggested they could have been

sloths.^[30]

In a 2012 study of the endocranial anatomy of Erlikosaurus and other therizinosaurs that preserve braincases, Stephan Lautenschlager and colleagues found these dinosaurs had well-developed senses of smell, hearing, and balance. The former two senses may have played a role in foraging, predator evasion, and social behavior. These senses were also well-developed in earlier coelurosaurs, so therizinosaurs may have inherited these traits from their carnivorous ancestors and used them for different dietary purposes.^[40] In a 2014 study of the function of therizinosaur hand claws, Lautenschlager found that these would not have been used for digging, which would have been done with the foot claws because, since as in other maniraptorans, feathers on the forelimbs would have interfered with this function. He could neither confirm nor disregard that the hand claws could have been used for defense, combat, stabilization by grasping tree trunks during high browsing, sexual display, or gripping mates during copulation. He largely ruled out that they dug burrows, due to their size.^[41]

site fidelity (always returning to the same site to breed).^[43]

Diet and feeding

The unusual features of therizinosaurs have led to several interpretations of their feeding behavior; there is no direct evidence of their diet, such as stomach contents and feeding traces. In 1970,

termite mounds or gather fruits from trees.^[12] Barsbold and Perle pointed out in 1979 and 1980 that their peculiar features probably reflected a different evolutionary direction than those of more typical theropods, many of which were considered effective, active predators. Their delicate jaws, small, weak teeth and beaks, and short, compact feet indicated they would not have used the armaments of other theropods to procure food but could have preyed on fish.^[16][5] In 1983, Barsbold said the horny beak at the front of the jaws and weakened teeth at the back were common features among herbivorous dinosaurs but not of carnivorous theropods, and speculated this might indicate segnosaurs had shifted to herbivory.^[3] In 1984, Paul suggested they were herbivorous due to the similarities of their skulls to those of prosauropod and ornithischian dinosaurs, which include horny beaks, inset rows of teeth, and a shelf at the side of the jaws that indicated the presence of cheeks. Like ornithischians, they could, therefore, crop, manipulate, and chew plants in a sophisticated manner. He also suggested the ilia of the pelvis had sideways-flaring blades at the front similar to those of sauropods to support a large gut that was used to ferment and process food.^[21] Norman stated in 1985 the possibility Segnosaurus was an aquatic fish-eater could explain its small, pointed teeth and broad and perhaps webbed feet, but found it mysterious why it should have a horny beak.^[22]

pubic bone

; therizinosaurs may have "sat" on their pelvises during feeding

In 1993, Russell and Dong considered the small size of the head, blunt beaks and large body weights of therizinosaurs consistent with herbivory.

social insects.^[30] In a 2006 conference abstract, Sara Burch presented the inferred range of motion in the arms of the therizinosaur Neimongosaurus and concluded the overall motion at the glenoid-humeral joint at the shoulder was roughly circular, and directed sideways and slightly downwards, which diverged from the more oval, backwards-and-downwards-directed ranges of other theropods. This ability to extend their arms considerably forwards may have helped therizinosars reach and grasp for foliage.^[44]

In 2009, Zanno and colleagues stated therizinosaurs were the most-widely regarded candidates for herbivory among theropods and listed features associated with this diet. These included small, densely packed, coarse serrations; lanceolate (lance-shaped) teeth with a low replacement rate; a beak at the front of the jaws; an inset tooth row that suggests fleshy cheeks; an elongated neck; a small skull; a very large gut capacity as indicated by the rib circumference at the trunk and the outwards flaring processes of the ilia; and the loss of

archosaurs may also have precluded the evolution of a complete beak.^[45] Lautenschlager found in 2014 the hands of therizinosaurs would have had to be able to extend the range of the animal to a point that could not be reached by the head if they were used for browsing and pulling down vegetation. In genera where both neck and forelimb elements are preserved, however, the necks were equal in length or longer than the forelimbs, so pulling vegetation would only make sense if lower parts of long branches were pulled down to access out-of-reach parts of trees.^[41]

Zanno and colleagues stated in 2016 that therizinosaurs were generally accepted to fall within the spectrum of

niche partitioning between them could have played a role in the group's evolutionary success. This is supported by Segnosaurus with its highly specialized dentition being a contemporary of Erlikosaurus with its relatively indistinct teeth, indicating their partitioning in food acquisition, processing, or resources. This conclusion is also strengthened by the large difference in estimated body masses of sympatric therizinosaurs in the Bayan Shireh Formation, which was up to 500%.^[6]

In a 2017 study of niche partitioning in therizinosaurs through digital simulations, Lautenschlager found the dentaries of Segnosaurus experienced one of the lowest stress magnitudes during extrinsic feeding scenarios. Segnosaurus and Erlikosaurus were aided by the down-turned tip of the lower jaws and symphyseal regions, and probably also by beaks, which are known to mitigate stress and strain. By contrast, the straighter and more elongated dentaries of basal therizinosaurs—typical of their coelurosaurian ancestors—had the highest magnitudes of stress and strain. A downwards-pulling motion of the head while gripping vegetation was more likely than a sideways or upwards movement, though such behavior would be more likely in Segnosaurus and Erlikosaurus with their stress-mitigating jaws. Difference in relative bite force between the sympatric Segnosaurus and Erlikosaurus show the former would have been able to feed on tougher vegetation while the overall robustness of the latter suggests greater flexibility in its manner of feeding, because stress levels stayed low across feeding simulations. Lautenschlager agreed the two taxa were adapted to different modes of feeding and food selection; Segnosaurus was more adapted to using its dentition to procure or process food while Erlikosaurus mostly used its beak for cropping and its neck musculature while foraging. The difference in size between Segnosaurus and Erlikosaurus (the former of which is estimated to have weighed more than the latter) indicates these effects were increased and that there were further mechanisms partitioning their resources, such as different heights. Because other therizinosaur taxa were more divided in time and space, other factors than competition within their group may also have contributed to their variation, such as adaptations to different flora and competition with other kinds of herbivores.[46]

In 2018, Loredana Macaluso and colleagues pointed out that the hips of therizinosaurs were peculiar because the shaft of the pubic bone was rotated backwards whereas the pubic boot was strongly projected forwards. While the larger gut associated with herbivory was able to push the shaft backwards, they suggested the pubic boot was restrained by

caenagnathids, was found to be in the former category, whereas Erlikosaurus was more similar to some sauropodomorph and ornithischian taxa, indicating these two therizinosaurs were functionally separated and occupied different niches.^[49]

Paleoenvironment

Fossils of Segnosaurus have been recovered from the Bayan Shireh Formation in Mongolia, which has been dated to about 102–86 million years ago during the

semi-arid climate.^[52][53]

Therizinosaurs were the most abundant theropods in the Bayan Shireh Formation in terms of

ostracods and freshwater molluscs.^[50][26][53] The Bayan Shireh Formation is possibly coeval with the Iren Dabasu Formation of the Inner Mongolia region of China, from where therizinosaur fossils similar to those of Segnosaurus and Erlikosaurus have also been found.^[59]

See also

• Timeline of therizinosaur research

References

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External links

• Media related to Segnosaurus at Wikimedia Commons
• Data related to Segnosaurus at Wikispecies