|Diagram showing known remains|
History of discovery
In 1973, a joint Soviet-Mongolian expedition investigating the Amtgay locality northeast of Khan-Bogdsomona in the southeastern Gobi Desert Mongolia, discovered the partial skeleton of an unknown dinosaur, among other fossils. Through 1974 and 1975 more remains of the same dinosaur were uncovered at the Amtgay and Khara-Khutul localities; though the skeletons were incomplete, the recovered bones were well-preserved. These fossils were scientifically described in 1979 by the Mongolian paleontologist Altangerel Perle, who named the new genus and species Segnosaurus galbiensis based on them. The generic name is derived from the Latin word segnis, meaning "slow", and the Ancient Greek sauros, meaning "lizard". The specific name refers to the Galbin region of the Gobi Desert.
The holotype specimen (from the Amtgay locality) is housed at the Mongolian Academy of Sciences under the specimen number IGM 100/80 (formerly GIN), and includes the mandible (lower jaws), an incomplete humerus, a complete radius and ulna (lower arm bones), phalanges of the fingers, a forelimb ungual (claw bone), an almost complete pelvis, an incomplete right femur, six sacral vertebrae, ten caudal vertebrae from the front of the tail, fifteen from the hindmost part of the tail, the first abdominal rib, and fragments of the dorsal ribs. Two additional specimens were designated as paratypes: specimen IGM 100/82 (from the Khara Khutul locality) includes a femur, tibia, fibula (leg bones), tarsals and metatarsals, five toe phalanges including a foot ungual, rib fragments, complete ilia, the upper portion of an ischium, and the lower portion of a pubis. Specimen IGM 100/83 includes a left scapulocoracoid, a radius, an ulna, forelimb unguals, and a fragment of a cervical vertebra. In 1980, the Mongolian paleontologist Rinchen Barsbold and Perle assigned an additional specimen to Segnosaurus, IGM 100/81 (from the Amtgay locality), which included a left tibia and fibula.
In 1983, Barsbold listed additional specimens GIN 100/87 and 100/88, but in 2010, the American paleontologist Lindsay E. Zanno suggested these may refer to paratypes IGM 100/82 and IGM 100/83, as the article has several typographical errors in regard to specimen numbers. Zanno also noted that by the time of her study, there were numerous issues with the Segnosaurus IGM specimens, including damage caused since collection, elements of the holotype having gone missing, incorrect identification of assigned elements, as well as more than one individual bearing the same specimen number. Holotype elements Zanno was able to access included a severely damaged ilium, a sacrum missing the left sacral ribs (with damage so that it could not conjoin well with the rest of the ilium), a pubis missing the upper portion, and an ischium missing the upper portion. Additional bones bearing the specimen number IGM 100/82 were located (though they were not mentioned in Perle's description), while the whereabouts of various paratype elements was unknown. In a 2016 re-description of the holotype mandible (the first detailed study of this element since 1979) Zanno and colleagues reported that the majority of the tooth crowns had been damaged after collection, most of them missing their apices (tips). Of the two hemimandibles (halves of the lower jaw), the right is nearly complete, missing only the hindmost part and the upper front of its mandibular symphysis (the area where the halves of the mandible meet). The left hemimandible is fragmented and preserves the front part, with some displacement of bone due to crushing.
Segnosaurus is estimated to have been about 6 m (20 ft) long, and to have weighed about 1.3 t (1.4 short tons). As a therizinosaurid, it would have been robustly built, with the trunk of the body tilted upwards compared to other theropods. The head was small, and the neck long and slender. Their fingers were not particularly long, but bore large claws. The front part of their pelvis was enlarged and flared sideways to support a large belly. Therizinosaurs are known to have had simple, primitive feathers, as evidenced by fossils of Beipiaosaurus (the second known non-bird dinosaur preserved with such integuments, after Sinosauropteryx).
The mandible of Segnosaurus was relatively robust and shapeless in general, compared to that of Erlikosaurus, which was more gracile and superficially detailed. The nearly complete right hemimandible is 379 mm (14.9 in) long from front to back, 55.5 mm (2.19 in) at the highest point, and 24.5 mm (0.96 in) at the lowest. The dentary bone (the tooth-bearing bone forming most of the mandible's front part) was complex in shape compared to early therizinosaurians. The tooth-bearing part was almost triangular and downwards sloping in side view, with a pronounced arc at the upper front throughout its length, more extreme than what is known in other therizinosaurians. When each hemimandible is articulated with each other, they form a broadly U-shaped, toothless mandibular symphysis, which projects upwards towards the front as in Erlikosaurus and Neimongosaurus. The expansive, toothless front region of the dentary spans 25.5 mm (1.00 in) on the right hemimandible of the holotype. Proportionally, the toothless part of the dentary is 20% of its tooth row, which is 150.3 mm (5.92 in) long. By comparison, the toothless region of Erlikosaurus is more reduced, about 12% of the tooth row's length, while it is almost absent in Jianchangosaurus.
The height of the dentary diminished towards the hindmost extend of the tooth row, where after it sharply fanned out to contact the surangular bone; by contrast, the hind part of the dentary in Erlikosaurus gradually approached the surungular by a gentle arc. The teeth were restricted to the front two thirds of the dentary in Segnosaurus, which bore 24 alveoli (tooth sockets), similar to Jianchangosaurus, but different from Erlikosaurus, where nearly the entire dentary was toothed, bearing 31 alveoli.
The in total forty-eight mandibular teeth are markedly peg-like and only slightly recurved: the front or mesial edge is curved and the back or distal edge is straight. Also the latero-dorsal shelf on the dentary, a flat bone surface at the upper outside of the lower jaw, starts at the fourteenth dentary tooth position and runs backwards for half the length of the lower jaw, unlike the shelf in Erlikosaurus, which starts at the fifth tooth position. This would have indicated that Segnosaurus did not have as extensive 'cheeks' as Erlikosaurus is believed to have had.
Segnosaurus can be distinguished from all other therizinosaurs on the basis of two unique derived traits (autapomorphies). The second autapomorphy is that the claws of the hand are rather flat instead of very narrow. In the same formation the closely related Erlikosaurus is found; Segnosaurus can be distinguished from this species by its moderate transverse compression of the pedal unguals or foot claws.
Perle noted in 1979 that the Segnosaurus fossils were possibly representative of a new family of dinosaurs, which he named Segnosauridae (with Segnosaurus as its type genus), and tentatively classified as theropods. The next year, in a description of the new taxon Erlikosaurus, Barsbold & Perle named the new order for the family, Segnosauria. At the time, the only two known genera were Segnosaurus and Erlikosaurus. Later discoveries made it clear that Therizinosaurus was a close relative. The group was renamed the Therizinosauridae, that name having priority, and these forms were determined to be plant-eaters, instead of fish-eaters as had originally been presumed by Perle.
The following cladogram is based on the phylogenetic analysis by Phil Senter et al., 2012.
Diet and feeding
Therizinosaurs are generally accepted to be omnivorous or herbivorous, showing herbivorous features like elongated necks, reduced skull size, broadening of the pelvis cavity, reduced bite force, reduced skull musculature, anterior tooth loss, development of a rhamphotheca, and a downturned anterior portion of the dentary, all of which are present, or presumed to be present in Segnosaurus. However, there has been no complexity in tooth morphology, which is found in many other herbivorous groups. A few of the dental modifications include increased symmetry in Erlikosaurus, enlargement of denticles in Segnosaurus, with the only previously known unique tooth morphologies being the cusped, incisiform anterior teeth of Falcarius and the outwards convex teeth of Jianchangosaurus.
However, in a 2016 study on the holotype dentaries of Segnosaurus, an additional tooth morphology was identified. One of these modifications is the presence of an additional row of denticles on the lateral side of posterior teeth, the other being a denticulated facet on the distal tooth crown, which shows that Segnosaurus had unique dental features within Theropoda, and a higher degree of oral food processing compared to other known therizinosaurids where teeth are known. This supports the idea that therizinosaurs were dietary-partitioned, which is also based on the very high number of therizinosaurs present in the same formations, and the highly varying bodymass (500% difference in mass in the Bayan shireh formation).
The four specimens were recovered from the Bayan Shireh Formation of the Mongolian People's Republic, in sediments dated between the Cenomanian to Turonian stages (Late Cretaceous Period, about 93 million years old).
- Perle, A. (1979). "Segnosauridae - novoe semeistvo teropod is posdnego mela Mongolii" [Segnosauridae - a new family of Theropoda from the Lower Cretaceous of Mongolia] (PDF). Trudy - Sovmestnaya Sovetsko-Mongol'skaya Paleontologicheskaya Ekspeditsiya (in Russian). 8: 45–55.
- Glut, D. F. (1997). Dinosaurs: The Encyclopedia. Jefferson: McFarland & Co. pp. 806–807. ISBN 978-0-89950-917-4.
- Zanno, L. E. (2010). "A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora)". Journal of Systematic Palaeontology. 8 (4): 503–543. doi:10.1080/14772019.2010.488045.
- Barsbold, R.; Perle, A. (1980). "Segnosauria, a new infraorder of carnivorous dinosaurs". Acta Palaeontologica Polonica. 25 (2): 187–195.
- Barsbold, R. (1983). "Carnivorous dinosaurs from the Cretaceous of Mongolia" (PDF). Transactions of the Joint Soviet-Mongolian Paleontological Expedition. 19: 5–119.
- Zanno, L. E.; Tsogtbaatar, K.; Chinzorig, T.; Gates, T. A. (2016). "Specializations of the mandibular anatomy and dentition of Segnosaurus galbinensis (Theropoda: Therizinosauria)". PeerJ. 4: e1885. doi:10.7717/peerj.1885. PMC 4824891. PMID 27069815.
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- Senter, P.; Kirkland, J. I.; Deblieux, D. D. (2012). Dodson, Peter (ed.). "Martharaptor greenriverensis, a New Theropod Dinosaur from the Lower Cretaceous of Utah". PLoS ONE. 7 (8): e43911. doi:10.1371/journal.pone.0043911. PMC 3430620. PMID 22952806.