Therizinosaurus is a dinosaur genus which lived about 70 million years ago, during the Late Creataceous period. The genus includes only one species, called Therizinosaurus cheloniformis, which is only known from a few bones. It has been named after its huge manual unguals, and the specific name “cheloni” is taken from Greek, meaning “turtle” as the fossils were thought to belong to a turtle-like reptile.
Therizinosaurus cheloniformis. By PaleoNeolitic - Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=90136941
Known occurrences of Therizinosaurus cheloniformis fossils.
This species was discovered in Southwestern Mongolia, in the Nemegt Formation of the Gobi Desert. It was found by a Mongolian field expedition, organized by the USSR Academy of Sciences, in 1948 and was the most remarkable find during the excavation. However, the new species was only described 6 years later, by the Russian paleontologist, Evgeny Maleev. Until 1980, more remains were found in the Nemegt Formation, including fore- and hindlimb elements. In 2013, a nesting colony was found which contained at least 17 clutches of eggs, with in all around 75 eggs. No embryos were found in any of them, most of the eggs seemed to have hatched. These discoveries lead us to conclude that these were social animals that guarded their nests.
Replica of a fossil claw of Therizinosaurus cheloniformis. Found in the Upper Cretaceous Nemegt Formation (about 70 million years old), Mongolia. Collection of the Sauriermuseum Aathal, Aathal, Switzerland. By Woudloper - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6973101
Taxonomy and valid species
As there were only a few pieces of the big bone puzzle, the taxonomy of Therizinosaurus was a mystery for a long time. After Maleev described the species, he thought it was a giant marine turtle, and created an entirely new family, Therizinosauridae, for its classification. In 1970 Anatoly Konstantinovich Rozhdestvensky suggested that the findings belong to a carnosaurian theropod. He highlighted the possibility that the supposed ribs of the holotype were belonged to another dinosaur. After Segnosaurus was discovered, a new family was placed within the theropods, called Segnosauridae. Paleontologists found similar the Segnosaurus to the Thereizinosaurus, and considered them for being sauropodomorphs. After 1994, the discovery of the Alxasaurus and later the Beipiaosaurus species helped to clarify the taxonomic position of the Therezinosaurus. After a long discussion, they are now placed in Theropoda clades, specifically as maniraptorans. According to the current scientific opinion Therizinosaurus is recovered within Therizinosauridae, Therizinosauria (formerly known as Segnosauria), and it is considered one of the last representative of its group. 
Fossil Therizinosaurus cheloniformis arms from Mongolia, exhibited in Experimentarium, Denmark. By FunkMonk (Michael B. H.) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8122332
Description and anatomy
Despite of the few fossils, based on more complete therizinosaurids, the outlook species can be inferred from its relatives appearance. According to estimates, it had a length between 9 and 10 m, a height from 4 to 5 m, and weighed between 3 and 5 tons. From these data it becomes obvious that Therizinosaurus was the largest known maniraptoran and theriziniosaur. It had relatively small skull and a long neck bearing a rhamphotheca at the top of its neck, which was a horny beak. It is interesting to imagine how could this enormous, long-necked creature move bipedally. Practically it must had a huge belly to digest foliage. It wore sparse plumage as it can be inferred from the data of its close relatives. Fossils show, this theropod had presumably 3 m long legs and 2.4 m long arms. The robust structure of its bones requires powerful muscles.
The manual unguals were especially straigtht and enormous, around 52 cm long, which is record in length among the currently known terrestrial animals. This is a well-known distinctive feature of the species. The manual unguals had sharp curvatures only at the tips, and unlike other therizinosaurs they were side to side flattened. The structure of the feet of Therizinosaurus and other therizinosaurids was unique, as the general theropod formula contain tridactyl feet in which the first toe was reduced to a dewclaw and held off the ground. The first pedal digit was short and weight-bearing.
Restoration of Therizinosaurus, Tylocephale (actually Prenocephale) and Velociraptor (actually Adasaurus). By ABelov2014 - https://www.deviantart.com/abelov2014/art/Therizinosauridae-Tylocephale-Velociraptor-559088991, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=96017434
Paleobiology and paleoenvironment
The feeding habits of Theriziniosaurus were analyzed by Dale A. Russell and Donald E. Russell in 1993, and they concluded that it was herbivorous, based on its physique and the data of its relatives. However, since his skull was never found, this cannot be declared with 100% certainty. Nevertheless, they reconstructed its feeding behavior in such a way that this theropod could eat foliage of trees while sitting on the ground. The dinosaur had reduced bite force, which was useful to tear off the vegetation. Thanks to its long neck and strong legs, it could reach for the higher leaves, and probably the plant material could have been harvested with its hands. It is conceivable that this dinosaur could rise from a prone position with the help of its long arms. Therizinosaurus was slow-moving and it had less capable of great precision in movement coordination. Presumably they formed groups until at least the time of nesting.
In connection with the function of arms and claws, there are several hypotheses. Some thought that the huge claws were used to harvest vegetation, others said, these were ideal to dig and open termite mounds – this would assume that this dinosaur was insectivorous. Several researcher suggested, that they were used for defense. Lautenschlager tested the possible functions of the claws with digital simulations, in 2014. As it turned out, the claws were not suitable for digging, due to the fact that they proved to be fragile. According to the current scientific consensus, their role was the protection against the predators, and claws were also useful in arboreal locomotion. With the help of them, Theriziniosaurus most likely grabbed the plants and scraped up plant material, as pandas do nowadays. Nevertheless, it cannot be ruled out that claws were important in sexual dimorphism, mating or intraspecific competition. Due to the incomplete remains, the function is still unclear.
As for the environment where Theriziniosaurus lived, dry, cold winters and rainy, warm summers and monsoons characterized the landscape. Altogether it can be said, the climate was temperate, the oasis-like area was covered by rivers and extensive forests and this was ideal for a great variety of creatures. It seems likely that Therizinosaurus lived along waterfronts as its remains were found in fluvial sediments. Based on fossils, due to its long neck, this theropod did not need to compete with other herbivores, and presumably it had only one predator in the area, the Tarbosaurus.
Author: Csenge Lugosi, Biologist BSc student
Restoration of Therizinosaurus. By Johnson Mortimer - http://www.deviantart.com/art/Therizinosaurus-6-618917652, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=49880832
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