'', specimen IGM 100/15 displayed at
Nagoya City Science Museum Therizinosaurs were long considered an enigmatic group, whose mosaic of features resembling those of various different dinosaur groups, and scarcity of their fossils, led to controversy over their evolutionary relationships for decades after their initial discovery. The first genus,
Therizinosaurus, was originally identified as a turtle when described from forelimb elements in 1954. Later in 1979, Barsbold and Perle found the pelvic features of segnosaurids and dromaeosaurids 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). 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. Combined, 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 were distinct when examined in detail. While they were essentially different from other theropods (perhaps due to diverging from them relatively early), and therefore warranted a new infraorder, they did show similarities with them. Since the
Erlikosaurus specimen lacked a pelvis, the authors were unsure if that of the undetermined segnosaurian could belong to it, in which case they would consider it part of a separate family. Though
Erlikosaurus was difficult to compare directly to
Segnosaurus due to the incompleteness of their remains, Perle stated in 1981 that there was no justification for separating it into another family. of the holotype of
Segnosaurus, which together with
Erlikosaurus became the basis of the new
infraorder Segnosauria; this group is now a
synonym of Therizinosauria In 1982, Perle reported 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 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, which he placed in its own family, Enigmosauridae, within Segnosauria. Though the structure of the pelvis of
Erlikosaurus was unknown, Barsbold considered it unlikely the
Enigmosaurus pelvis belonged to it, since
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 that they went "beyond the borders" of this group, but opted to retain them within Theropoda. In the same year, Barsbold stated that the segnosaurian pelvis deviated strongly from the theropod norm, and found the configuration of their ilia generally similar to those of
sauropods. -like, quadrupedal
Erlikosaurus. Therizinosaurs were often depicted this way until they were definitively identified as
theropods Paleontologist
Gregory S. Paul concluded in 1984 that segnosaurs did not possess any theropodan features, but were instead derived, late-surviving
Cretaceous prosauropods with adaptations similar to those of ornithischians. He found segnosaurs similar to prosauropods in the morphology of their snout, mandible, and hindfoot, and to ornithischians in their cheek, palate, pubis, and ankle, and similar to early dinosaurs in other respects. He proposed that ornithischians were descended from prosauropods, and that the segnosaurs were an intermediate relict of this transition, which supposedly took place during the
Triassic period. In this way, he considered segnosaurians to be to herbivorous dinosaurs what
monotremes are to mammals. He did not rule out that segnosaurs could be derived from theropods, or that segnosaurs, prosauropods and ornithischians were each independently derived from early dinosaurs, but found these options unlikely. He considered the common descent of these groups as support for the idea that dinosaurs were a
monophyletic (natural) group, which was contested by some paleontologists at the time (who instead thought different dinosaurs groups evolved independently from
thecodonts). Paleontologist
David B. Norman considered Paul's idea a contentious claim "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 that paleontologist
Robert Bakker had created in 1985 to retain all plant-eating dinosaurs. In a 1986 study of the interrelationships of
saurischian dinosaurs, paleontologist
Jacques Gauthier concluded that segnosaurs were prosauropods. While he conceded they had similarities with ornithischians and theropods, he proposed these featured had evolved independently. In a 1989 conference abstract about sauropodomorph interrelationships, paleontologist
Paul Sereno also considered segnosaurs as prosauropods, based on skull features. from China, the completeness of which confirmed therizinosaurs as theropods in 1993 In a 1990
review article, Barsbold and paleontologist
Teresa Maryańska found Segnosauria to be a rare and aberrant group of saurischians, in an unresolved position among sauropodomorphs and theropods, probably closer to the former. They therefore listed them as Saurischia
sedis mutabilis ("position subject to change"). Though they agreed the hindlimbs assigned to
Therizinosaurus in 1982 were segnosaurian, they did not consider this justification for
Therizinosaurus itself being a segnosaur, since it was only known from forelimbs. In 1993, paleontologists
Dale A. Russell and
Dong Zhi-Ming described the new genus
Alxasaurus from China, at the time the most complete large theropod from its time and place. While it was similar to prosauropods in some respects, the detailed morphology of its limbs linked it to
Therizinosaurus and segnosaurs. Since it preserved both fore and hindlimbs,
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 Therizinosauridae (since the latter name was older), with
Alxasaurus being the most completely known representative so far, providing a better understanding of the group. They also named the new higher taxonomic rank
Therizinosauroidea to contain
Alxasaurus and Therizinosauridae (since the new genus was somewhat different from its relatives), which they placed in the group
Tetanurae within Theropoda. They considered therizinosaurs most closely related to ornithomimids, troodontids, and oviraptorids, which they placed together in the group
Oviraptorosauria (since they found
Maniraptora, the conventional grouping of these, invalid, and the higher level taxonomy of theropods was in flux at the time). therizinosaur
Beipiaosaurus with impressions of
feather structures,
Paleozoological Museum of China The synonymy of Segnosauridae with Therizinosauridae was accepted by Perle himself and co-authors of a redescription of the holotype skull of
Erlikosaurus in 1994, and they considered therizinosaurs maniraptoran theropods, the group that also includes modern birds (since they did find Maniraptora to be valid through their analysis). They also discussed the previous ornithischian and sauropod hypotheses for therizinosaur affinities in detail and demonstrated various faults with them. Palaeontologist Lev Alexandrovich Nessov rejected that therizinosaurs were theropods in 1995, and instead considered them a distinct group within saurischia. In 1996, paleontologist
Thomas R. Holtz Jr. found therizinosaurs to group with oviraptorosaurs in a
phylogenetic analysis of coelurosauria. In 1999, paleontologist Xing Xu and colleagues described a small, basal therizinosauroid from China,
Beipiaosaurus, which confirmed that the group belonged among the coelurosaurian theropods, and that similarities with prosauropods had evolved independently. They published the first ever
cladogram showing the evolutionary relationships of Therizinosauria, and demonstrated that
Beipiaosaurus had features of more basal theropods, coelurosaurs, and therizinosaurs. Sereno found Therizinosaurs to be basal
Ornithomimosaurian theropods during the year 1999. By the early 21st century, many more therizinosaur taxa had been discovered, including outside Asia (the first being
Nothronychus from North America), as well as various basal taxa that helped understanding of the early evolution of the group (such as
Falcarius, also from North America). Therizinosaurs were not considered as rare or aberrant anymore, but more diverse than previously thought (including in size), and their classification as maniraptoran theropods was generally accepted. The placement of Therizinosauria within Maniraptora continued to be unclear; in 2007, paleontologist Alan H. Turner and colleagues found them to group with oviraptorosaurs, while Zanno and colleagues found them to be the most basal clade within Maniraptora in 2009, bracketed by
Ornithomimosauria and
Alvarezsauridae. 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 until 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. Wills, Underwood & Barrett (2023) assigned specimen GLCRM G167-32, a tooth from the
Bathonian-aged
Chipping Norton Limestone in
England, to the Therizinosauroidea, making this the oldest record of Therizinosauroidea and also the first record of Therizinosauroidea in
Europe. Barsbold and Maryanska in 1990 considered
C. zheziangensis as a tentative segnosaur (later known as therizinosaurs) based on its relatively short and robust pedal phalanges and enlarged, strongly curved unguals, mostly similar to
Segnosaurus. As this taxon may lie outside the genus
Chilantaisaurus, they listed this species as
"Chilantaisaurus" zheziangensis. Although Glut (1997) stated this specimen may have been based on part of the holotype of Nanshiungosaurus brevispinus (based on a pers. comm from Dong to Molnar in 1984), Dong in 1979 described both taxa from largely different formations and localities. Hartman with team in 2019 added
"C". zheziangensis to a phylogenetic analysis and recovered it within Therizinosauroidea in a polytomy with
Alxasaurus,
Enigmosaurus and therizinosaurids. Li and colleagues in their 2007 description of
Suzhousaurus pointed out that
N. bohlini might be synonymous with the former, as both are found in the same geological group and also incompletely known. As per terms of
taxonomic priority, the species name would be
Suzhousaurus bohlini. However, they noted that a direct comparison between specimens is difficult to near impossible because there is no overlapping material (besides dorsal vertebrae) and the holotype of
N. bohlini is apparently lost. Li and team disagree in that this species belong to
Nanshiungosaurus and listed it as
"Nanshiungosaurus" bohlini. Zanno in 2010 indicated that the anatomical traits that were originally used to characterize
"N." bohlini are now known to be present in other therizinosaur taxa. The specimen was later described in depth in 2001 and used as the holotype specimen for the new genus and species
Eshanosaurus deguchiianus, named by Xing and colleagues. The team reinforced therizinosaur relationships, arguing that the teeth morphology of
Eshanosaurus can be differentiated from
sauropodomorphs. In the same 2001 however,
James I. Kirkland and
Douglas G. Wolfe noted that the holotype of
Eshanosaurus preserves traits only seen in sauropodomorphs. ==Description==