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.

It includes the mandible (lower jaws), an incomplete humerus, a complete radius and ulna (lower arm bones), phalanges of the fingers, a forelimb ungual (claw bone), an almost-complete pelvis, an incomplete right femur, six sacral vertebrae, ten caudal vertebrae from the front of the tail, fifteen from the hindmost part of the tail, the first gastral rib, and fragments of the dorsal ribs.

[10][11] 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.

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 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.

Their mosaic of features resembling those of different dinosaur groups and the scarcity of their fossils led to controversy over their evolutionary relationships for decades after their initial discovery (the forelimb elements of Therizinosaurus itself were originally identified as belonging to a giant turtle when described in 1954).

Using features of their humeri and hand claws, he distinguished Segnosauridae from the theropod families Deinocheiridae and Therizinosauridae, which were then only known from the genera Deinocheirus and Therizinosaurus, respectively, mainly represented by large forelimbs found in Mongolia.

[1] Later in 1979, Barsbold and Perle found the pelvic features of segnosaurids and dromaeosaurids were so different from those of "true" theropods that they should be separated into three taxa of the same rank, possibly at the level of infraorder within Saurischia, one of the two main divisions of dinosaurs—the other being Ornithischia.

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.

[25] 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.

[27][28] 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).

[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.

[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] Falcarius Jianchangosaurus Beipiaosaurus Alxasaurus Erliansaurus Nanshiungosaurus Neimongosaurus Segnosaurus Erlikosaurus Suzhousaurus Enigmosaurus Therizinosaurus Nothronychus 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.

[23] Based on the more complete remains of Alxasaurus and the articulation of its vertebral column, Russell concluded in 1993 that Paul's skeletal restoration was inaccurate and that the arms of therizinosaurs were held clear off the ground.

[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.

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.

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.

[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.

[27] In 1993 and 1997, Russell suggested therizinosaurs would have "sat" on their pelvises and supported their bodies on their hind limbs while using their long arms, claws, and flexible necks to reach leaves from trees and bushes with their beaks.

This parallels the way some herbivorous mammals use their forelimbs to manipulate vegetation; Russell considered the extinct chalicotheres and ground sloths, as well as gorillas, adaptively convergent with therizinosaurs.

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.

Because multiple geological formations show evidence of sympatric therizinosaur species—related species that lived in the same area at the same time—it is possible 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.

[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.

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.

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.

[49] 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.

[52][53] Therizinosaurs were the most abundant theropods in the Bayan Shireh Formation in terms of biodiversity; in addition to Segnosaurus, members of the group included Erlikosaurus, Enigmosaurus, and possibly a fourth type.