Therizinosaurus

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Therizinosaurus
Temporal range:
Ma
Mounted forelimbs of specimen MPC-D 100/15 at Nagoya City Science Museum
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Superfamily: †
Therizinosauroidea
Family: †Therizinosauridae
Genus: †Therizinosaurus
Maleev, 1954
Type species
†Therizinosaurus cheloniformis
Maleev, 1954

Therizinosaurus (

period in what is now the Nemegt Formation around 72.1 million years ago to 66 million years ago. It contains a single species, Therizinosaurus cheloniformis. The first remains of Therizinosaurus were found in 1948 by a Mongolian field expedition at the Gobi Desert and later described by Evgeny Maleev in 1954. The genus is only known from a few bones, including gigantic manual unguals
(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 a

sauropodomorphs
.

It was one of the last and the largest representative of its unique group, the

ornithischians, 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 as maniraptorans
. Therizinosaurus is widely recovered within Therizinosauridae by most analyses.

The unusual arms and body anatomy (extrapolated after relatives) of Therizinosaurus have been cited as an example of

predators like Tarbosaurus
.

History of discovery

Cretaceous fossil localities of Mongolia; Therizinosaurus fossils have been collected from the Altan Uul, Hermiin Tsav, and Nemegt localities at the area A (Nemegt Formation)

In 1948, several Mongolian Paleontological expeditions organized by the

ribs fragments. It was labelled under the specimen number PIN 551-483 and later on, these fossils were described by the Russian paleontologist Evgeny Maleev in 1954 who used them to scientifically name the new genus and type species Therizinosaurus cheloniformis, becoming the holotype specimen. The generic name, Therizinosaurus, is derived from the Greek θερίζω (therízo, meaning scythe, reap or cut) and σαῦρος (sauros, meaning lizard) in reference to the enormous manual unguals, and the specific name, cheloniformis, is taken from the Greek χελώνη (chelóni, meaning turtle) and Latin formis as the remains were thought to belong to a turtle-like reptile. Maleev also coined a separate family for this new and enigmatic taxon: Therizinosauridae. Since little was known of Therizinosaurus 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]

Holotype unguals PIN 551–483 at the Moscow Paleontological Museum; note left ungual cast

Though it was not fully understood to what general kind of animal these fossils belonged, in

sauropod remains.[2] These theropodan affinities were also followed by the Polish paleontologist Halszka Osmólska and co-author Ewa Roniewicz in 1970 during their naming and description of Deinocheirus—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]

Additional specimens

Arms of specimen MPC-D 100/15 with unguals 100/16 and 100/17 at CosmoCaixa Barcelona

Further expeditions in the Nemegt Formation unearthed more fossils of Therizinosaurus. 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.

metacarpus including a complete digit Il, and some ribs with gastralia (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 to Therizinosaurus by the Mongolian paleontologist Rinchen Barsbold in 1976. 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 represented Therizinosaurus as the ungual in this specimen shared the elongation and flattened morphology of all previous specimens. He concluded that Therizinosaurus was a theropod taxon since MPC-D 100/15 matched multiple theropodan characters.[4]

Composite skeletal of the known remains of Therizinosaurus

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 fragmented

therizinosaurian (then called segnosaurians) given that the metatarsus was stocky and the astragalus had a laterally arched ascending process (bony extension), but cast doubt with his referral of it to Therizinosaurus 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 not Therizinosaurus.[7]

In 2010 however, the

Lindsay E. Zanno in her large taxonomic reevaluation of Therizinosauria considered the referral of MPC-D 100/45 to Therizinosaurus 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 as Segnosaurus. 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 not Therizinosaurus itself.[8]

Description

Size of two specimens (holotype in yellow, right) compared to a 1.8 m (5.9 ft) tall human

For

bipedal gaits; a large belly for foliage processing; and sparse feathering. Other traits that were likely present in Therizinosaurus include a heavily pneumatized (air-filled) vertebral column and a robustly-built, ophistopubic (backwards oriented) pelvis.[13][14][15]

Life restoration

In 2010, Senter and James used hindlimb length equations to predict the total length of the hindlimbs in Therizinosaurus and Deinocheirus. They concluded that an average Therizinosaurus may have had approximately 3 m (9.8 ft) long legs.[16] More recently, Mike Taylor and Matt 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 the cervical vertebrae series of Nanshiungosaurus.[17] The most distinctive feature of Therizinosaurus 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 in Therizinosaurus, and they are considered as the longest known from any terrestrial animal.[12]

Forelimbs

Diagram of the right arm and shoulder blade (rotated) of MPC-D 100/15

The arm of Therizinosaurus covered 2.4 m (7.9 ft) in total length (humerus, radius and second metacarpal with phalanges lengths).

biceps muscle was prominently developed by a large tubercle with a stocky top, indicating powerful muscles in life.[4][18][8]

The

epicondyles were very broad and projected over the limits of the articular areas. The ulna 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. The radius was 55.04 cm (21.67 in) long and slightly S-curved. Its upper end was flattened in a lateral direction, very wide, and the distal end was highly robust.[4][18][8]

unguals
of Therizinosaurus

The first lower

carpus. 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][18][8]

The metacarpal 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 a unique trait of Therizinosaurus. 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][18][8]

Highlighted regions of Therizinosaurus MPC-D 100/15 ungual

Only the second

therizinosaurs they were very straight, side to side flattened, and had sharp curvatures only at the tips, a unique feature of Therizinosaurus. 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][18][8]

Hindlimbs

Labelled diagram of the referred pes specimen MPC-D 100/45

Therizinosaurus had a rather stocky and robust

tetradactyl (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 of Therizinosaurus and other therizinosaurids was unique, as the general theropod formula includes tridactyl (three-toed) feet in which the first toe was reduced to a dewclaw and held off the ground.[6][15]

Classification

See also: Timeline of therizinosaur research
Outdated quadrupedal, sauropodomorph-like restoration of Erlikosaurus

Maleev originally classified Therizinosaurus as a giant marine turtle and the genus was assigned by him to a separate family, Therizinosauridae given how enigmatic the specimen was.

sauropod dinosaurs.[21]

Royal Tyrrell Museum
. Alxasaurus helped to resolve important aspects about the affinities of the long-enigmatic therizinosaurs

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.[22] He maintained his position in 1988 by placing the Segnosauria into the now obsolete Phytodinosauria, and was one of the first to suggest a segnosaur assignment for the enigmatic Therizinosaurus.[23] Other prominent paleontologists like Jacques Gauthier or Paul Sereno supported this vision.[24][25] In 1990, Barsbold and Teresa Maryanska agreed in that the hindlimb material from Hermiin Tsav referred to Therizinosaurus 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 classified Deinocheirus as a segnosaur.[7] In the same year, David B. Norman considered Therizinosaurus to be a theropod of uncertain classification.[26]

Feather impressions from the holotype of Beipiaosaurus

However, with the unexpected discovery and description of

Therizinosauroidea, 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.[27] In 1994, Clark and colleagues redescribed the very complete skull of Erlikosaurus 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.[28] In 1997, Rusell coined the infraorder Therizinosauria 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.[29] Though some uncertainties remained, a small and feathered therizinosauroid from China was described in 1999 by Xu Xing and colleagues: the new genus Beipiaosaurus. It confirmed the placement of therizinosaurs among theropods and also their taxonomic place on the Coelurosauria. The discovery also indicated that feathers were highly distributed among theropod dinosaurs.[30]

In 2010, 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 to Therizinosaurus 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 not Therizinosaurus.[8] In 2019, 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. The cladogram below shows the placement of Therizinosaurus within Therizinosauria according to Hartman and colleagues in 2019:[31]

Skeletal reconstructions of various therizinosaurian genera (not to scale); Therizinosaurus in top left
Therizinosauridae
unnamed

Suzhousaurus

unnamed

Neimongosaurus

unnamed

Therizinosaurus

Erliansaurus

unnamed

Nanchao embryos

unnamed

Nanshiungosaurus

unnamed
unnamed

Segnosaurus

AMNH 6368

Erlikosaurus

unnamed

Nothronychus mckinleyi

unnamed

Nothronychus graffami

Paleobiology

Feeding

Life restoration of Erliansaurus in a sitting stance. Therizinosaurus may have sat on its pelvis during feeding

In 1993 Dale A. Russell and Donald E. Russell analyzed Therizinosaurus and

muscular capacities.[33]

Anthony R. Fiorillo and colleagues in 2018 suggested that Therizinosaurus had a reduced bite force that may have been useful for cropping vegetation or foraging, based on relative therizinosaurids such as Erlikosaurus and Segnosaurus. 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.[34]

Arms and claws function

foliage

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.

sloths or hoatzin chicks.[35]

In 2014, Lautenschlager tested the function of various therizinosaur hand

ground 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 of Therizinosaurus 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.[36]

Manus of Therizinosaurus tested in a hook and pull scenario

Scott A. Lee and Zachary Richards in 2018 based on bending resistance measurements of several dinosaur humeri, found the humeri of

ornithomimosaurs who avoided predation with speed, Therizinosaurus and relatives relied on arms and claws to face threats (and were generally slow-runners to begin with).[37]

A 2023 study by Qin, Rayfield, Benton et al., regarding the claw function in therizinosaurids and

alvarezsaurids, which represent the extremes of theropod claw morphology, suggest that there was no mechanical function identifiable for Therizinosaurus, suggesting the claws on its forelimbs were merely decorative rather than functional and a result of peramorphic growth resulting from increased body size.[38]

Paleoenvironment

The Nemegt Formation may have had similar environments to those of the swampy Okavango Delta

The remains of Therizinosaurus have been found in the well-known

lacustrine, and fluvial sedimentation.[39][40]

The environments that Therizinosaurus inhabited have been determined by the sedimentation across the formation, the

oviraptorids from arid neighbour localities such as the Barun Goyot Formation, as evidenced on the presence of Nemegtomaia in both regions.[42] It has been previously suggested that the Nemegt Formation may have been similar to the modern-day Okavango Delta, which is also composed of mesic (well-watered) surroundings.[43]

Life restoration of a Therizinosaurus pair along with a small group of Prenocephale, and Adasaurus

The paleofauna of the Nemegt Formation was diverse and rich, composed of other dinosaurs such as the

crocodylomorph Paralligator.[47][48][49][50]

As the sediments in which Therizinosaurus remains have been found are fluvial-based, it is suggested that it may have preferred to forage on

predators like dromaeosaurids and troodontids did not represent a threat to Therizinosaurus, the only other predator rivaling in size was Tarbosaurus. Because of the greater height of Therizinosaurus, a large Tarbosaurus may have been not able to bite any higher than the thighs or belly of an adult standing Therizinosaurus. The elongated claws may have been useful for self-defense or to intimidate the predator during this situation.[16] It is also possible that Therizinosaurus competed for other various resources with Deinocheirus, Saurolophus, Nemegtosaurus and Opisthocoelicaudia.[43]

See also

References

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External links
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