Therizinosaur, group of theropod dinosaurs that lived during the Late Cretaceous (roughly 100 million to 66 million years ago) in Asia and North America and were characterized by their relatively small skulls, leaf-shaped teeth, and extended fingers with extremely long and robust claws. Therizinosaurs also lacked teeth in the front half of their upper jaws, and they had long necks, wrist bones similar to those of birds, widely spaced hips, a backward-pointing pubis bone, and four widely spread toes similar to those of sauropod dinosaurs. Fossil specimens have been known since the 1950s, but their unusual combination of skeletal features (especially their teeth, hips, and toes) made their relationships to other dinosaur groups contentious. By the mid-1990s, the discovery of new, more complete specimens had confirmed their theropod ancestry. Therizinosaurs are divided into five genera (Beipiaosaurus, Falcarius, Alxasaurus, Erlikosaurus, and Therizinosaurus).

Unlike most other theropods, therizinosaurs were most likely herbivorous. It is likely that the transition from carnivory to herbivory occurred early in the evolution of the group. The transition involved changes in dentition and changes to the hips and hind limbs—which allowed more room and better support for the larger gut needed to digest plants. The most primitive therizinosaur, Falcarius, has been described as a transitional species because it has herbivorous dentition and wider hips; however, it also possessed a pubis bone and legs that resembled those of its running, carnivorous ancestors.

Some therizinosaur fossils show remarkable preservation. For example, Beipiaosaurus specimens show large patches of featherlike integument on the chest, forelimbs, and hind limbs. Several embryonic therizinosaur skeletons have been found inside fossilized eggs. These embryos show several unambiguous theropod characteristics that are lost by adulthood; they provide insight into the order of bone formation in dinosaurs.

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 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.

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.

Paleoenvironment

The remains of Therizinosaurus have been found in the well-known Nemegt Formation of the Gobi Desert. Although this formation has never been dated radiometrically because of the discontinuity of exposures and absence of datable volcanic rock facies, the vertebrate fossil assemblage suggests an early Maastrichtian stage possibly between 70 million and 68 million years ago. The Nemegt Formation is separated into three informal members. The lower member is mainly composed by fluvial sediments, while middle and upper members consist of alluvial plain, paludal, lacustrine, and fluvial sedimentation.

The environments that Therizinosaurus inhabited have been determined by the sedimentation across the formation, the δ13C level preserved on the tooth enamel of many herbivorous dinosaurs and the numerous petrified wood across the formation. They consisted of large meandering and braided rivers with extensive woodlands composed of large, enclosed, canopy-like forests of Araucarias that supported diverse herbivorous dinosaurs like Therizinosaurus. The climate of the formation was relatively temperate (mean annual temperatures between 7.6 and 8.7 °C), characterized by monsoons with cold, dry winters and hot, rainy summers with the addition of mean annual precipitations between 775 mmm and 835 mmm, a precipitation that was subject to prominent seasonal fluctuations. The wet environments of the Nemegt Formation may have acted as an oasis-like area that attracted oviraptorids from arid neighbour localities such as the Barun Goyot Formation, as evidenced on the presence of Nemegtomaia in both regions. 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.

The paleofauna of the Nemegt Formation was diverse and rich, composed of other dinosaurs such as the alvarezsaurs Mononykus and Nemegtonykus; deinonychosaurs Adasaurus and Zanabazar; ornithomimosaurs Anserimimus and Gallimimus; oviraptorosaurs Avimimus, Rinchenia and Elmisaurus; tyrannosaurids Alioramus and possibly Bagaraatan; ankylosaurids Saichania and Tarchia; and pachycephalosaurids Homalocephale and Prenocephale. The Nemegt megafauna included the ornithomimosaur Deinocheirus; hadrosaurids Barsboldia and Saurolophus; titanosaurs Nemegtosaurus and Opisthocoelicaudia; and the apex predator Tarbosaurus.

As the sediments in which Therizinosaurus remains have been found are fluvial-based, it is suggested that it may have preferred to forage on riparian areas. 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 significant competition with other herbivores over the foliage, however, a niche partitioning with the titanosaurs—also long-necked dinosaurs—of the formation could have occurred. If Therizinosaurus was a grazer, on the other hand, it would have competed with contemporary grazers such as Saurolophus. Although small 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. It is also possible that Therizinosaurus competed for other various resources with Deinocheirus, Saurolophus, Nemegtosaurus and Opisthocoelicaudia.

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