[D] Segnosaurus galbinensis [Eg] [sG] [T]
Cretaceous Late Cenomanian Turonian
Saurischia Theropoda Tetanurae Coelurosauria Therizinosauroidea Therizinosauridae
Baynshirenskaya Svita, Amtgai, Baishin Tsav, Khar Hötöl; Urilge Khudag Mongolia, Iren Dabasu Formation, Inner Mongolia, China
4 - 8 meter
Genus - Typespecies - Eggs
A heavy-bodied biped as long as the largest modern crocodile, \\\\\\\"slow lizard\\\\\\\"is often thought of having a small, narrow skull witk a shallow lower jaw, a toothless beak, rather sharp yet prosauropod-like cheek teeth, and fleshy cheek pouches.
Segnosaurus had muscular arms, and its three-fingered hands probably, bore huge, curved claws. At first considered as aberant theropod but subsequently designated as Late Cretaceous relict of the \\\\\\\"prosauropod-ornithischian transition\\\\\\\"(Paul, 1984) or as the sister-group of sauropodomorphs (Barsbold & Maryanska, 1990)
Recently, therizinosaurids have been unambiguously shown to be theropods (Russel & Dong, 1993; Clarck et al. 1994) Clearly, the morphology of group is very peculiar.
3 specimens, including mandible, pelvis, hind limb, scapulocoracoid incomplete fore limb, fragmentary vertebrae. The front of the jaw is toothless, and the anterior teeth are somewhat curved, whereas the posterior ones are smaller and straight.
PST GIN AN MNR no. 100/90; lower jaw, remains of the humerus, forearm, separate manual phalanges and unguals, almost complete pelvis, incomplete right femur, remains of carpus, incomplete caudal vertebrae.
Occlusal edge of the lower jaw is slightly concave anteriorly. Dentary with large angular process. Splenial does not reach symphysis, and has elongate, wing-like, narrow anterior and angular processes. Anterior and posterior sections of lower jaw do not have a groove articulation. Weakly expressed coronoid process. Narrow symphysis. 24-25 teeth in each mandibular ramus. Anterior teeth are slightly recurved, while posterior are smaller and slightly straight. Edentulous area in anterir lower jaw corresponds to the width of approximately four presymphyseal teeth.
Caudal vertebrae with disproportionately massive centra and low neural arches. Six fused sacral vertebrae with widened centra and highly elongated transverse processes. Unequal, relatively narrow processes reach the dorsal edge of the [ilium] and fuse [to the] top. Proximal caudal vertebrae platycoelous, with large centra, long neural arches, and massive zygapophyses. Prezygapophyses turned dorsomedially and widely separated, postzygapophyses directed somewhat ventrally and externally. Posterior caudal vertebrae have short, massive centra.
Coracoids very wide with [rectangular] outline. Attachment point for biceps muscle clearly indicated by a rise. Small coracoid foramen. Straight, proximally flattened scapula. Acromial process sharply expressed and elongated along the entire contact with the coracoid. Dorsal and distal edges of scapula subparallel. Lower distal end of scapula forms a large part of glenoid cavity, directed posterolaterally. Forelimbs somewhat shortened. Massive humerus with widened epiphyses.
Deltopectoral crest displaced proximally, and moderately developed. Ulna considerably more massive and slightly longer than radius. Olecranon process highly developed. Manus supposed to be tridactyl, with highly curved, laterally compressed ungual phalanges. The ilia are widely positioned, with a tall, laterally curved preacetabular process, and a highly curved exterior.
Postacetabular process is highly reduced, with the upper part occupied by a sharply protruding, large process that is tube-like in outline, the upper surface of which corresponds to the dorsal edge of the blade. The pubis is directed posteroventrally, with an elongated, flattened shaft that bears a medial crest on the posterior symphyseal edge.
The distal end has an ellipsoidal shape and a protruding anterior portion, is laterally compressed, and has a narrow dorsal symphyseal edge. Ischium parallel to and much shorter than the pubis, with a widened distal end. Obturator process subrectangular in outline, and connected to the posterior medial crest of the pubis above its distal end. A round, low, longitudinal crest-like projection is located on the posterior edge of the ischium, across from the obturator process.
Pubic peduncle of ischium shortened. Proximal pubic opening large, with open edges, and formed to a large extent by the ilium and pubis. Femur with weak sigmoidal curvature. It has a massive head and elongated neck. Fourth trochanter found somewhat above midshaft. Tibia slightly shorter than femur. Astragalus with relatively short ascending process that is wide at its base.
Calcaneum massive, relatively large, and possibly did not fuse with the astragalus. Pes shortened, metatarsus massive and has wide ends, with the proximal ends forming a very compact structure. Metatarsal I is the most shortened, participating in the formation of the laterally widened proximal articular surface of the pes. Metatarsal V rudimentary.
Digit I the most shortened, II and III nearly subequal in size, and IV the [thinnest]. The phalanges of the three main digits are very similar in construction, relatively shortened, and massive. In digit IV, phalanges 2 and 3 are highly shortened, while the ungual phalanges are curved and slightly laterally compressed.
In many basic features, Segnosaurus is distinct from other carnivorous dinosaurs. The nature of the construction of the lower jaw and teeth, the disproportionately massive and elongated cervical vertebrae, the unusual construction of the pelvis as expressed by altilia and the retroverted pubes, and the shortened and noncompact structure of the tarsus and entire pes all define the great peculiarity of segnosaurids, differentiating them from all other theropods.
Besides the holotype, fragments of postcranial skeletons (including a pelvis) from two specimens (PST GIN AN MNR no. 100/87, 100/88).
Source: Polyglot Paleontologist