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
thumb|right|upright=1.4|[[Cretaceous-aged dinosaur fossil localities of Mongolia; Segnosaurus was found by areas C and D (right, Amtgay and Khara-Khutul localities).]]
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.
Description
thumb|Size compared to a human
Segnosaurus was a large-bodied therizinosaur that is estimated to have been about long and to have weighed about . 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. 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.
left|thumb|Reconstructed [[holotype pelvis in left side view and in top view]]
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.
thumb|left|[[Therizinosaurus, the first known therizinosaur, was originally known only from forelimb bones from Mongolia (cast shown here, in Aathal Dinosaur Museum), which created confusion about its affinities with other 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 ornithomimosaur). Barsbold retained Segnosaurus and Erlikosaurus in the family Segnosauridae in 1983 and named the new genus Enigmosaurus based on the previously undetermined segnosaurian pelvis. The structure of the pelvis of Erlikosaurus was unknown but Barsbold considered it unlikely the Enigmosaurus pelvis belonged to it because Erlikosaurus and Segnosaurus were so similar in other respects while the pelvis of Enigmosaurus was very different from that of Segnosaurus. Barsbold found that segnosaurids were so peculiar compared to more typical theropods that they were either a very significant deviation in theropod evolution, or were possibly outside the group; he nevertheless retained them within Theropoda.
thumb|Outdated restoration of a [[prosauropod-like, quadrupedal Erlikosaurus. "Segnosaurs" were often depicted this way until they were definitively identified as theropods.]]
Gregory S. Paul concluded in 1984 that segnosaurs had no theropodan features but were derived, late-surviving Cretaceous prosauropods with adaptations similar to those of ornithischians. He found segnosaurs to be similar to prosauropods in the morphology of their snout, mandible, and hindfoot; to ornithischians in their cheek, palate, pubis, and ankle; and to early dinosaurs in other respects. He proposed that ornithischians were descended from prosauropods and that the segnosaurs were an intermediate relic of this transition, which supposedly took place during the Triassic period. In this way, he considered segnosaurians to have a comparable position to herbivorous dinosaurs in general, as monotremes have to mammals. He found it unlikely but did not rule out that segnosaurs could have derived from theropods or that segnosaurs, prosauropods and ornithischians were each independently derived from early dinosaurs. David B. Norman considered Paul's idea contentious and "bound to provoke much argument" in 1985. In 1988, Paul maintained that segnosaurs were late-surviving, ornithischian-like prosauropods and proposed a segnosaurian identity for Therizinosaurus. He also placed segnosauria within Phytodinosauria, a superorder Robert Bakker had created in 1985 to contain all plant-eating dinosaurs. 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. In a 1989 conference abstract about sauropodomorph inter-relationships, Paul Sereno also considered segnosaurs to be prosauropods, based on skull features.
thumb|left|Reconstructed skeleton of [[Alxasaurus from China in Royal Tyrrell Museum, the completeness of which confirmed therizinosaurs as theropods]]
In a 1990 review article, Barsbold and Teresa Maryańska found Segnosauria to be a rare and aberrant group of saurischians in an unresolved position among sauropodomorphs and theropods, and probably closer to the former. Accordingly, they listed them as Saurischia sedis mutabilis (position subject to change). They agreed the hindlimbs assigned to Therizinosaurus in 1982 were segnosaurian but did not consider this a sufficient justification for Therizinosaurus itself being a segnosaur because it was only known from forelimbs. In 1993, Dale A. Russell and Dong Zhi-Ming described the new genus Alxasaurus from China; at the time this was the most complete large theropod from its time and place. While Alxasaurus was similar in some respects to prosauropods, the detailed morphology of its limbs linked it to Therizinosaurus and segnosaurs. Because its fore and hindlimbs were preserved, Alxasaurus showed that Perle's assignment of segnosaurian hindlimbs to Therizinosaurus was probably correct. Russell and Dong, therefore, proposed that Segnosauridae was a junior synonym of the older name Therizinosauridae, and that Alxasaurus was the most completely known representative so far. They also named the new higher taxonomic rank 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). In 1995, Lev A. Nessov rejected the idea therizinosaurs were theropods; he considered them a distinct group within Saurischia. Russell coined the name Therizinosauria for the wider group in 1997. 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. By 2015, Segnosaurus remained one of the best known therizinosaurs, according to Christophe Hendrickx and colleagues.
Paleobiogeography
thumb|upright=1.8|Known elements of various therizinosaurs shown to scale, with Segnosaurus in the upper middle
The basalmost definite therizinosaur is Falcarius from the Early Cretaceous of North America; it showed the pelvis and dentition were the first features that were modified away from the more general maniraptoran plan in therizinosaurs, probably reflecting their transition from carnivory to herbivory. Therizinosaurs are mainly known from the Cretaceous of Asia and North America, and possible remains from other ages and places are controversial. Since therizinosaurs are known to have lived across the supercontinent Laurasia (which consisted of what are now North America, Europe, and Asia), Zanno suggested two scenarios for their paleobiogeographic distribution in 2010. One possibility is they dispersed through vicariance, whereby therizinosaurs were present in the areas that became Asia and North America before the rifting that divided these areas in the Late Triassic. The other possibility is that basal therizinosaurs dispersed between Asia and North America via Europe after the rifting event but before the middle Barremian; between 132 and 138 million years ago, a temporary land bridge connected North America and Europe, whereafter the landmasses were again isolated from each other, explaining why the basal therizinosaurs Beipiaosaurus from Asia and Falcarius from North America were so morphologically divergent from each other, though coeval. The presence of the derived therizinosaurid Nothronychus, which was most-closely related to Asian genera, in North America during the Turonian stage of the early Late Cretaceous also shows there would have been a 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.
In a 2012 study of the anatomy of Erlikosaurus and other therizinosaurs that preserve , 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. 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.
thumb|Nest attributed to therizinosaurs, [[Dinosaurland Fossil Museum]]
Dinosaur eggs with embryos of the Dendroolithidae type from the Nanchao Formation of China were identified as belonging to therizinosaurs and described by Martin Kundrát and colleagues in 2007. The development of the embryos and the fact that no adults were found in association with the nests indicate that therizinosaur hatchlings were precocial (capable of locomotion from birth) and able to leave their nests to feed alone, independently of their parents.
Diet and feeding
thumb|left|Right half of the holotype mandible shown from the inner side (A), the top (C), and bottom (D), and the left half shown from the inner side (B), compared with the mandibles of other therizinosaurs (to scale in lower right)
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, Anatoly K. Rozhdestvensky suggested Therizinosaurus—the only member of the group known at the time—used its large claws to open termite mounds or gather fruits from trees. Based on the assemblage of fossils in the Bissekty Formation of Uzbekistan, Nessov suggested in 1995 that therizinosaurs could have been part of its nutrient-rich aquatic ecosystems, though perhaps indirectly, by feeding on wasps which had themselves fed on carrion of aquatic vertebrates. He found this consistent with Rozhdestvensky's suggestion that therizinosaurs may have fed on social insects.
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 cursorial (related to running) adaptations in the hind limbs, including development of functionally tetradactyl feet. Zanno and colleagues found the clades at the base of Maniraptora—Ornithomimosauria, Therizinosauria, and Oviraptorosauria—had either direct or morphological evidence for herbivory, which would mean either this diet evolved independently multiple times in coelurosaurian theropods or that the primitive condition of the group was at least facultative herbivory with carnivory only emerging in more derived maniraptorans. Zanno and Peter J. Makovicky found, in 2011, therizinosaurs and some other groups of herbivorous dinosaurs that had beaks and retained teeth were unable to lose their teeth completely because they lacked gastric mills (gizzards) and needed the teeth to process food, and that the high-fiber folivorous (leaf-based) diet of therizinosaurs and other archosaurs may also have precluded the evolution of a complete beak. 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.
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 ventilatory muscles that were crucial for cuirassal ventilation—breathing with extra air sacs—which shows the importance of this mode of respiration. In a 2019 study of jaw musculature, Ali Nabavizadeh concluded therizinosaurs were mainly orthal feeders—moving their jaws up and down—and raised their jaws isognathously whereby the upper and lower teeth of each side occluded (contacted each other) at once. The origin and insertion sites of their jaw muscles also added strength to their jaw closure. David J. Button and Zanno found in 2019 herbivorous dinosaurs mainly followed two distinct modes of feeding, either processing food in the gut—characterized by gracile skulls and low bite forces—or the mouth, characterized by features associated with extensive processing. Segnosaurus, along with diplodocoid and titanosaur sauropods, deinocheirid and ornithomimid ornithomimosaurs, and 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.
Paleoenvironment
thumb|upright=1.5|Dinosaurs from the [[Bayan Shireh Formation compared in size (Segnosaurus fifth from right, in dark red)]]
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 Cenomanian to Turonian stages of the Late Cretaceous period, based on paleomagnetic analysis and calcite U–Pb measurements. The remains were found in poorly cemented, gray sands containing intraformational conglomerates, gravel, and gray claystone. Other theropods included the tyrannosaur Khankhuuluu, the ornithomimosaur Garudimimus, and the dromaeosaur Achillobator. Other dinosaurs included the ankylosaur Talarurus, the sauropod Erketu, and the ceratopsian Graciliceratops. Dinosaur eggs, some of which were identified as Dendroolithidae, as well as footprints of dinosaurs and crocodyliforms, have also been found. The formation is distinctive for its variety and abundance of turtles, and invertebrates include ostracods and freshwater molluscs.
See also
- Timeline of therizinosaur research