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 and western North America. 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 Latin segnis meaning 'slow' or 'sluggish', and Greek σαυρος, sauros, meaning 'lizard'.
|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 performng 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).