| Therizinosaurs | |
|---|---|
 | |
| Collection of five therizinosaurs, clockwise from top left:Suzhousaurus,Erliansaurus,Nothronychus,Falcarius andJianchangosaurus. | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Maniraptora |
| Clade: | †Therizinosauria Russell, 1997 |
| Subgroups | |
| Synonyms | |
| |
Therizinosaurs (ⓘ; once calledsegnosaurs) are an extinct group of large herbivoroustheropoddinosaurs whose fossils have been mainly discovered fromCretaceous deposits inAsia andNorth America. Potential fragmentary remains have also been found inJurassic deposits ofAsia andEurope.[1][2] Various features of the forelimbs, skull and pelvis unite these finds as both theropods andmaniraptorans, making them relatives ofbirds. The name of the representative genus,Therizinosaurus, is derived from theGreekθερίζω (therízō, 'to reap' or 'scythe')[4] andσαῦρος (saûros, 'lizard'). The older representative,Segnosaurus, is derived from theLatinsēgnis ('slow') and the Greekσαῦρος.
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.[4] Perle noted in 1979 that theSegnosaurus fossils were possibly representative of a new family of dinosaurs, which he tentatively classified astheropods (traditionally thought of as the "meat-eating" dinosaurs). He named the family Segnosauridae, withSegnosaurus astype genus and sole member. He distinguished Segnosauridae from the theropod familiesDeinocheiridae and Therizinosauridae (then only known from the generaDeinocheirus andTherizinosaurus, both mainly represented by large forelimbs found in Mongolia) by features of their humeri and hand claws.[5] 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 ofinfraorder withinSaurischia (one of the two main divisions of dinosaurs, the other beingOrnithischia).[6]
In 1980, Barsbold and Perle named the new theropod infraorder Segnosauria, containing only Segnosauridae. In the same article, they named the new genusErlikosaurus (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 asSegnosaurus. 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 theErlikosaurus 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.[7] ThoughErlikosaurus was difficult to compare directly toSegnosaurus due to the incompleteness of their remains, Perle stated in 1981 that there was no justification for separating it into another family.[8]
In 1982, Perle reported hindlimb fragments similar to those ofSegnosaurus, and assigned them toTherizinosaurus, 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 andTherizinosaurus were particularly similar, leading Perle to suggest they belonged in a family to the exclusion of Deinocheiridae (today,Deinocheirus is recognized as anornithomimosaur).[9][10] Barsbold retainedSegnosaurus andErlikosaurus in the family Segnosauridae in 1983, and named the new genusEnigmosaurus based on the previously undetermined segnosaurian pelvis, which he placed in its own family, Enigmosauridae, within Segnosauria. Though the structure of the pelvis ofErlikosaurus was unknown, Barsbold considered it unlikely theEnigmosaurus pelvis belonged to it, sinceErlikosaurus andSegnosaurus were so similar in other respects, while the pelvis ofEnigmosaurus was very different from that ofSegnosaurus. 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.[11] 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 ofsauropods.[12]
PaleontologistGregory S. Paul concluded in 1984 that segnosaurs did not possess any theropodan features, but were instead derived, late-survivingCretaceous 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 theTriassic period. In this way, he considered segnosaurians to be to herbivorous dinosaurs whatmonotremes 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 amonophyletic (natural) group, which was contested by some paleontologists at the time (who instead thought different dinosaurs groups evolved independently fromthecodonts).[13] PaleontologistDavid B. Norman considered Paul's idea a contentious claim "bound to provoke much argument" in 1985.[14] In 1988, Paul maintained that segnosaurs were late surviving ornithischian-like prosauropods, and proposed a segnosaurian identity forTherizinosaurus. He also placed segnosauria withinPhytodinosauria, asuperorder that paleontologistRobert Bakker had created in 1985 to retain all plant-eating dinosaurs.[15] In a 1986 study of the interrelationships ofsaurischian dinosaurs, paleontologistJacques Gauthier concluded that segnosaurs were prosauropods. While he conceded they had similarities with ornithischians and theropods, he proposed these featured had evolved independently.[16] In a 1989 conference abstract about sauropodomorph interrelationships, paleontologistPaul Sereno also considered segnosaurs as prosauropods, based on skull features.[17]
In a 1990review article, Barsbold and paleontologistTeresa 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 Saurischiasedis mutabilis ("position subject to change"). Though they agreed the hindlimbs assigned toTherizinosaurus in 1982 were segnosaurian, they did not consider this justification forTherizinosaurus itself being a segnosaur, since it was only known from forelimbs.[18] In 1993, paleontologistsDale A. Russell andDong Zhi-Ming described the new genusAlxasaurus 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 toTherizinosaurus and segnosaurs. Since it preserved both fore and hindlimbs,Alxasaurus showed that Perle's assignment of segnosaurian hindlimbs toTherizinosaurus was probably correct. Russell and Dong therefore proposed that Segnosauridae was ajunior synonym of Therizinosauridae (since the latter name was older), withAlxasaurus being the most completely known representative so far, providing a better understanding of the group. They also named the new higher taxonomic rankTherizinosauroidea to containAlxasaurus and Therizinosauridae (since the new genus was somewhat different from its relatives), which they placed in the groupTetanurae within Theropoda. They considered therizinosaurs most closely related to ornithomimids, troodontids, and oviraptorids, which they placed together in the groupOviraptorosauria (since they foundManiraptora, the conventional grouping of these, invalid, and the higher level taxonomy of theropods was in flux at the time).[19][20]
The synonymy of Segnosauridae with Therizinosauridae was accepted by Perle himself and co-authors of a redescription of the holotype skull ofErlikosaurus 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.[21] Palaeontologist Lev Alexandrovich Nessov rejected that therizinosaurs were theropods in 1995, and instead considered them a distinct group within saurischia.[22] In 1996, paleontologistThomas R. Holtz Jr. found therizinosaurs to group with oviraptorosaurs in aphylogenetic analysis of coelurosauria.[23] 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 evercladogram showing the evolutionary relationships of Therizinosauria, and demonstrated thatBeipiaosaurus had features of more basal theropods, coelurosaurs, and therizinosaurs.[24] Sereno found Therizinosaurs to be basalOrnithomimosaurian theropods during the year 1999.[25]
By the early 21st century, many more therizinosaur taxa had been discovered, including outside Asia (the first beingNothronychus from North America), as well as various basal taxa that helped understanding of the early evolution of the group (such asFalcarius, 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.[26][27][28] 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 byOrnithomimosauria andAlvarezsauridae.[29][30] 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.[31]
Wills, Underwood & Barrett (2023) assigned specimen GLCRM G167-32, a tooth from theBathonian-agedChipping Norton Limestone inEngland, to the Therizinosauroidea, making this the oldest record of Therizinosauroidea and also the first record of Therizinosauroidea inEurope.[1]
In 1979 Dong Zhiming named a new species of themegalosauridChilantaisaurus,C. zheziangensis, based on specimen ZhM V.001. This specimen was recovered in 1972 from theTangshang Formation and consists of a partial tibia and partial right pes (foot) largely lacking metatarsals. Dong referred it to the genus mainly based on similarities between the unguals of this specimen and those ofC. tashuikouensis.[32] Barsbold and Maryanska in 1990 consideredC. 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 toSegnosaurus. As this taxon may lie outside the genusChilantaisaurus, they listed this species as"Chilantaisaurus" zheziangensis.[33] 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),[34] Dong in 1979 described both taxa from largely different formations and localities.[32] Zanno in 2010 argued that the notorious side to side compression of the unguals reflect therizinosaur affinities, although examinations to the preserved tibia are required for further conclusions.[31] In 2012 Mai-Ping Qian and colleagues placed"C". zheziangensis in the family Therizinosauridae based on its pes morphology, which is consistent to other therizinosaurids. They also illustrated most of the preserved pes.[35] Hartman with team in 2019 added"C". zheziangensis to a phylogenetic analysis and recovered it within Therizinosauroidea in a polytomy withAlxasaurus,Enigmosaurus and therizinosaurids.[36]
In 1997 Dong Zhiming and You Hailu named and described a supposed second species ofNanshiungosaurus,N. bohlini, based on specimen IVPP V 11116 found in 1992 atEarly Cretaceous strata from theZhonggou Formation,Xinminbao Group. It consists of 11 cervical and 5 dorsal vertebrae with someribs. In order to contain bothN. brevispinus andN. bohlini they coined the Nanshiungosauridae family. Dong and Yu presented no clear evidence regarding the assignment of this new species toNanshiungosaurus.[37] Li and colleagues in their 2007 description ofSuzhousaurus pointed out thatN. bohlini might be synonymous with the former, as both are found in the same geological group and also incompletely known. As per terms oftaxonomic priority, the species name would beSuzhousaurus 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 ofN. bohlini is apparently lost. Li and team disagree in that this species belong toNanshiungosaurus and listed it as"Nanshiungosaurus" bohlini.[38] 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.[31] Hartman with colleagues in 2019 recovered"N." bohlini as a therizinosaurid in a clade joined bySegnosaurus andNothronychus.[36]
Around 2005 partial therizinosaur material was collected from theLaijia Formation and later used to represent "Tiantaiosaurus sifengensis" (alternatively "Tiantaisaurus"), which is a currently unpublished and informal therizinosaur taxon. Qian and team in 2012 noted that a whole manuscript describing the taxon was written in 2007 but never officially published.[35]
In 1998Zhao Xijin andXu Xing briefly discovered a partial lower jaw with teeth (IVPP V11579) from theEarly Jurassic-agedLufeng Formation inYunnan, and concluded that this specimen represented the oldest known record of a coelurosaurian theropod. Based mainly on teeth morphology, they indicated therizinosaur affinities.[39] The specimen was later described in depth in 2001 and used as the holotype specimen for the new genus and speciesEshanosaurus deguchiianus, named by Xing and colleagues. The team reinforced therizinosaur relationships, arguing that the teeth morphology ofEshanosaurus can be differentiated fromsauropodomorphs.[40] In the same 2001 however,James I. Kirkland andDouglas G. Wolfe noted that the holotype ofEshanosaurus preserves traits only seen in sauropodomorphs.[26]Paul M. Barrett in 2009 examined the specimen in detail, noting six features shared with therizinosaurs but not exhibited by prosauropods, agreeing in thatEshanosaurus is a therizinosaur.[41]
Therizinosaurs spanned a large range of sizes, from the smallerBeipiaosaurus (2.2 m (7.2 ft) long)[24] andJianchangosaurus (2 m (6.6 ft) long)[42] to the large-sized 6–7 m (20–23 ft) longSegnosaurus andSuzhousaurus.[43][44]Therizinosaurus itself, obtained the top dimensions of the group, growing up to 10 m (33 ft) long and weighing over 5 t (11,000 lb), dimensions that make the genus among thelargest-known theropods.[44]
Therizinosaurs had a very distinctive, often confusing set of characteristics. Their long necks, wide torsos, and hind feet with four toes used in walking resembled those of basalsauropodomorph dinosaurs. Their unique hip bones, which pointed backwards and were partially fused together, initially reminded paleontologists of the "bird-hipped"ornithischians. Among the most striking characteristics of therizinosaurs are the enormous claws on their hands, which reached lengths of around one meter inTherizinosaurus. The unusual range of motion in therizinosaur forelimbs, which allowed them to reach forward to a degree other theropods could not achieve, also supports the idea that they were mainly herbivorous. Therizinosaurs may have used their long reach and strongly curved claws to grasp and shear leafy branches, in a manner similar to large mammals that lived later on, such aschalicotheres,ground sloths,great apes, andgiant pandas.[45] Skin impressions fromBeipiaosaurus indicate that therizinosaurs were covered with a coat of primitive,down-likefeathers similar to those seen in thecompsognathidSinosauropteryx, as well as longer, simpler, quill-like feathers that may have been used in display.[24][46]
Barsbold and Perle named the group Segnosauria as an infraorder of Theropoda in 1980.[7]Dong Zhiming went further, placing the segnosaurs in their own order,Segnosaurischia.[47] This name has been abandoned since the discovery that segnosaurs are a specialized group within the suborder Theropoda. Clark et al. 2004 considered Segnosaurischia a synonym of Therizinosauroidea.[21]
The cladeTherizinosauria was first coined byDale Russell in 1997—effectively replacing the older name Segnosauria, which has not yet been defined as a clade—to contain all therizinosaurian dinosaurs.[48] The superfamilyTherizinosauroidea was first coined in 1993 as a superfamily with no phylogenetic definition.[19] The familyTherizinosauridae had been established by Maleev in 1954 to include only the bizarre, giant-clawed theropodTherizinosaurus.[4] Subsequent analyses have proven this family to be more diverse and synonymous with Segnosauridae.[31][36]
The following taxonomy follows Zanno 2010, unless otherwise noted.[31]
Therizinosauria is defined asAlxasaurus,Enigmosaurus,Erlikosaurus,Nanshiungosaurus,Segnosaurus,Therizinosaurus, and all taxa closer to them than tooviraptorosaurs,ornithomimids, andtroodontids.[48]Paul Sereno, in 2005, modified this definition to the most inclusive clade containingTherizinosaurus but notOrnithomimus,Oviraptor,Shuvuuia,Troodon, orTyrannosaurus.[50]
When it was later realized thatTherizinosaurus was an advanced therizinosaur more related toAlxasaurus than other dinosaur lineages, Therizinosauroidea was coined to includeAlxasaurus and Therizinosauridae, and has largely replaced the use of the older name Segnosauria in phylogenetic studies, mainly because of the association of the name Segnosauria with the discredited idea that these animals were relatives ofprosauropods.[19] Therizinosauroidea was first defined by Zhanget al. in 2001, as the clade containing all theropods more closely related toTherizinosaurus than to birds. This definition, however, defines the same group as the pre-existing Therizinosauria. An alternate definition was given by Clark in 2004 (as the last common ancestor ofTherizinosaurus andBeipiaosaurus and all its descendants), comprising a narrower group that excludes more primitive therizinosaurs, such asFalcarius, and allows the name Therizinosauria to remain in use for the larger group comprising all therizinosaurs.[51] This definition was followed bySereno (2005), Zannoet al. (2009) and Zanno (2010),[31][50][52] though other subsequent studies, such as Senter (2007, 2012) have continued to use Therizinosauroidea for the therizinosaur "total group".[53]
The cladogram below follows the extensive phylogenetic analysis of the Therizinosauria byLindsay E. Zanno in 2010.[31]
| Therizinosauria | |
Below is the recently performed phylogenetic analysis performed by Hartman et al.2019 using the data provided by Zanno in 2010:[36]
| Therizinosauria |
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CT scans published by Stephan Lautenschlager et al. 2012 focused on the skull and brain cavity ofErlikosaurus, revealing it to have a large forebrain, and suggesting it had well developed senses of balance, hearing and smell, all of which would have been useful in evading predators, finding food, or in performing complexsocial behavior. These senses were also well-developed in earlier coelurosaurs and other theropods, indicating that therizinosaurs may have inherited many of these features from their carnivorous ancestors and used them for their specialized dietary purposes.[54]
Dinosaur eggs with embryos of theDendroolithidae type from theNanchao Formation were identified as belonging to therizinosaurs based on anatomical features, and described by paleontologist Martin Kundrát and colleagues in 2007. The development of the embryos (their bones were extensively ossified) and the fact that no adults were found in association with the nests indicate that therizinosaur hatchlings wereprecocial, capable of locomotion from birth, and able to leave their nests to feed alone, independently of their parents. The development of the teeth of the hatchlings was consistent with an omnivorous diet. Subterraneously constructed nests are also indicative of a lack of parental care during the incubation period.[55][56]
In a 2013 conference abstract, paleontologist Yoshitsugu Kobayashi and colleagues reported a nesting ground of theropod dinosaurs at theJavkhlant Formation, which contained at least 17 egg clutches from the same layer within an area of 22 m by 52 m. Each clutch contained 8 spherical eggs with rough surfaces which were in contact with each other and arranged in a circular structure without a central opening. Based on microscopical features in the eggshells, the researchers identified the eggs as the type Dendroolithidae, which had previously been attributed to therizinosaurs. Though therizinosaurs are not known from the Javkhlant Formation, it overlies the Bayan Shireh Formation, whereSegnosaurus,Erlikosaurus, andEnigmosaurus were found. The multiple clutches indicate that some therizinosaurs were colonial nesters, likehadrosaurs, prosauropods,titanosaurs, and birds, and the fact that they were found in a single stratigraphic layer suggests the dinosaurs nested at the site on a single occasion, and therefore did not exhibitsite fidelity. The discovery was the first record of colonially nesting non-avian theropods from Asia, as well as the largest known non-avian theropod colony.[57]
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