Therizinosaurs (or segnosaurs) were theropod dinosaurs belonging to the clade Therizinosauria. Therizinosaur fossils have been found in Early through Late Cretaceous deposits in Mongolia, the People's Republic of China, western North America and possibly Australia. Various features of the forelimbs, skull and pelvis unite these finds as both theropods and as maniraptorans, close relatives to birds.
The name therizinosaur is derived from the Greek θερίζω therízein, meaning 'to reap' or 'to cut off', and σαῦρος saûros meaning 'lizard'. The older name segnosaur is derived from the Latin segnis meaning 'slow' or 'sluggish', and σαῦρος.
|Reconstructed skeleton of Nothronychus graffami|
Segnosauria Barsbold, 1980
Therizinosaurs had a very distinctive, often confusing set of characteristics. Their long necks, wide torsos, and hind feet with four toes used in walking resembled those of basal sauropodomorph dinosaurs. Their unique hip bones, which pointed backwards and were partially fused together, initially reminded paleontologists of the "bird-hipped" ornithischians. Among the most striking characteristics of therizinosaurs are the enormous claws on their hands, which reached lengths of around one meter in Therizinosaurus. The unusual range of motion in therizinosaur forelimbs, which allowed them to reach forward to a degree other theropods could not achieve, also supports the idea that they were mainly herbivorous. Therizinosaurs may have used their long reach and strongly curved claws to grasp and shear leafy branches, in a manner similar to large mammals that lived later on, such as chalicotheres, ground sloths, great apes, and giant pandas.
Skin impressions from Beipiaosaurus indicate that therizinosaurs were covered with a coat of primitive, down-like feathers similar to those seen in the compsognathid Sinosauropteryx, as well as longer, simpler, quill-like feathers that may have been used in display. Therizinosaurs spanned a large range of sizes, from the small Beipiaosaurus (which measured 2.2 m [7 ft 3 in] in length), to the gigantic Therizinosaurus, which at an approximate 10 m (33 ft) long and an estimated weight of 5 tonnes, was among the largest-known theropods.
Because early finds were incomplete, the strange suite of anatomical features combining features typical of theropods, prosauropods and ornithischians led some scientists, such as Gregory S. Paul, to conclude that segnosaurs (as they were called before Therizinosaurus was recognized as part of the group) represented a late-surviving suborder of primitive dinosaurs, sometimes thought of as intermediates between prosauropods and ornithischians. Because of their suspected relationship with prosauropods, early depictions of segnosaurs (including illustrations by Paul) portrayed them as semi-quadrupedal, a mode of locomotion now known to have been impossible given the bird-like nature of their wrists. It also led Paul to include segnosaurs within paleontologist Robert T. Bakker's Phytodinosauria in 1986, a superorder which was to include ornithischians, prosauropods, and sauropods, typified by their "blunt, spoon-crowned teeth suitable for cropping plants."
It was not until the mid-1990s, after Alxasaurus was discovered and shown to possess more typically theropod features, and Therizinosaurus was recognized as a member of the segnosaur group, that their true identity as herbivorous descendants of the carnivorous theropods became generally accepted. The relation between the more derived therizinosaurids and other theropods was greatly elucidated by the discovery of primitive members of the group, such as Beipiaosaurus in 1999 and Falcarius in 2005. The scientists who described Falcarius noted that it seemed to represent an intermediate stage between carnivorous and herbivorous theropods, a sort of "missing link" between predatory maniraptorans and plant-eating therizinosaurs. Although they are now classified as theropods, therizinosaurs had skulls similar to those of sauropods and the shape of their teeth and jaws make it likely that they were herbivores.
Barsbold and Perle named the group Segnosauria as an infraorder of Theropoda in 1980. Dong Zhiming (1992) went further, placing the segnosaurs in their own order, Segnosaurischia. This name has been abandoned since the discovery that segnosaurs are a specialized group within the suborder Theropoda. Clark et al. in 2004 considered Segnosaurischia a synonym of Therizinosauroidea.
The superfamily Therizinosauroidea had been established by Maleev in 1954, to include only the bizarre, giant-clawed theropod Therizinosaurus. When it was later realized that Therizinosaurus was an advanced segnosaur, Therizinosauroidea was given a phylogenetic definition to include both groups, and has largely replaced the use of the older name Segnosauria in phylogenetic studies, mainly because of the association of the name Segnosauria with the discredited idea that these animals were relatives of prosauropods.
The following taxonomy follows Zanno, 2010 unless otherwise noted.
The clade Therizinosauria was first defined by Dale Russell in 1997 as Alxasaurus, Enigmosaurus, Erlikosaurus, Nanshiungosaurus, Segnosaurus, Therizinosaurus, and all taxa closer to them than to oviraptorosaurs, ornithomimids, and troodontids. Paul Sereno, in 2005, modified this definition to the most inclusive clade containing Therizinosaurus but not Ornithomimus, Oviraptor, Shuvuuia, Tyrannosaurus, or Troodon.
Therizinosauroidea, previously named as a superfamily with no phylogenetic definition, was first defined by Zhang et al. in 2001, as the clade containing all theropods more closely related to Therizinosaurus than to birds (effectively replacing the older name Segnosauria, which has not yet been defined as a clade). This definition, however, defines the same group as the pre-existing Therizinosauria. An alternate definition was given by Clark in 2004 (as the last common ancestor of Therizinosaurus and Beipiaosaurus and all its descendants), comprising a narrower group that excludes more primitive therizinosaurs, such as Falcarius, and allows the name Therizinosauria to remain in use for the larger group comprising all therizinosaurs. This definition was followed by Teresa Maryańska and Barsbold (2004), Sereno (2005), Zanno et al. (2009) and Zanno (2010), though other subsequent studies, such as Senter (2007, 2012) have continued to use Therizinosauroidea for the therizinosaur "total group".
The cladogram below follows the extensive phylogenetic analysis of Therizinosauria, by Lindsay E. Zanno, 2010.
The following cladogram is based on the phylogenetic analysis by Phil Senter et al., 2012.
Therizinosauroid behavior is quite poorly understood, but 20th-century studies and subsequent finds have revealed some aspects of their behavior. Nests with sub-spherical eggs have been found, and evidence points to the eggs being buried and abandoned by the parents. And studies of neonates indicate they were well developed and likely precocial—able to leave the nest shortly after birth and suggesting little to no parental care.
CT scans published in 2012 by Stephan Lautenschlager et al. focused on the skull and brain cavity of Erlikosaurus, revealing it to have a large forebrain, and suggesting it had well developed senses of balance, hearing and smell, all of which would have been useful in evading predators, finding food, or in performing complex social behavior.
In 2011, a nesting ground containing 17 clutches of eggs was found in Mongolia's Gobi Desert, with a total of 75 eggs uncovered during excavation. The eggs were 5 inches in diameter and contained no embryos; and there was evidence—in the form of egg shells that had been broken out of—that the young had hatched and left, presumably with their parents. The presence of so many fossilized eggs in one venue implies that therizinosaurs probably were social animals that came together for nesting; and that some genera may have performed parental care. This find, described by Yoshitsugu Kobayashi et al., and mass-death quarries such as those containing Falcarius, augments evidence that therizinosauroids were social, herding animals. Adult therizinosaurs are estimated to have weighed around 500 kg (1,100 lb).
Alxasaurus (; "Alxa Desert lizard") is a genus of therizinosauroid alxasaurid theropod dinosaur from the Early Cretaceous (Albian) Bayin-Gobi Formation in Inner Mongolia. It is one of the earliest known members of the superfamily Therizinosauroidea, but it already possessed the body shape - including the long neck, short tail, and long hand claws - of later therizinosauroids. Like other members of this group, it was a bipedal herbivore with a large gut to process plant material. Several specimens are known and the largest was a little over 3.8 metres (12 feet) long. According to Gregory S. Paul, it was 4 metres (13 feet) long and its weight was about 400 kilograms (880 pounds).Beipiaosaurus
Beipiaosaurus is a genus of therizinosauroid theropod dinosaur from the Early Cretaceous of China. Before the discovery of Yutyrannus, it was among the largest dinosaurs known from direct evidence to be feathered.
The exact classification of therizinosaurs had in the past been hotly debated, since their prosauropod-like teeth and body structure indicate that they were generally herbivorous, unlike typical theropods. Beipiaosaurus, being considered to be a primitive therizinosauroid, has features which suggest that all therizinosauroids, including the more derived Therizinosauridae, to be coelurosaurian theropods, not sauropodomorph or ornithischian relatives as once believed.Bostobe Formation
The Bostobe Formation (Kazakh: Boztóbe svıtasy) is a geological formation in Kazakhastan whose strata date back to the Late Cretaceous. Dinosaur remains are among the fossils that have been recovered from the formation. It is about 45 metres thick and consists primarily of clay with interbeds of sand, representing an estuarine environment.Enigmosaurus
Enigmosaurus (meaning "Riddle Lizard") is a genus of therizinosauroid theropod dinosaur from the Late Cretaceous of Mongolia, central Asia. It is a herbivorous bipedal dinosaur.
The holotype, IGM 100/84, was collected in the Bayan Shireh Formation (=Baynshirenskaya) in Khara Khutul, southeastern Mongolia, dating from the Cenomanian and the Turonian stages, around 98-89.8 million years ago. It was first reported in 1979. It consists of a partial skeleton, lacking the skull, which includes a nearly complete pelvis with part of the right ischium missing.The type species, Enigmosaurus mongoliensis, was named in 1983, by Rinchen Barsbold and Altangerel Perle. The generic name was stated to be derived from Greek αἴνιγμα, ainigma, "riddle", after the puzzling and unusual shape of its pelvis (hips); at the time little was known about therizinosauroids. The specific name refers to the provenance from Mongolia.Enigmosaurus reached a length of five to seven metres, and a weight of one tonne. It is a considerably larger dinosaur than the related Erlikosaurus. The obturator process on the front edge of the ischium is horizontally elongated and low.
Enigmosaurus was by the describers assigned to a separate Enigmosauridae but later considered a member of the Segnosauridae which are today called the Therizinosauridae. Lindsay Zanno in 2010 recovered a position more basal in the Therizinosauroidea.Erliansaurus
Erliansaurus is a genus of therizinosauroid theropod dinosaur from the Late Cretaceous of China.Erlikosaurus
Erlikosaurus is a genus of herbivorous theropod dinosaur from the late Cretaceous Period, belonging to the Therizinosauridae. Its fossils, a skull and some post-cranial fragments, were found in the Bayan Shireh Formation of Mongolia, dating to around 90 million years ago.Eshanosaurus
Eshanosaurus is a genus of therizinosaurian dinosaur from the early Jurassic Period. It is known only from a fossil partial lower jawbone, found in China. It may be the earliest known coelurosaur.Falcarius
Falcarius is a genus of therizinosaurian dinosaur found in the Cretaceous of east-central Utah, United States. Its name is derived from the word "sickle", falcarius in Latin being a sickle cutter, which scientists have used to describe its large clawed hands.
Falcarius was a 4-meter-long (13 ft), bipedal herbivore. It had a small head and a long neck and tail.
The description of Falcarius (2005), following that of therizinosauroid Beipiaosaurus from the Early Cretaceous of China in 1999, clarifies the early evolution of the Therizinosauria and their relationship with the larger group of theropod dinosaurs, because Falcarius is a transitional form between older theropods and the much changed Therizinosauridae.Jianchangosaurus
Jianchangosaurus is a genus of therizinosaurian dinosaur that lived approximately 126 million years ago during the early part of the Cretaceous Period from the Yixian Formation in what is now China. The nearly complete juvenile specimen was missing only the distal tail. Jianchangosaurus was a small, lightly built, bipedal, ground-dwelling herbivore, that could grow up to an estimated 2 m (6.6 ft) long and was 1 m (3.3 ft) high at the hips.Lingyuanosaurus
Lingyuanosaurus (meaning "the lizard of Lingyuan") is a genus of therizinosaurian theropod dinosaur from the Early Cretaceous of China. It contains a single species, L. sihedangensis. It was found within a layer of the Jehol Group.
According to Mickey Mortimer, Lingyuanosaurus may have been an oviraptorosaur.Maniraptora
Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the non-avian dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Deinonychosauria, Oviraptorosauria and Therizinosauria. Ornitholestes and the Alvarezsauroidea are also often included. Together with the next closest sister group, the Ornithomimosauria, Maniraptora comprises the more inclusive clade Maniraptoriformes. Maniraptorans first appear in the fossil record during the Jurassic Period (see Eshanosaurus), and are regarded as surviving today as living birds.Maniraptoriformes
Maniraptoriformes is a clade of dinosaurs with pennaceous feathers and wings that contains ornithomimosaurs and maniraptors. This group was named by Thomas Holtz, who defined it as "the most recent common ancestor of Ornithomimus and birds, and all descendants of that common ancestor."Nanshiungosaurus
Nanshiungosaurus is a genus of therizinosaurian theropod dinosaur from the Cretaceous of China.
Two species have been named in the genus: the type species Nanshiungosaurus brevispinus and Nanshiungosaurus bohlini.Neimongosaurus
Neimongosaurus ("Nei Mongol lizard") is a genus of herbivorous therizinosaur theropod dinosaur known from the Upper Cretaceous of Nei Mongol, China.
Neimongosaurus is a therizinosauroid which is known from the holotype LH V0001, a partially preserved skull and skeleton, consisting of a partial braincase, the front of the right lower jaw, a nearly complete axial column, a furcula, the shoulder girdle, both humeri, a left radius, parts of the pelvis and parts of the hindlimbs. The specimen was in 1999 collected in the Sanhangobi in Inner Mongolia from the Iren Dabasu Formation dating from the Santonian stage about 85 million years ago. A second specimen, LH V0008, consisting of a sacrum with ilia, was assigned as the paratype.The type species, Neimongosaurus yangi, was formally named and described by Zhang Xiaohong, Xu Xing, Paul Sereno, Kwang Xuewen and Tan Lin in 2001. The generic name is derived from Nei Mongol, the Chinese name for Inner Mongolia. The specific name honours Yang Zhongjian.Neimongosaurus was bipedal. The teeth in its deep lower jaw, coarsely serrated, indicate an herbivorous diet. It had elongated cervical vertebrae. Its scapula had a tapering end. Neimongosaurus is thought to have been about 2.3 meters in length.
The original describers of the genus assigned Neimongosaurus to the Therizinosauroidea, in a basal position. Subsequent cladistic analyses have indicated a position in the more derived Therizinosauridae, but an analysis in 2010 by Lindsay Zanno confirmed the original placement.Nothronychus
Nothronychus is a genus of theropod dinosaur classified in the group Therizinosauria, from the Cretaceous of North America.
The type species of this dinosaur, Nothronychus mckinleyi, was described by James Kirkland and Douglas G. Wolfe in 2001. It was recovered near New Mexico's border with Arizona, in an area known as the Zuni Basin, from rocks assigned to the Moreno Hill Formation, dating to the late Cretaceous period (mid-Turonian stage), around 91 million years ago. A second specimen, described in 2009 as a second species, Nothronychus graffami, was found in the Tropic Shale of Utah, dating to the early Turonian, between one million and a half million years older than N. mckinleyi.
The name Nothronychus, is derived from Greek, meaning "slothful claw."Nothronychus was a herbivorous theropod with a beak, a bird-like hip (resembling that of the non-related ornithischians) and four-toed feet, with all four toes facing forward.Segnosaurus
Segnosaurus ('slow lizard') is a genus of herbivorous theropod dinosaur belonging to the Therizinosauridae from the Cretaceous of Mongolia.Suzhousaurus
Suzhousaurus is a genus of herbivorous therizinosauroid theropod dinosaur from the Early Cretaceous-age Xinminbao Group of the Yujingzi Basin, Gansu, China.Therizinosauridae
Therizinosauridae ("reaper lizards") is a family of theropod dinosaurs whose fossil remains have been dated to the Mid-to-Late Cretaceous period (100 to 70 mya). Even though representative fossils have only been found throughout China, Mongolia, and the United States, the range of Therizinosauridae was believed to have spanned much of the supercontinent of Laurasia at its height.Therizinosauridae was named after the large, claw-bearing ungual found on the manus of members in the group. This feature has led to little insight about the ecology of the family, and the purpose of the claw remains unknown. Other notable aspects of the physiology of these animals include a modified pelvis, robust hind-limbs, and a highly derived, nearly avian inner-ear. Moreover, the larger superfamily of Therizinosauroidea is believed to be the earliest group in which simple feathers have been documented.Research into therizinosaurids has also focused on uncovering more about the unique ecology and paleobiology of the family. A fair portion of modern research has concentrated on the feeding-patterns of these dinosaurs, as they are considered to be the best regarded candidate for the emergence of herbivory within Theropoda. While many closely related taxa are carnivorous, it is thought that the members of Therizinosauroidea, including Therizinosauridae, diverged and adopted either an herbivorous or omnivorous lifestyle.The current scientific consensus is that therizinosaurids evolved from small, bird-like maniraptorans, and thus they fall within the coelurosaurian clade called Maniraptora. Most studies have concluded that within Maniraptora, Therizinosaurians were the first of five major groups to diverge.Therizinosaurus
Therizinosaurus (; 'scythe lizard', from Ancient Greek θερίζω, meaning 'to reap', and σαῦρος, meaning 'lizard') is a genus of very large theropod dinosaurs. Therizinosaurus comprises the single species T. cheloniformis, which lived in the late Cretaceous Period (early Maastrichtian stage, around 70 million years ago), and was one of the last and largest representatives of its unique group, the Therizinosauria. Fossils of this species were first discovered in Mongolia and were originally thought to belong to a turtle-like reptile (hence the species name, T. cheloniformis – "turtle-formed"). It is known only from a few bones, including gigantic hand claws, from which it gets its name.