Therizinosaurus

''Therizinosaurus'' (; meaning 'scythe lizard') is a genus of very large therizinosaurid that lived in Asia during the Late Cretaceous period in what is now the Nemegt Formation around 70 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 have been discovered during the 1960s–1980s.

''Therizinosaurus'' was a colossal therizinosaurid that could grow up to long and tall, and weigh possibly over . Like other therizinosaurids, it would have been a slow-moving, long-necked, high browser equipped with a rhamphotheca (horny beak) and a wide torso for food processing. Its forelimbs were particularly robust and had three fingers 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 in length. Its hindlimbs ended in four functionally weight-bearing toes differing from other theropod groups in which the first toe was reduced to a dewclaw and also resembling the unrelated sauropodomorphs.

It was one of the last and the largest representative of its unique group, the Therizinosauria (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 of ''Therizinosaurus'' to belong to a large turtle-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 Cretaceous sauropodomorphs or transitional 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 convergent evolution with chalicotheriines and other primarily herbivorous mammals, suggesting similar feeding habits. The elongated hand claws of ''Therizinosaurus'' were more useful when pulling vegetation 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 to stress, which suggests a robust use of these limbs. ''Therizinosaurus'' was a very tall animal, likely having a reduced competition over the foliage in its habitat and outmatching predators like ''Tarbosaurus''.

History of discovery

In 1948, several Mongolian Paleontological expeditions organized by the USSR Academy of Sciences were conducted in the Nemegt Formation of the Gobi Desert, Southwestern Mongolia, with the main objective of new fossils findings. The expeditions unearthed numerous dinosaur and turtle fossil remains from the stratotype locality Nemegt (also known as Nemegt Valley), but the most notable elements collected were three partial manual unguals (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 several 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 (, meaning scythe, reap or cut) and (, meaning lizard) in reference to the enormous manual unguals, and the specific name, ''cheloniformis'', is taken from the Greek (, 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, long turtle-like reptile that relied on its giant hand claws to harvest seaweed.Translated paper


Though it was not fully understood to what general kind of animal these fossils belonged, in 1970, the Russian paleontologist Anatoly K. Rozhdestvensky was one of the first authors to suggest that ''Therizinosaurus'' was a theropod and not a turtle. He made comparisons between '' Chilantaisaurus'' and the holotype unguals of ''Therizinosaurus'' to propose that the appendages actually came from a carnosaurian dinosaur, thereby interpreting ''Therizinosaurus'' as a theropod. Rozhdestvensky also illustrated the three holotypic manual unguals and re-identified the metacarpal fragment as a metatarsal bone, and based on the unusual shape of both metatarsal and ribs fragments he listed them as sauropod remains. These theropodan affinities were also followed by the Polish paleontologist Halszka Osmólska and co-author Ewa Roniewicz