| Therizinosaurus Temporal range:Late Cretaceous,Maastrichtian | |
|---|---|
 | |
| Mounted forelimbs of specimen MPC-D 100/15 atNagoya City Science Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Therizinosauroidea |
| Family: | †Therizinosauridae |
| Genus: | †Therizinosaurus Maleev,1954 |
| Type species | |
| †Therizinosaurus cheloniformis Maleev, 1954 | |
Therizinosaurus (/ˌθɛrəˌzɪnoʊˈsɔːrəs/ ⓘ; meaning 'scythe lizard') is agenus of very largetherizinosauriddinosaurs that lived during theLate Cretaceousperiod in what is nowAsia. It contains a single species,Therizinosaurus cheloniformis, known from thefossiliferousNemegt Formation. The first remains ofTherizinosaurus were found in 1948 by a Mongolian field expedition in theGobi Desert and later described byEvgeny Maleev in 1954. The genus is only known from a few bones, including gigantic manualunguals (claw bones), from which it gets its name, and additional findings comprising fore and hindlimb elements that were discovered from the 1960s through the 1980s.
Therizinosaurus was a colossal therizinosaurid that could grow up to 9–10 m (30–33 ft) long and 4–5 m (13–16 ft) tall, and weigh possibly over 5 t (5.5 short tons). Like other therizinosaurids, it would have been a slow-moving, long-necked, high browser equipped with arhamphotheca (horny beak) and a wide torso for food processing. Itsforelimbs were particularly robust and had threefingers that bore unguals which, unlike other relatives, were very stiffened, elongated, and only had significant curvatures at the tips.Therizinosaurus had the longest known manual unguals of any land animal, reaching above 50 cm (20 in) in length. Itshindlimbs ended in four functionally weight-bearing toes differing from other theropod groups in which the first toe wasreduced to adewclaw and also resembling the very distantly relatedsauropodomorphs.
It was one of the last and the largest representative of its unique group, theTherizinosauria (formerly known as Segnosauria; the segnosaurs). During and after its original description in 1954,Therizinosaurus had rather complex relationships due to the lack of complete specimens and relatives at the time. Maleev thought the remains ofTherizinosaurus to belong to a largeturtle-like reptile, and also named a separate family for the genus:Therizinosauridae. Later on, with the discovery of more complete relatives,Therizinosaurus and kin were thought to represent some kind of Late Cretaceoussauropodomorphs or transitionalornithischians, even though at some point it was suggested that it may have been a theropod. After years of taxonomic debate, nevertheless, they are now placed in one of the major dinosaur clades,Theropoda, specifically asmaniraptorans.Therizinosaurus is widely recovered within Therizinosauridae by most analyses.
The unusual arms and body anatomy (extrapolated after relatives) ofTherizinosaurus have been cited as an example ofconvergent evolution withchalicotheriines and other primarilyherbivorous mammals, suggesting similar feeding habits. The elongated handclaws ofTherizinosaurus were more useful when pullingvegetation within reach rather than being used for active attack or defense because of their fragility, however, they may have had some role for intimidation. Its arms also were particularly resistant tostress, which suggests a robust use of these limbs.Therizinosaurus was a very tall animal, likely having a reducedcompetition over thefoliage in its habitat and outmatchingpredators liketyrannosauridTarbosaurus.
In 1948, several Mongolian Paleontological expeditions organized by theUSSR Academy of Sciences were conducted in theNemegt Formation of theGobi Desert, SouthwesternMongolia, with the main objective of newfossils findings. The expeditions unearthed numerousdinosaur andturtle fossil remains from thestratotype locality Nemegt (also known as Nemegt Valley), but the most notable elements collected were three partial manualunguals (claw bones) of considerable size. This set of unguals was found on a subdivision of the Nemegt locality designated as Quarry V near the skeleton of a large theropod, but also in association with other elements including a metacarpal fragment and severalribs fragments. It was labelled under the specimen numberPIN 551-483 and later on, these fossils were described by theRussian paleontologistEvgeny Maleev in1954 who used them to scientifically name the newgenus andtype speciesTherizinosaurus cheloniformis, becoming theholotype specimen. Thegeneric name,Therizinosaurus, is derived from theGreekθερίζω (therízo, meaning scythe, reap or cut) andσαῦρος (sauros, meaning lizard) in reference to the enormous manual unguals, and thespecific name,cheloniformis, is taken from the Greekχελώνη (chelóni, meaning turtle) andLatin formis as the remains were thought to belong to a turtle-likereptile. Maleev also coined a separate family for this new and enigmatictaxon:Therizinosauridae. Since little was known ofTherizinosaurus at the time of the original description, Maleev thought PIN 551-483 belonged to a large, 4.5 m (15 ft) long turtle-like reptile that relied on its giant hand claws to harvest seaweed.[1]
Though it was not fully understood to what general kind of animal these fossils belonged, in1970, the RussianpaleontologistAnatoly K. Rozhdestvensky was one of the first authors to suggest thatTherizinosaurus was atheropod and not a turtle. He made comparisons betweenChilantaisaurus and theholotype unguals ofTherizinosaurus to propose that the appendages actually came from acarnosaurian dinosaur, thereby interpretingTherizinosaurus as a theropod. Rozhdestvensky also illustrated the three holotypic manual unguals and re-identified the metacarpal fragment as ametatarsal bone, and based on the unusual shape of both metatarsal and ribs fragments he listed them assauropod remains.[2] These theropodan affinities were also followed by the Polish paleontologistHalszka Osmólska and co-authorEwa Roniewicz in 1970 during their naming and description ofDeinocheirus—another large and enigmatic theropod from the formation that was initially known from partial arms. Similar to Rozhdestvensky, they suggested that the holotype unguals were more likely to have belonged to a carnosaurian theropod, rather than a large marine turtle.[3]
Further expeditions in the Nemegt Formation unearthed more fossils ofTherizinosaurus. In 1968 prior to Rozhdestvensky, Osmólska and Roniewicz statements, the upper portion of a manual ungual was found in the Altan Uul locality and labeled as MPC-D 100/17 (formerly IGM or GIN). In 1972, another fragmented ungual (specimen MPC-D 100/16) was discovered at the Upper White Beds of the Hermiin Tsav locality, only preserving its lower portion.[4][5] During the year 1973, a much more complete, larger, and articulated specimen was collected also from Hermiin Tsav. This specimen was labelled as MPC-D 100/15 and consists of both left and right arms including thescapulocoracoids, bothhumeri (upper arm bones), rightulna withradius and left ulna, two rightcarpals, the rightmetacarpus including a completedigit Il, and some ribs withgastralia (belly ribs). As common with fossils, some elements were not entirely preserved such as the scapulocoracoids with broken ends, and the left arm is less complete than the right one. All of these specimens were first described and referred toTherizinosaurus by the Mongolian paleontologistRinchen Barsbold in1976. In this new monograph, he pointed out that the rib fragments in MPC-D 100/15 were more slender than the ones from the holotype, and identified MPC-D 100/16 and 100/17 as pertaining to digits I and III, respectively. It was clear to Barsbold that MPC-D 100/15 representedTherizinosaurus as the ungual in this specimen shared the elongation and flattened morphology of all previous specimens. He concluded thatTherizinosaurus was a theropod taxon since MPC-D 100/15 matched multiple theropodan characters.[4]
Also during the year 1973, the specimen MPC-D 100/45 was discovered by the Joint Soviet-Mongolian Paleontological Expedition at the Hermiin Tsav locality. Unlike the previous findings, MPC-D 100/45 is represented by a right hindlimb composed of a very fragmentedfemur with the lower end of thetibia,astragalus,calcaneum,tarsal IV, a functionaltetradacyl feet (four-toed) compromising four partialmetatarsals, partially preserved digits I and III, and nearly complete digits II and IV. These newer remains were described by the also Mongolian paleontologistAltangerel Perle in1982. He regarded the referral ofTherizinosaurus and Therizinosauridae toChelonia (turtlesorder) to be unlikely, and hypothesizedTherizinosaurus andSegnosaurus—at the time of this description regarded as a theropod dinosaur—to be particularly similar based on their respective scapulocoracoid morphology, only differing in size. Perle referred MPC-D 100/45 toTherizinosaurus given that this specimen was found near the location of MPC-D 100/15 and was virtually similar to the described pes forSegnosaurus.[6] In 1990, Barsbold and Teresa Maryanska agreed with Perle in that the hindlimb material from Hermiin Tsav he described in 1982 wastherizinosaurian (then called segnosaurians) given that the metatarsus was stocky and the astragalus had a laterally arched ascendingprocess (bony extension), but cast doubt with his referral of it toTherizinosaurus and the segnosaurian identity for this taxon since it was only known from the pectoral girdle and other forelimb elements, making direct comparisons between specimens impossible. They considered this specimen to represent a Late Cretaceous representative of the Segnosauria, but notTherizinosaurus.[7]
In 2010 however, theNorth American paleontologistLindsay E. Zanno in her large taxonomic reevaluation ofTherizinosauria considered the referral of MPC-D 100/45 toTherizinosaurus to be likely based on the rationale that it was collected in the same stratigraphic context (Nemegt Formation) as the holotype, and shared the robust and four-toed morphology of other therizinosaurids such asSegnosaurus. She also excluded the rib material from the holotype as it was re-identified by Rozhdestvensky to likely have come from a sauropod dinosaur, and notTherizinosaurus itself.[8]
Formaniraptoran standards,Therizinosaurus obtained enormous sizes, estimated to have reached 9 to 10 m (30 to 33 ft) in length.[9] In 2010,Gregory S. Paul estimated in his bookThe Princeton Field Guide to Dinosaurs that the animal weighed around 3 tons.[10] However, the estimate has been revised to 5 to 10 t (5.5 to 11.0 short tons) in the latest revision of the book in 2024.[9] In 2012,Thomas R. Holtz Jr. claimed that the animal weighs as much as anelephant.[11] In 2013, Lindsay E. Zanno and Peter J. Makovicky suggested that the therizinosaurus weigh around 6.6 t (7.3 short tons).[12] These dimensions makeTherizinosaurus the largest therizinosaur known and the largest known maniraptoran. Along with the contemporaneousornithomimosaurDeinocheirus, it was the largestmaniraptoriform.[13] Though the body remains ofTherizinosaurus are relatively incomplete, inferences can be made about its physical characteristicsbased on more complete and relatedtherizinosaurids. Like other members of its family,Therizinosaurus had a proportionally small skull bearing arhamphotheca (horny beak) atop its long neck;bipedal gaits; a large belly for foliage processing; and sparsefeathering. Other traits that were likely present inTherizinosaurus include a heavilypneumatized (air-filled)vertebral column and a robustly-built,ophistopubic (backwards oriented)pelvis.[14][15][16]
In 2010, Senter and James used hindlimb length equations to predict the total length of the hindlimbs inTherizinosaurus andDeinocheirus. They concluded that an averageTherizinosaurus may have had approximately 3 m (9.8 ft) long legs.[17] More recently,Mike Taylor andMatt Wedel suggested that the whole neck would be 2.9 times the size of the humerus, which was 76 cm (760 mm), resulting in a 2.2 m (7.2 ft) long neck based on comparisons with thecervical vertebrae series ofNanshiungosaurus.[18] The most distinctive feature ofTherizinosaurus was the presence of gigantic unguals on each of the three digits of its hands. These were common among therizinosaurs but particularly large and stiffened inTherizinosaurus, and they are considered as the longest known from any terrestrial animal.[13]
The arm ofTherizinosaurus covered 2.4 m (7.9 ft) in total length (humerus, radius and second metacarpal with phalanges lengths).[17] Thescapula measured 67 cm (26 in) long with a stocky and flattened dorsal blade, wide acromialprocess (bony extension) and a very widened ventral surface. Near the anterior edge of the scapular widening and near thescapulocoracoidsuture (bone joint), aforamen was located; it likely functioned as a channel forblood vessels andnerves. The posterior edge of the scapula was robust and theacromion was lightly built, likely fused into acartilaginous system with its periphery in life. Thecoracoid measured 36 cm (14 in) in length, it had a broad and convex lateral surface that formed a slightly inclined concavity near of the scapulocoracoid suture. This concavity bent down towards the scapular widening. Near the scapulocoracoid suture, this edge turned very thin and possibly into cartilage along with the periphery of the coracoid in life, as the case of the scapular edge. A large foramen was also present on the coracoid. Theglenoid was broad and deep, slightly pointing to the outer lateral side. It had robust and convex crest-like borders. The supraglenoid thickness was developed in a convex crest-shaped form, it was divided across the top of the scapulocoracoid suture. The attachment for thebiceps muscle was prominently developed by a largetubercle with a stocky top, indicating powerful muscles in life.[4][19][8]
Thehumerus was robustly built, measuring 76 cm (30 in) long. It had a broad upper end. Thedeltopectoral crest (deltoid muscle attachment) was particularly long and thick, with its top located approximately 1/3 from the upper end. The length of the crest was no less than 2/3 the length of the whole bone element. The lower end of the humerus was very expanded and flared. Thecondyles were developed onto the anterior side of the lower expansion while theepicondyles were very broad and projected over the limits of the articular areas. Theulna measured 62.02 cm (24.42 in) and most of its length was occupied by its straight shaft. The ulnar process was very wide. The upper articular area was divided into inner and outer lateral sides. The lateral side had a triangular-shaped border and was slightly concave; it was limited in a top view by the depression for the upper articulation of the radius. The inner side formed a semilunar-shaped depression that covered the lunar-shaped condyle of the humerus. Theradius was 55.04 cm (21.67 in) long and slightlyS-curved. Its upper end was flattened in a lateral direction, very wide, and the distal end was highly robust.[4][19][8]
The first lowercarpal bone measured 8.23 cm (3.24 in) tall and 8.53 cm (3.36 in) wide and had two articulation surfaces on its lowermost end. The upper surface of this carpal was divided by a broad depression that formed the articulation of thecarpus. On its inner side, it had a triangular-shaped outline that attached to the upper surface of metacarpal I, occupying a little bit less than the lateral side, which articulates to metacarpal II. These areas were separated by an oblique bony projection. The second lower carpal was smaller than the first one, measuring 5.6 cm (2.2 in) tall and 5.93 cm (2.33 in) wide. Its lower surface was flattened and the articular surface of the carpus extended from the first carpal to the second carpal over the articulation of the two bones.[4][19][8]
Themetacarpal I was 14.55 cm (5.73 in) long and compared to the others it was more stockier. Its lateral side was broad, especially on the uppermost area; the inner border was thin and narrow. The upper articulation was configured into three parts. The lower articular surface was somewhat asymmetric and bent to the inner side from the left one, along with a wide and deep opening. The total length of this metacarpal was larger than 2/3 the length of metacarpal III, which may have been aunique trait ofTherizinosaurus. The metacarpal II measured 28.68 cm (11.29 in) in length and was the most elongated and robust metacarpal. It had an inclined, square-shaped, and flattened upper articulation. The articulation on the lower head had very symmetrical condyles, being divided by a broad, deep depression. The lateral connecting openings were poorly developed. The metacarpal III covered 19.16 cm (7.54 in) in length and had a very thin shaft compared to the other metacarpals. Its upper articulation was divided into three parts. The lower articular head was asymmetrical with deep and broad openings. As in metacarpal II, the lateral connecting openings were poorly developed.[4][19][8]
Only the seconddigit of the manus is known inTherizinosaurus. It consisted of twophalanges and a largeungual. The first and second phalanges were somewhat equal in shape and length (14.17 cm (5.58 in) and 14.38 cm (5.66 in), respectively), and shared the robust and stocky structure. The upper articular facets were very symmetrical and had a crest—particularly taller in the first phalanx. The top border of this crest was very pointed and thick; it likely served as the site for attachment of theextensor tendons in life. The lower heads were nearly symmetrical, but the central depression was considerably wider and deeper in the first phalanx. The manual unguals ofTherizinosaurus were especially enormous and long, estimated to have covered approximately 52 cm (20 in) in length. Unlike othertherizinosaurs they were very straight, side to side flattened, and had sharp curvatures only at the tips, a unique feature ofTherizinosaurus. The lower tubercle, where the flexor tendons attached to the ungual, was thick and robust, indicating a large pad in life. The articulation surface that connected the preceding phalanx was slightly concave and divided into two by a central ridge.[4][19][8]
Therizinosaurus had a rather stocky and robusttibia that was very wide on its lower end. Themetatarsus was robust and short (almostsauropodomorph-like), and composed of fivemetatarsals. The first four were functional and terminated in weight-bearing digits, hence having atetradactyl (four-toed) condition. The last or fifth metatarsal was highly reduced bone located at the lateral side of the metatarsus and had no functional significance. Unlike most other theropods groups, the first pedal digit was—though shorter than the others—functional and weight-bearing. The second and third were equally long while the fourth was smaller and somewhat thinner. The pedal unguals were side to side flattened and likely sharp. The morphology of the feet ofTherizinosaurus and other therizinosaurids was unique, as the general theropod formula includes tridactyl (three-toed) feet in which the first toe was reduced to adewclaw and held off the ground.[6][16]
Maleev originally classifiedTherizinosaurus as a giant marine turtle and the genus was assigned by him to a separate family, Therizinosauridae given how enigmatic the specimen was.[1] The fossils remained with uncertainty among the scientific community; however, in 1970 Rozhdestvensky was one of the first paleontologists to suggest thatTherizinosaurus was actually a theropod dinosaur instead of a turtle. He also suggested that the supposed ribs of the holotype were likely from a different dinosaur, possibly a sauropodomorph.[2] In 1976 Barsbold concluded thatTherizinosaurus was a theropod because MPC-D 100/15 matched numerous theropodan characters, and that Therizinosauridae andDeinocheiridae were probablysynonyms.[4] With the discovery and description ofSegnosaurus, in 1979 Perle named a new family of dinosaurs, theSegnosauridae. He tentatively placed the family withinTheropoda given the similarities of the mandible and dentition to other members.[20] A year later, the new genusErlikosaurus was named byBarsbold and Perle in 1980. They named a new infraorder called the Segnosauria, composed ofErlikosaurus andSegnosaurus. They also noted that while aberrant and having ornithischian-like pelves, segnosaurs featured similar traits to other theropods.[21] With the discovery of the referred hindlimb toTherizinosaurus in 1982 by Perle, he concluded thatSegnosaurus was very similar to the latter based on the morphology and they possibly belonged to a single, if not the same, group.[6] In 1983, Barsbold named a new genus of segnosaur,Enigmosaurus. He analyzed the pelvis of the new genus and pointed out that segnosaurids were so different from other theropods that they could be outside the group or represent a different lineage of theropod dinosaurs.[19] Later on the same year, he intensified the exclusion of segnosaurs from being theropods by noting that their pelves resembled those ofsauropod dinosaurs.[22]
Consequently, the assignment of segnosaurs started to shift towards sauropodomorphs. In 1984,Gregory S. Paul claimed that segnosaurs, rather than being theropods, were indeed sauropodomorphs that successfully managed to remain in the Cretaceous period. He based the idea on anatomical traits such as the skull and similar configuration.[23] He maintained his position in 1988 by placing the Segnosauria into the now obsoletePhytodinosauria, and was one of the first to suggest a segnosaur assignment for the enigmaticTherizinosaurus.[24] Other prominent paleontologists likeJacques Gauthier orPaul Sereno supported this vision.[25][26] In 1990, Barsbold and Teresa Maryanska agreed in that the hindlimb material from Hermiin Tsav referred toTherizinosaurus in 1982 was segnosaurian since it matched several traits, but considered it unlikely to belong to the genus and species as there was no overlapping material among specimens. Barsbold and Maryanska also disagreed with previous researchers who classifiedDeinocheirus as a segnosaur.[7] In the same year, David B. Norman consideredTherizinosaurus to be a theropod of uncertain classification.[27]
However, with the unexpected discovery and description ofAlxasaurus in 1993, the widely accepted sauropodomorph affinities of segnosaurs were questioned by paleontologistsDale Russell andDong Zhiming. This new genus was far more complete than any other segnosaur and multiple anatomical features indicated that it was related toTherizinosaurus. With this, they identified the Therizinosauridae along with the Segnosauridae to be the same group, the former name having taxonomicpriority. Due to some primitive characters present inAlxasaurus they coined a new taxonomic rank, theTherizinosauroidea, containing the new taxon and Therizinosauridae. All of the new information provided data on the affinities of the new-named therizinosauroids. Russell and Dong concluded that they were theropods with unusual features.[28] In 1994, Clark and colleagues redescribed the very complete skull ofErlikosaurus and even more theropod traits were found this time. They also validated the synonymy of the Segnosauridae with Therizinosauridae and considered therizinosauroids as maniraptoran dinosaurs.[29] In 1997, Rusell coined the infraorderTherizinosauria in order to contain all segnosaurs. This new infraorder was composed of Therizinosauroidea and the more advanced Therizinosauridae. Consequently, Segnosauria became a synonym of Therizinosauria.[30] Though some uncertainties remained, a small and feathered therizinosauroid fromChina was described in 1999 byXu Xing and colleagues: the new genusBeipiaosaurus. It confirmed the placement of therizinosaurs among theropods and also their taxonomic place on theCoelurosauria. The discovery also indicated that feathers were highly distributed among theropod dinosaurs.[31]
In2010,Lindsay Zanno revised the taxonomy of therizinosaurs in extensive detail. She found that many parts on therizinosaur holotype and referred specimens were lost or damaged, and sparse specimens with no overlapping elements were disadvantages when concluding the relationships of the members. Zanno accepted the referral of the specimen IGM 100/45 toTherizinosaurus since it matches multiple therizinosaurid traits, but decided not to include the specimen in her taxonomic analysis due to the lack of comparative forelimb remains. She also excluded the supposed ribs that were present on the holotype since they likely came from a different animal and notTherizinosaurus.[8] In2019, Hartman and colleagues also performed a large phylogenetic analysis of Therizinosauria based on the characters provided by Zanno in her revision. They found similar results to Zanno regarding the family Therizinosauridae but this time with the inclusion of more taxa and specimens. Thecladogram below shows the placement ofTherizinosaurus within Therizinosauria according to Hartman and colleagues in 2019:[32]
| Therizinosauridae |
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In 1993 Dale A. Russell and Donald E. Russell analyzedTherizinosaurus andChalicotherium,[33] and noted similarities in their respectivebody plan, even though they form part of different groups. Both genera had large, well-developed, and relatively strong arms; the pelvic girdle was robust and suited for a sitting behavior; and the hindlimb (particularly the foot) structure was robust and shortened. They considered these adaptations to represent an example ofconvergent evolution—a condition where organisms evolve similar traits without necessarily being related—between extinct mammal and dinosaur genera. Moreover, the body plan is somewhat exhibited by the modern-daygorillas. Because the animals with this type of body plan are known to representherbivores, the authors suggested this lifestyle forTherizinosaurus. Russell and Russell reconstructed the feeding behavior ofTherizinosaurus as being able to sit while consumingfoliage from largeshrubs andtrees. The plant material would have been harvested with its hands and this action was likely favored by its elongated neck which prevented the use of large amounts of force and effort. As its arms were long enough to have touched the ground during certain stances, they could have helped the dinosaur to rise from a prone position. Ifbrowsing in abipedal stance,Therizinosaurus may have been able to reach even higher vegetation supported by its short and robust feet. WhereasChalicotherium was more suited to hook branches,Therizinosaurus was better at pushing large clumps of foliage because of its long claws. It is also possible thatTherizinosaurus was less capable of great precision in its movements than wasChalicotherium, due to the latter having more developedbrain,dental andmuscular capacities.[34]
Anthony R. Fiorillo and colleagues in 2018 suggested thatTherizinosaurus had a reduced bite force that may have been useful for cropping vegetation orforaging, based on relative therizinosaurids such asErlikosaurus andSegnosaurus. As the bite force started to decrease from primitive to derived therizinosaurians,Therizinosaurus, being a derived member, would have been subject to the evolutionary relationship.[35]
When the genus was first described by Maleev in 1954, he considered that the unusually large claws were used to harvest seaweed. This was however, based on the assumption of a giant marine turtle.[1] In 1970, Rozhdestvensky re-examined the claws and suggested a possible function specialized in openingtermite mounds or afrugivore diet.[2] Barsbold in 1976 suggested that the unusual claws ofTherizinosaurus may have been employed to impale or dig up loose terrain, however, he pointed out their notorious fragility upon impact.[4] In 1995, Lev A. Nessov suggested the elongated claws were used fordefense against predators and juveniles could have used their claws forarboreal locomotion, in a similar way to the modern-daysloths orhoatzin chicks.[36]
In 2014, Lautenschlager tested the function of various therizinosaur handclaws—includingTherizinosaurus—through digital simulations. Three different functional scenarios were simulated for each claw morphology with a force of 400 N applied in each scenario: scratch/digging; hook-and-pull; and piercing. Though the stocky claws ofAlxasaurus resulted in low-stress magnitudes, the stress was greater with the curvature and elongation of the claws inFalcarius,Nothronychus andTherizinosaurus. Some of the highest stress, deformation, and strain magnitudes were obtained in the scratch/digging scenario; the hook-and-pull scenario, in contrast, resulted in lower magnitudes, and lesser ones were found in the piercing scenario. Particularly, the overall stress was most pronounced in the unusual claws ofTherizinosaurus, which may represent an exceptional case of elongationspecialization. Lautenschlager noted the more strongly curved and elongate claws of some therizinosaurian taxa were poorly functional in a scratch/digging fashion, indicating this as the most unlikely function. Thoughfossorial (digging) behavior has been reported in several dinosaur species, the large body size largely rules out the possibility of burrow digging in therizinosaurs. Nevertheless, an overall digging action would have been done with the foot claws because, since as in other maniraptorans,feathers on the arms would have interfered with this function. Instead of being used for fossorial behavior, it is more likely thatTherizinosaurus make use of its hands in a hook-and-pull fashion to pull or grasp vegetation within reach. This herbivorous behavior would make therizinosaurs mostly similar to the extantanteaters and the extinctground sloths. Lautenschlager could neither confirm nor disregard that the hand claws could have been used for defense,intraspecific competition, stabilization by grasping tree trunks during high browsing,sexual dimorphism, or gripping mates during mating given the lack of more specimens. He clarified that there is no evidence that the long claws ofTherizinosaurus would have been used in active defense or attack; however, it is possible that these appendages could have had some role when facing a threat, such as intimidation.[37]
Scott A. Lee and Zachary Richards in 2018 based on bending resistance measurements of several dinosaur humeri, found the humeri ofcarnosaur,therizinosaur, andtyrannosaur dinosaurs to be relatively resilient to stress. This increased ability to withstand stress supports the idea thatTherizinosaurus and other therizinosaurians used their arms in a robust fashion that generated significant forces. They also suggested that the prominent claws of some members could have been used as a defense against predators and other various functions. Unlike the generally light and agileornithomimosaurs who avoided predation with speed,Therizinosaurus and relatives relied on arms and claws to face threats (and were generally slow-runners to begin with).[38]
A 2023 study by Qin, Rayfield, Benton,et al., regarding the claw function in therizinosaurids andalvarezsaurids, which represent the extremes of theropod claw morphology, suggest that there was no mechanical function identifiable forTherizinosaurus, suggesting the claws on its forelimbs were merely decorative rather than functional and a result of peramorphic growth resulting from increased body size.[39]
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The remains ofTherizinosaurus have been found in the well-knownNemegt Formation of theGobi Desert. Although this formation has never been datedradiometrically because of the discontinuity of exposures and absence of datablevolcanic rockfacies, the vertebrate fossil assemblage suggests aMaastrichtian stage (and possiblyLate Campanian). The Nemegt Formation is separated into three informal members. The lower member is mainly composed byfluvialsediments, while middle and upper members consist ofalluvial plain,paludal,lacustrine, and fluvial sedimentation.[40][41]Therizinosaurus is known from the Altan Uul, Hermiin Tsav, and Nemegt localities, belonging to the lower and middle members of the formation.[41]
The environments thatTherizinosaurus inhabited have been determined by the sedimentation across the formation, theδ13C level preserved on thetooth enamel of many herbivorous dinosaurs and the numerouspetrified wood across the formation. They consisted of largemeandering andbraided rivers with extensivewoodlands composed of large, enclosed,canopy-likeforests ofAraucarias that supported diverse herbivorous dinosaurs likeTherizinosaurus. The climate of the formation was relativelytemperate (mean annual temperatures between 7.6 and 8.7 °C), characterized bymonsoons with cold, dry winters and hot, rainy summers with the addition of mean annualprecipitations between 775 mmm and 835 mmm, a precipitation that was subject to prominentseasonal fluctuations.[41][42] The wet environments of the Nemegt Formation may have acted as anoasis-like area that attractedoviraptorids from arid neighbour localities such as theBarun Goyot Formation, as evidenced on the presence ofNemegtomaia in both regions.[43] It has been previously suggested that the Nemegt Formation may have been similar to the modern-dayOkavango Delta, which is also composed ofmesic (well-watered) surroundings.[44]
The area would have been semi-arid during certain times of the year.[42] While the environment was likely dominated byAraucarian conifer forests,[42] it also containedginkgos, reed grasses,fagalean trees, cycad-like plants,sycamores (plane trees),bald cypresses, katsura relatives,pondweeds, tupelos, duckweeds, lotuses, and sedges.[45]
The paleofauna of the Nemegt Formation was diverse and rich, composed of other dinosaurs such as thealvarezsaursMononykus andNemegtonykus;deinonychosaursAdasaurus, andZanabazar;ornithomimosaursAnserimimus andGallimimus;oviraptorosaursAvimimus,Conchoraptor,Rinchenia, andElmisaurus;tyrannosauridsAlectrosaurus,Alioramus, and possiblyBagaraatan;ankylosauridsSaichania andTarchia; andpachycephalosauridsHomalocephale andPrenocephale. The Nemegtmegafauna included theornithomimosaurDeinocheirus;hadrosauridsBarsboldia andSaurolophus;titanosaursNemegtosaurus andOpisthocoelicaudia; and theapex predatorTarbosaurus.[46][43][47][48] Additional paleofauna includesbirds likeJudinornis orTeviornis; abundantfreshwaterostracods at numerous localities;fish; terrestrial and aquaticturtles such asMongolochelys andNemegtemys; and thecrocodylomorphParalligator.[49][50][51][52]
As the sediments in whichTherizinosaurus remains have been found are fluvial-based, it is suggested that it may have preferred to forage onriparian areas.[34]Therizinosaurus due to its prominent height and high-browsing lifestyle, was one of the tallest dinosaurs in the Nemegt Formation paleofauna. It probably had no significantcompetition with other herbivores over the foliage; however, aniche partitioning with the titanosaurs—also long-necked dinosaurs—of the formation could have occurred. IfTherizinosaurus was a grazer, on the other hand, it would have competed with contemporary grazers such asSaurolophus. Although smallpredators like dromaeosaurids and troodontids did not represent a threat toTherizinosaurus, the only other predator rivaling in size wasTarbosaurus. Because of the greater height ofTherizinosaurus, a largeTarbosaurus may have been not able to bite any higher than the thighs or belly of an adult standingTherizinosaurus. The elongated claws may have been useful for self-defense or to intimidate the predator during this situation.[17] It is also possible thatTherizinosaurus competed for other various resources withDeinocheirus,Saurolophus,Nemegtosaurus andOpisthocoelicaudia.[44]
Media related toTherizinosaurus at Wikimedia Commons
Data related toTherizinosaurus at Wikispecies
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