Therizinosaur

Skeletal reconstruction of Erliansaurus

• Asia
• North America

The Therizinosauroidea were a group of two to eight meter long theropod dinosaurs from the Cretaceous of North America and Asia. The most striking feature of this bipedal ( two-legged ) dinosaurs are provided with very long claws forelimbs.

The group was first described in 1980 by the Mongolian paleontologists bars Bold and Pearl as Segnosauria and because an accurate classification was not possible to distinguish between lizards Beck dinosaurs ( Saurischia ) and bird Beck dinosaurs ( Ornithischia ) classified. Later discoveries, among which are also indications of a simple fletching, allowed the assignment to the theropods. Fossils of Therizinosauroidea originate mainly from the Lower and Upper Cretaceous of Asia (China, Mongolia, Buryatia, Kazakhstan, Uzbekistan). The genus Nothronychus comes from the western United States (Utah, New Mexico). A single lower jaw fragment described as Eshanosaurus could extend the stratigraphic range of the group up in the Hettangian (earliest Jurassic ). The Therizinosauroidea be defined as the clade ( ancestry ) from Therizinosaurus and Beipiaosaurus, their last common ancestor and all its descendants.

• 3.1 Outer systematics
• 3.2 Internal systematics

Features

The skull of Therizinosauroidea is known only from Alxasaurus, Beipiaosaurus and Erlikosaurus. He was unusually small for a Theropodenschädel in comparison to the body. The skull bones were heavily pneumatized ( with air spaces interspersed ). The premaxillary bone ( premaxilla, the foremost bone of the upper jaw ) was toothless and ended in a sharp edge. In the living animal they may have been provided with a horned beak. The teeth of the upper jaw bone ( maxilla ) and the dental, the tooth- carrying portion of the mandible were cut, flattened side and, seen from the side, symmetrical. The teeth of the dentary became smaller from front to back. The tooth roots were cylindrical.

The external nares ( nostrils ) were very elongated and were, as in the Troodontidae, back and bounded below by the maxilla. The Antorbitalfenster, a skull window in front of the eye sockets, was in front, bounded above and below by a projecting edge. On the palate, the vomer ( vomer ) was greatly extended, the wing bone ( pterygoid ) snout upward reduced.

Fuselage skeleton and limbs

The spine is known only from a few species. The cervical vertebrae were elongated, tall and with excavations at the front ( pleurocoel ) provided. In some genera the cervical ribs were fused with the vertebral bodies. The vertebrae of the cervical spine, the vertebrae and the sacral vertebrae had low Neuralfortsätze. The vertebrae of the back spine had, if known, lateral indentations and may have been pneumatized. In Neimongosaurus, the only genus in the entire tail is preserved fossil, it is short and has 23 caudal vertebrae.

The forelimbs had three fingers ( Phalangenformel 2,3,4), the finger bone of the third were much narrower than the other two. The Fingerendglieder most forms were large, laterally flattened and bent. In Therzinosaurus they were greatly extended, the longest more than 60 inches long.

In very broad pelvis, the ilium ( Ilia ) were far apart and were with the transverse processes ( diapophyses ) of the sacrum fused. The pubis ( pubic ) was inclined backwards. The ischium ( ischium ) was shorter than the pubis and had by a transverse process of connecting to the pubic bone. This ophistopubisch called pelvic anatomy occurs otherwise only in birds and their closest relatives within the Coelurosaurier and was also the reason for the difficulties systematically classify the Therizinosauriden.

Basin of a Therizinosauriers; A = ilium, B = pubic bone, C = ischial

Idealized basin of a bird Beck dinosaur

The foot of the Therizinosaurier is always shorter and wider than that of other theropods. In contrast to all other Tetanurae they had four instead of three toes. Segnosaurus even had five-pointed metatarsals, the fifth but without phalanges.

In the well-preserved fossil of the basal representative Beipiaosaurus were in the area of ​​the front limbs and shoulders tight, up to 70 mm long filaments discovered, which were interpreted as proto- feathers.

Paleoecology

Fossils of Therizinosauriern were found only in deposits from non- arid areas and were absent in the Late Cretaceous in the Gobi as there prevailed a desert climate.

The strange anatomy and long claws on the front legs have led to speculation about their lifestyle. Rozhdestvensky suspected that they were intended to open termite mounds, or stripe of fruit trees. Bars Bold and Pearl believed that they were suitable for fishing. Nessow adopted an arboreal lifestyle and presented a Segnosaurus reconstruction, which hung like a sloth with his back down from a tree branch. The American paleontologist Dale Russell Alan pointed out the similarities to the Chalicotherien, extinct, related with the horses mammals. Today, most differing from all other theropods of life is assumed to be slow herbivores, like the Chalicotherien or the also extinct giant sloths. However, there is no traditional fossil stomach contents, supporting the thesis of a herbivorous diet.

System

Outer systematics

The Therizinosauroidea are usually regarded as the sister group of the Oviraptorosauria and united with them in a taxon Oviraptoriformes. Phil Senter she is after his phylogenetic analysis of Coelurosauria in a sister group relationship to a common taxon from Alvarezsauridae, Oviraptorosauria, Deinonychosauria and Avialae.

Inside systematics

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