Information about Therizinosauria

Therizinosaurs Fossil range: Cretaceous
Model of the therizinosauroid Beipiaosaurus Model of the therizinosauroid Beipiaosaurus
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Sauropsida Superorder: Dinosauria Order: Saurischia Suborder: Theropoda Superfamily: Therizinosauroidea Maleev, 1954
Families
Alxasauridae Therizinosauridae

Therizinosaurs (or Segnosaurs) were theropod dinosaurs and members of the clade Therizinosauroidea. 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 them quite comfortably, both as theropods and, as maniraptorans, close relatives to birds.

The name therizinosaur is derived from the Greek therizo meaning 'to reap' or 'to cut off' and sauros meaning 'lizard'. The older name segnosaur is derived from Latin segnis meaning 'slow' or 'sluggish' and Greek sauros meaning 'lizard'.

Description

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 prosauropod dinosaurs. Their unique hip bones, which pointed backwards and were partially fused together, initially reminded paleontologists of the "bird-hipped" ornithischians. Their claws and hands, however, were most like the carnivorous theropods. Because early finds were incomplete, this strange suite of anatomical features led some scientists, such as Gregory S. Paul, to conclude that segnosaurs (Therizinosaurus was not yet 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.^[1]

It was not until the mid-1990s, after Alxasaurus was discovered and shown to posess more typically theropod features, and Therizinosaurus was recognized as a member of the segnosaur group, that their true identity as herbivorous descendents of the carnivorous theropods became generally accepted.^[2] 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.^[3] 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.^[4] 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.

Among the most striking characteristics of therizinosaurs are the enormous claws on their hands, which reached lengths of three feet 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 the prehistoric ground sloths.^[5]

Skin impressions from Beipiaosaurus indicate that therizinosaurs were covered in primitive feathers, similar to those seen in the compsognathid Sinosauropteryx.^[3] Therizinosaurs spanned a large range of sizes, from the small Beipiaosaurus (which measured 2.2m, or 7.3 ft in length), to the gigantic Therizinosaurus, which at an approximate 10-12m (33-40 ft) long and an estimated weight of 6.2 tonnes, was among the largest known theropods.

Systematics

Taxonomy

Barsbold and Perle named the group Segnosauria as an infraorder of Theropoda in 1980.^[6] Dong Zhiming (1992) went further, placing the segnosaurs in their own suborder, 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.

• Superfamily Therizinosauroidea
• Beipiaosaurus
• Falcarius
• Nanshiungosaurus
• Suzhousaurus
• Family Alxasauridae
• Alxasaurus
• Family Therizinosauridae
• Enigmosaurus
• Erliansaurus
• Erlikosaurus
• Neimongosaurus
• Nothronychus
• Segnosaurus
• Therizinosaurus

Other possible therizinosaurs include Thecocoelurus, and the Early Jurassic Eshanosaurus, which would be the earliest known coelurosaur if it belongs to this group.

Phylogeny

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.^[7]

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 Maryanska and Barsbold (2004) and Sereno (2005), though other subsequent studies, such as Senter (2007) have continued to use Therizinosauroidea for the therizinosaur "total group".^[8][7][9]

The following cladogram follows an analysis by Phil Senter, 2007.^[9]

Therizinosauroidea	Erliansaurus Erlikosaurus Therizinosaurus

References

External links

• Therizinosauroidae - UCMP, Berkeley
• Therizinosauria
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