Editing Segnosaurus (section)

==Classification==
[[File:Erlikosaurus skull and foot.jpg|thumb|Skull and foot bones of ''[[Erlikosaurus]]'', which together with ''Segnosaurus'' (both from Mongolia) became the basis of the new [[infraorder]] Segnosauria; this group is now a [[junior synonym]] of [[Therizinosauria]].]]

''Segnosaurus'' and its relatives, which are now recognized as therizinosaurs ("scythe reptiles"), were long considered an enigmatic group. 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).<ref name="Zanno2006">{{cite journal |last=Zanno |first=L. E. |author-link=Lindsay Zanno |title=The pectoral girdle and forelimb of the primitive therizinosauroid ''Falcarius utahensis'' (Theropoda, Maniraptora): analyzing evolutionary trends within Therizinosauroidea |journal=Journal of Vertebrate Paleontology |date=2006 |volume=26 |issue=3 |page=636 |doi=10.1671/0272-4634(2006)26[636:TPGAFO]2.0.CO;2|s2cid=86166623 }}</ref><ref>{{cite book|last1=Holtz|first1=T. R.|chapter=Theropods|editor-last1=Farlow|editor-first1=J. O.|editor-last2=Brett-Surman|editor-first2=M. K.|title=The Complete Dinosaur|date=2000|publisher=Indiana University Press|location=Bloomington|isbn=978-0-253-21313-6|page=404}}</ref><ref name="Dinosauria2"/> In 1979, Perle noted the ''Segnosaurus'' fossils were possibly representative of a new family of dinosaurs, which he named Segnosauridae, ''Segnosaurus'' being the [[type genus]] and sole member. He tentatively classified Segnosauridae as theropods, traditionally thought of as the "meat-eating" dinosaurs, pointing to similarities in the mandible and its front teeth. 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.<ref name=Perle1979/> 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]].<ref name="Barsbold&Perle1979" />

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.<ref name="perle&barsbold80" /> 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.<ref name=Perle1981>{{cite journal | last=Perle | first=A. |author-link=Altangerel Perle |year=1981 |title=Noviy segnozavrid iz verchnego mela Mongolii |journal=Trudy – Sovmestnaya Sovetsko-Mongol'skaya Paleontologicheskaya Ekspeditsiya |volume=15 |pages=50–59|language=Russian|trans-title=New Segnosauridae from the Upper Cretaceous of Mongolia|translator-last=Siskron |translator-first=C. |translator2-last=Welles |translator2-first=S. P. |url=http://paleoglot.org/files/Perle_82.pdf}}</ref>
[[File:Therizinosaurus claw.jpg|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]]).<ref name="Perle1982">{{Cite journal |last=Perle |first=A. |author-link=Altangerel Perle |year=1982 |title=A hind limb of ''Therizinosaurus'' from the Upper Cretaceous of Mongolia |journal=Problems in Mongolian Geology |language=Russian |volume=5 |pages=94–98 |translator-last=Welsh |translator-first=W. R. |url=http://paleoglot.org/files/Perle_82.pdf |archive-date=October 1, 2020 |access-date=January 20, 2020 |archive-url=https://web.archive.org/web/20201001060111/http://paleoglot.org/files/Perle_82.pdf |url-status=live }}</ref><ref name="nature13874">{{cite journal | title=Resolving the long-standing enigmas of a giant ornithomimosaur ''Deinocheirus mirificus'' | last1=Lee | first1=Y. N. | last2=Barsbold | first2=R. | last3=Currie | first3=P. J. | last4=Kobayashi | first4=Y. | last5=Lee | first5=H. J. | last6=Godefroit | first6=P. | last7=Escuillié | first7=F. O. | last8=Chinzorig | first8=T. | journal=Nature | year=2014 | volume=515 | issue=7526 | pages=257–260 | doi=10.1038/nature13874| pmid=25337880 | bibcode=2014Natur.515..257L | s2cid=2986017 }}</ref> 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.<ref name=barsbold1983/> Later in 1983, Barsbold stated the segnosaurian pelvis deviated significantly from the theropod norm and found the configuration of their ilia generally similar to those of [[sauropods]].<ref name="Barsbold1983B">{{Cite journal |last=Barsbold |first=R. |year=1983 |title=O ptich'ikh chertakh v stroyenii khishchnykh dinozavrov |trans-title=“Avian” features in the morphology of predatory dinosaurs |journal=Transactions of the Joint Soviet Mongolian Paleontological Expedition |volume=24 |pages=96–103 |language=Russian |translator-last=Welsh |translator-first=W. R. |url=https://paleoglot.org/files/barsbold_83b.pdf |archive-date=September 29, 2020 |access-date=January 20, 2020 |archive-url=https://web.archive.org/web/20200929110800/http://paleoglot.org/files/Barsbold_83b.pdf |url-status=live }}</ref>

[[Image:Erlikosaurus.jpg|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.<ref name="Paul1984">{{cite journal |last1=Paul |first1=G. S. |title=The segnosaurian dinosaurs: relics of the prosauropod-ornithischian transition? |journal=Journal of Vertebrate Paleontology |date=1984 |volume=4 |issue=4 |pages=507–515 |jstor= 4523011 |issn=0272-4634|doi=10.1080/02724634.1984.10012026 |bibcode=1984JVPal...4..507P }}</ref> [[David B. Norman]] considered Paul's idea contentious and "bound to provoke much argument" in 1985.<ref name="DBN85">{{cite book |last=Norman |first=D. B.|title=The Illustrated Encyclopedia of Dinosaurs: An Original and Compelling Insight into Life in the Dinosaur Kingdom |chapter=Mischellaneous theropods |year=1985 |publisher=Crescent Books |location=New York |pages=52–53 |isbn=978-0-517-46890-6 }}</ref> 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.<ref name=GSP88a>{{cite book |last=Paul |first=G. S. |title=Predatory Dinosaurs of the World |year=1988 |publisher=Simon & Schuster |pages=185, 283 |location=New York |isbn=978-0-671-61946-6 }}</ref> 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.<ref name="Gauthier">{{cite journal |last1=Gauthier |first1=J. |title=Saurischian monophyly and the origin of birds |journal=Memoirs of the California Academy of Sciences |date=1986 |volume=8 |page=45 |url=https://www.biodiversitylibrary.org/page/15651737#page/331/mode/1up |archive-url=https://web.archive.org/web/20190816230537/https://www.biodiversitylibrary.org/page/15651737#page/331/mode/1up |archive-date=2019-08-16 |url-status=live }}</ref> In a 1989 conference abstract about sauropodomorph inter-relationships, [[Paul Sereno]] also considered segnosaurs to be prosauropods, based on skull features.<ref name="Sereno">{{cite conference |last1=Sereno |first1=P. |title=Prosauropod monophyly and basal sauropodomorph phylogeny |book-title=Abstract of Papers. Forty-Ninth Annual Meeting Society of Vertebrate Paleontology |journal=Journal of Vertebrate Paleontology |date=1989 |volume=9 |issue=3 Supplement|page=39A |jstor= 4523276 |issn=0272-4634}}</ref>

[[File:Alxasaurus Tyrrell.jpg|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.<ref name=Dinosauria1>{{cite book |last1=Barsbold |first1=R. |editor1-last = Weishampel |editor1-first = D. B. |editor2-last=Osmolska |editor2-first=H. |editor3-last = Dodson | editor3-first = P. |last2=Maryańska |first2=T.|title=The Dinosauria |edition=1st |year=1990 |publisher=University of California Press |location=Berkeley |isbn=978-0-520-06727-1|pages=408–415 |chapter=Saurischia Sedis Mutabilis: Segnosauria}}</ref> 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).<ref name="Alxasaurus">{{cite journal |last1=Russell |first1=D. A. |last2=Dong |first2=Z.-M. |title=The affinities of a new theropod from the Alxa Desert, Inner Mongolia, People's Republic of China |journal=Canadian Journal of Earth Sciences |date=1993 |volume=30 |issue=10 |pages=2107–2127 |doi=10.1139/e93-183|bibcode=1993CaJES..30.2107R }}</ref><ref name=Russell1997/>

[[File:Beipiaosaurus-Paleozoological Museum of China.jpg|thumb|Partial forelimb of the [[Basal (phylogenetics)|basal]] therizinosaur ''[[Beipiaosaurus]]'' with impressions of [[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.<ref>{{cite journal |last1=Clark |first1=J. M. |last2=Altangerel |first2=P. |last3=Norell |first3=M. |title=The skull of ''Erlicosaurus andrewsi'', a late Cretaceous "segnosaur" (Theropoda, Therizinosauridae) from Mongolia|issue=3115 |date=1994 |journal=American Museum Novitates|hdl=2246/3712 |pages=2, 32–36}}</ref> In 1995, Lev A. Nessov rejected the idea therizinosaurs were theropods; he considered them a distinct group within Saurischia.<ref name="Nessov" /> In 1996, [[Thomas R. Holtz Jr.]] found therizinosaurs to group with oviraptorosaurs in a [[phylogenetic analysis]] of coelurosaurian theropods.<ref name="Holtz96">{{cite journal |last1=Holtz |first1=T. R. |title=Phylogenetic taxonomy of the Coelurosauria (Dinosauria: Theropoda) |journal=Journal of Paleontology |date=1996 |volume=70 |issue=3 |pages=536–538 |issn=0022-3360|jstor=1306452 |doi=10.1017/S0022336000038506 |bibcode=1996JPal...70..536H |s2cid=87599102 }}</ref> Russell coined the name [[Therizinosauria]] for the wider group in 1997.<ref name=Russell1997/> In 1999, [[Xu Xing (paleontologist)|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.<ref name="Beipiaosaurus" />

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.<ref name="Nothronychus">{{cite journal |last1=Kirkland |first1=James I. |last2=Wolfe |first2=Douglas G. |title=First definitive therizinosaurid (Dinosauria; Theropoda) from North America |journal=Journal of Vertebrate Paleontology |date=2001 |volume=21 |issue=3 |pages=410–414 |issn=0272-4634|jstor=20061971 |doi=10.1671/0272-4634(2001)021[0410:FDTDTF]2.0.CO;2 |s2cid=85705529 }}</ref><ref name=":0">{{Cite journal|last1=Zhang|first1=X.-H.|last2=Xu|first2=X.|last3=Zhao|first3=Z.-J.|last4=Sereno|first4=P.|last5=Kuang|first5=X.-W.|last6=Tan|first6=L.|date=2001|title=A long-necked therizinosauroid dinosaur from the Upper Cretaceous Iren Dabasu Formation of Nei Mongol, People's Republic of China|url=http://www.ivpp.cas.cn/cbw/gjzdwxb/xbwzxz/200901/W020090813372120151973.pdf|journal=Vertebrata PalAsiatica|volume=39|issue=4|pages=282–290|archive-url=https://web.archive.org/web/20160304111348/http://www.ivpp.cas.cn/cbw/gjzdwxb/xbwzxz/200901/W020090813372120151973.pdf|archive-date=2016-03-04|url-status=live}}</ref><ref name="Falcarius">{{cite journal |last1=Kirkland |first1=J. I. |last2=Zanno |first2=L. E. |last3=Sampson |first3=S. D. |last4=Clark |first4=J. M. |last5=DeBlieux |first5=D. D. |author2-link=Lindsay Zanno |title=A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah |journal=Nature |date=2005 |volume=435 |issue=7038 |pages=84–87 |doi=10.1038/nature03468|pmid=15875020 |bibcode=2005Natur.435...84K |s2cid=4428196 |url=http://doc.rero.ch/record/15297/files/PAL_E2596.pdf }}</ref> 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]].<ref>{{cite journal |last1=Turner |first1=A. H. |last2=P. |first2=D. |last3=Clarke |first3=J. A. |last4=Erickson |first4=G. M. |last5=Norell |first5=M. A. |title=A basal dromaeosaurid and size evolution preceding avian flight |journal=Science |date=2007 |volume=317 |issue=5843 |pages=1378–1381 |doi=10.1126/science.1144066|pmid=17823350 |bibcode=2007Sci...317.1378T |doi-access=free }}</ref><ref name="Zanno2009"/> 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.<ref name=Zanno2010>{{Cite journal|last=Zanno |first=L. E. |author-link=Lindsay Zanno |year=2010 |title=A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora) |journal=Journal of Systematic Palaeontology |volume=8 |issue=4 |pages=507–508, 512–515, 522–524, 527, 533, 538–539 |doi=10.1080/14772019.2010.488045 |bibcode=2010JSPal...8..503Z |s2cid=53405097 }}</ref> By 2015, ''Segnosaurus'' remained one of the best known therizinosaurs, according to Christophe Hendrickx and colleagues.<ref name="Hendrickx"/>

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'':<ref name=os>{{Cite journal | last1 = Pu | first1 = H. | last2 = Kobayashi | first2 = Y. | last3 = Lü | first3 = J. | last4 = Xu | first4 = L. | last5 = Wu | first5 = Y. | last6 = Chang | first6 = H. | last7 = Zhang | first7 = J. | last8 = Jia | first8 = S. | editor1-last = Claessens | editor1-first = Leon | doi = 10.1371/journal.pone.0063423 | title = An unusual basal therizinosaur dinosaur with an ornithischian dental arrangement from Northeastern China | journal = PLOS ONE | volume = 8 | issue = 5 | pages = e63423 | year = 2013 | pmid =  23734177| pmc = 3667168| bibcode = 2013PLoSO...863423P | doi-access = free }}</ref>
[[File:Therizinosaur skeletons.jpg|thumb|upright=1.8|Known elements of various therizinosaurs shown to scale, with ''Segnosaurus'' in the upper middle]]
{{clade| style=font-size:90%;line-height:85%
|label1=[[Therizinosauria]]
|1={{clade
   |1=''[[Falcarius]]''
   |label2=<span style="color:white;">unnamed</span>
   |2={{clade
      |1=''[[Jianchangosaurus]]''
      |label2=[[Therizinosauroidea]]
      |2={{clade
         |1=''[[Beipiaosaurus]]''
         |label2=<span style="color:white;">unnamed</span>
            |2={{clade
               |1=''[[Alxasaurus]]''
               |label2=[[Therizinosauridae]]
               |2={{clade
                  |1=''[[Erliansaurus]]''
                  |2=''[[Nanshiungosaurus]]''
                  |3=''[[Neimongosaurus]]''
                  |4={{clade
                     |1='''''Segnosaurus'''''
                     |2=''[[Erlikosaurus]]''
                     |3=''[[Suzhousaurus]]''
                     |4=''[[Enigmosaurus]]''
                     |5=''[[Therizinosaurus]]''
                     |6=''[[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.<ref name="Falcarius"/><ref name="Osteology">{{cite journal |last=Zanno |first=L. E. |author-link=Lindsay Zanno |title=Osteology of ''Falcarius utahensis'' (Dinosauria: Theropoda): characterizing the anatomy of basal therizinosaurs |journal=Zoological Journal of the Linnean Society |date=2010 |volume=158 |issue=1 |pages=225 |doi=10.1111/j.1096-3642.2009.00464.x|doi-access=free }}</ref> 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&nbsp;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.<ref name=Zanno2010/>