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  This group of dinosaurs evolved from swift predators into lazy herbivores.
The revision will show the above image of 
Therizinosaurus cheloniformis.

    Therizinosaurs are the strangest dinosaurs ever discovered. These are incredibly weird animals that baffled paleontologists for many years. The first record of their existence was the 1948 discovery of giant 3-foot long claws that certainly did not below to any known species. Maleev thought they may have come from an otherwise unknown giant chelonian (turtle).  In 1954, he named it Therizinosaurus cheloniformis
   Around ten years later another giant Therizinosaur claw was found, but this time along with other fossils, including a tooth, parts of the front and rear limbs and even a four-toed foot. The forelimbs had all the characteristics of the fierce meat-eating theropods, such as tyrannosaurs and velociraptors, but they were much longer than any known species. The pelvic bones were thought to belong
late survivor descendents of early Mesozoic prosauropods.  Displaying a mix of characters from across Dinosauria. Therizinosaursus skeletons look as if they were assembled by a drunken designer who got quite confused as to what belonged where.
   More Therizinosaurus fossils were found in the late 1970s.  There was enough difference between them and the earlier fossils to determine that they were from a different, but very closely related, species. The partial specimen was named Erlikosaurus. The limbs established that it was a theropod.  More importantly, it yielded the full full skull of any therizinosaur.  It was the massive skull with powerful jaws and sharp, blade-like teeth found on other theropods.  Erlikosaurus had a small skull, but the big surprise was that it had small peg-like teeth and the anterior part of the muzzle was modified into a toothless, bill-like structure similar to a bird's beak.
   The 1994 discovery of Alxasaurus helped to solve many of the mysteries because it was the most complete Therizinosaurus found to date.  Taken together, the three species provided enough material to reconstruct this strange group of animals.  Except for the basal form Beipiaosaurus inexpectus, they were large, heavily built creatures that were probably quite slow.  Their pelvis was perfectly shaped for sitting, the same way a ground sloth sits when feeding from trees.  Another hint as to a sitting way of life, lies in the dorsal vertebrae, as they are tall, and built for keeping a big animal vertically - most probable sitting.  The bird-like beak and lack of teeth were unsuitable for a meat-eater, which meant that Therizinosaurus was a herbivore.  They had long arms with enormous claws on their manus (hands); some were up to 3 feet long. These provided an excellent tool to strip vegetation from trees and bring it into the mouth.  They would have also provided defense against predators, as running away was not an option.  It has been suggested that they used these giant claws to strip leaves from trees. If Therizinosaurs were plant-eaters, then they are the only such critters classified with the fierce, meat-eating theropods.
   The four specimens shown below cover the evolution of these strange animals over 30 million years.  As they grew in size, they become slow and lazy, evolving from predators into herbivores.  They are the only herbaceous theropods.
   The proverbial icing on the cake was the 1999 discovery of Beipiaosaurus, as it was covered with feathers, the longest feathers found on any dinosaur to date.  By extension, it is believed that all of its closely-related cousins had them too.  What emerges is a really strange dinosaur - a herbivore theropod that looked somewhat like a giant goose and probably ate and behaved somewhat like the now-extinct ground sloth.
Canadian Goose - Branta canadensis Giant Extinct Sloth - Megatherium

   T herizinosauridae 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 probable theropods and, as Maniraptora, close relatives to birds. These strange creatures were are bipedal, and have hands bearing three fingers each.  Their bodies have radically changed to accommodate their vegetarian lifestyle. The hips of a therezinosaur are much wider then is normal for a maniraptor, and the pubis is swept backward in the manner of a bird or ornithischian dinosaur, to make room the large belly needed to digest plant matter. Because of its tremendous gut, the center of balance of a therezinosaur is shifted backward along its spine, and these creatures squat like sumo wrestlers, with their torsos held further up then is normal for a maniraptor. To make matters even more confusing, the tiny, non-functional hallux claw possessed by most maniraptors has, in the therizinosaurs, been greatly enlarged to make a decidedly un-maniraptorian four-toed foot. Misfits even in the varied and bizarre clade Maniraptora, and in the face of stiff competition from the hadrosaurs, ceratopsians, and other ornithischian herbivores, the therizinosaurs have nonetheless flourished in the past 130 million years, and have spread themselves across Eurasia and the Americas. Eggs attributed to therizinosaurs are found from Cretaceous sediments of Mongolia.
Therizinosaur eggs sometimes enclose embryos. Two main types of embryo have been found.
    The history of Therizinosauria begins in the early Cretaceous, when small browsers like Beipiaosaurus lived in the lush forests of Asia. By the end of the Mesozoic, the therizinosaurs had expanded far into their herbivorous life-style, evolving into huge, lumbering forms like Segnosaurus and Therizinosaurus, itself.  

Falcarius utahensis - 125 MYA
Found in Utah, USA
This newly discovered species appears to be the missing link between early carnivores and herbaceous Therizinosaurs.  First of its kind found outside of Asia.

Beipiaosaurus inexpectatus - 130 MYA
Found in Liaoning, China.  7 ft long.  Carnivore.
   Described in 1999, its fossils include the largest feathers found on any dinosaur.
Alxasaurus elesitaiensis - 112 MYA
Found in Mongolia, China. 13 feet long. Carnivore.
  Most complete fossil specimen found in Asia.
Erlikosaurus andrewsi - 95 MYA
Found in Mongolia, China.  20 feet long.  Herbivore?
  The only therizinosaur with a fully known skull (at least prior to the recent discovery of Falcarius utahensis. See: Discoveries
Therizinosaurus cheloniformis - 77-71 MYA
Mongolia, China.  26 feet long. Herbivore.
Its enormous claws were discovered in 1948.  They were so unique that it was obviously a new species.  It was named in 1954.  A few years later more fossils were found that provided a rough pictures of its appearance, but additional finds in the 1980's helped to fill in the details

Other described species

  Chilantaisaurs zheziangensis (Dong, 1979)
Time: Late Cretaceous.  Place: Mongolia. Size: unknown
Remains: Claws  This species has been classified under "Segnosaurus" zheziangensis and might really be any Mongolian therizinosaur that fits it's size.
  Enigmosaurus mongoliensis (Barsbold & Perle, 1983)
Time: Cenomanian - Late Cretaceous, 93-89 MYA
Place: Mongolia
Length: 6 meters.  Fossil consists of partial hip.  It may be the pelvis of Erlikosaurus.  The Pubic boot is shaped much sharper than the one of Segnosaurus, and so is the ischium, which is totally fused to the pubis, making it one single pelvic bone.
  Eshanosaurus deguchiianus (Xu, Zhao & Clark, 2001)
Time: Early Jurassic, 200 MYA.  Place: China
Size: ca 1m. (if therizinosaur) or 1,5-2m (if prosauropod)
Remains: Mandible.  Thought to be a Therizinosaur due to the mandible shape, but the time it lived, points to that it may really have been a Prosauropod of some kind. There's still debate going on about this.
  Segnosaurus galbinensis (Perle, 1979).
Segnosaurus (meaning "slow lizard") was a meat-eating dinosaur that lived during the late Cretaceous period, about 97.5 to 88.3 million years ago. Segnosaurus walked on two long legs, a long flexible neck, long head, three-toed feet, a short body, a long tail, long, strong legs, and clawed fingers and toes. Segnosaurus was perhaps 13 to 30 feet (4-9 m) long; its size is uncertain. Unlike other therizionsaurs, It appears to have been a carnivore that ate small animals, like insects. Three partial skeletons have been found in Mongolia.
  Nanshiungosaurus brevispinus (Dong, 1979)
Time: Campanian, Senonian, Late Cretaceous, 71 MYA
Place: China.  Size: 4m. Remains: Complete cervical/dorsal vertebral column, incomplete pelvis. This is the only therizinosaur with cervical ribs shrinked to small bumps and fused to the cervical vertebrae. The neck was robust, and so was the dorsal column.  The pubis is only known by a little bony knob, but was probably similar to the one of Segnosaurus, while the ischium was much broader, and spoonshaped. The ilium is much taller as well. The downward turning of the preacetabular process in this
species goes to the extreme, but they are about the same as in Segnosaurus when seen from above.
  Neimongosaurus (Zhang, Xu, Sereno, Kuang & Tang, 2001)
Time: Late Cretaceous
Place: China
Size: ca 2m.
Remains: (to be updated)
Notes: This species is a bit difficult to classify, as it's got more
cervical vertebrae than both Alxasaurus and Nanshoungosaurus.
At the same time, it's pelvis is fairly fragmental, and cannot be used to classify much. It is almost entirely the vertebral structures, the length of the tail, the mandibular fragment and the general structure of the animal that helps put a diagnosis on this species.
  Nothronychus mckinleyi (Kirkland & Wolfe vide Stanley, 2001)
pronounced "no-thron-EYE-kus"
Time: Late Cretaceous, 89 MYA
Place: New Mexico, Utah, USA.  Size: 4,5-6m.
Discovered in 2001, 
It was the first therizinosaurs found in the Americas. The others all came from China and Mongolia. Nothronychus apparently evolved into a plant-eater, said Jim Kirkland, a paleontologist with the Utah Geological Survey.  No fossil evidence was found of feathers, but Wolfe noted that similar dinosaurs from Asia were found with feathers and speculated that this one had "a loose gaggle of feathers around the head and along the spine, back of the arms and legs."
  Segnosaurus galbinensis (Perle, 1979)
Time: Late Cretaceous, 93-89 MYA. Place: Mongolia
Size: 4-9m.  Remains: Mandible, pelvis girdle, spine fragments, metatarsals, phalanges, partial scapula, coracoid, humerus, tibia,
fibula, complete pelvis.   Notes: The few remains of this species indicate low, broad pes and a wide pelvis, the same features as Alxasaurus, only more prominent. The pubis is clearly backwards pointing, and the ischium is in direct contact with the pubis.
The lower jaw has a strong downwards bend in it, and got tall peg-like teeth. 


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