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Temporal range: Late Cretaceous, 102–83 Ma
Alectrosaurm olseni.jpg
Right pes of A. olseni, specimen AMNH 6554, American Museum of Natural History
Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Clade: Eutyrannosauria
Genus: Alectrosaurus
Gilmore, 1933
A. olseni
Binomial name
Alectrosaurus olseni
Gilmore, 1933

Alectrosaurus (/əˌlɛktrˈsɔːrəs/; meaning "alone lizard") is a genus of tyrannosauroid theropod dinosaur that lived in Asia during the Late Cretaceous period. It was a medium-sized, moderately-built, ground-dwelling, bipedal carnivore, with a body shape similar to its much larger advanced relative, Tyrannosaurus.[1]

Discovery and naming

Excavating the right hindlimb of A. olseni, specimen AMNH 6554, 1923. George Olsen on the right

In 1923, the Third Asiatic Expedition of the American Museum of Natural History, led by chief palaeontologist Walter W. Granger was hunting for dinosaur fossils in Mongolia. On April 25, assistant palaeontologist George Olsen excavated and recovered the holotype AMNH 6554, a nearly complete right hindlimb. This included a virtually complete right hindlimb with some elements from the left pes and two manual unguals. On May 4, Olsen discovered another specimen approximately 30 m (98.4 ft) away from his first find, catalogued as AMNH 6368. This specimen included a right humerus, two incomplete manual digits, four fragmentary caudal vertebrae, and other two or three unspecified elements that were discarded due to badly preservation.[1][2] These discoveries were made at the Iren Dabasu Formation in what is now the Inner Mongolia Autonomous Region (Nei Mongol Zizhiqu) of China.[1]

The generic name, Alectrosaurus, can be translated as "alone lizard" or "mateless lizard", derived from the Greek words ἄλεκτρος (meaning alone or unmarried) and σαῦρος (meaning lizard). The specific name, olseni, is in honor of George Olsen, who discovered the first specimens. Both genus and species were described and named by American palaeontologist Charles Gilmore in 1933.[1]

Skeleton reconstruction of AMNH 6554

Since then, more material has been referred to Alectrosaurus: the specimen IGM 100/50 and IGM 100/51. Consisting of a partial maxilla, scapulocoracoid and manual ungual (50), and a fragmentary skull, lower jaws, incomplete ilium, and metatarsals of the right foot (51). These fossils were found in the Bayan Shireh Formation of Outer Mongolia.[3] Estimates suggest it was deposited from Cenomanian through Santonian stages, between 102 million to 86 million years ago.[4] Iren Dabasu and Bayan Shireh dinosaur faunas are similar, but Van Itterbeeck et al. 2005 claimed that the Iren Dabasu is probably Campanian-Maastrichtian in age and possibly correlated with the Nemegt Formation, so it is not surprising that a species of Alectrosaurus would be found there.[5] Furthermore, several partial skeletons found in both Inner and Outer Mongolia might belong to Alectrosaurus.[6] However, Alexander Averianov and Hans-Dieter Sues 2012 have estimated that the Iren Dabasu Formation is Santonian in age, correlating the Upper Bayan Shireh Formation.[7]

Near the holotype, the specimen AMNH 6556 was found in the same strata but at different points. Consisting of a premaxillary and lateral teeth, incomplete left lacrimal, maxillary process of the left jugal, partial right quadratojugal, jugal process of the right ectopterygoid and the quadrate ramus to the right pterygoid. Although the specimen seems to represent a smaller individual.[8]


Life restoration, based on the holotype specimen

The lectotype AMNH 6554 is fragmentary, consisting of a nearly complete right hindlimb only lacking the distal tarsal elements; left metatarsals II, III and IV, and a fragmentary distal foot of a pubis, however it is unknown which pubis represents.[1][2]

It was a medium-sized tyrannosauroid, reaching probably a length between 5 to 6 m (16 to 20 ft), and a weight ranging from 454 to 907 kg (1,000.9 to 1,999.6 lb).[9][10] The length of its tibia (lower leg bone) and femur (thighbone) are very close, in contrast to the majority of other tyrannosauroids, where the tibia is longer. The metetarsals are also closer in size to the tibia than in most other tyrannosauroids, where they are usually longer.[1]

Alectrosaurus was originally characterized as a long-armed theropod, but Perle 1977, Mader and Bradley 1989 observed that the forelimbs of the specimen AMNH 6368 did not belong to the genus, as they do not share characteristics with Tyrannosauroidea, and assigned them to the Therizinosauria.[3][2] The remaining material, AMNH 6554 represents the hind limb with characteristics of a true tyrannosauroid, and were assigned as the lectotype for Alectrosaurus olseni.[2] Additionally, four small caudal vertebrae were associated with the specimen AMNH 6368, the vertebrae were not included in the original description. Nevertheless, in 1984 they were catalogued as AMNH 21784. Mader and Bradley described these vertebrae, and were provisionally identified as caudal vertebrae of a small theropod dinosaur that is not referable to either the Tyrannosauroidea or Therizinosauridae as they show resemblance to the caudal vertebrae of Deinonychus and Plateosaurus.[2]

Distinguishing anatomical features

Following the original description of Alectrosaurus, it can be distinguished by the following traits: long slender-limbed type of tyrannosauroid; humerus long and slender; ungual and phalanx of digit I robust, laterally compressed and strongly curved; femur and tibia subequal in length; length of astragalus onefourth the combined length of astragalus and tibia.[1]

According to Carr 2005, Alectrosaurus can be distinguished based on unique traits present in the hindlimbs, such as the spike-like process extending from the caudodorsal surface of the medial condyle of the femur, the presence of an abrupt expansion in length of the anterior margin of the joint surface for the tibia on the fibula, tendon pit adjacent to the ventrolateral buttress of the astragalus undercutting the medial surface of the buttress, the dorsal margin of the proximal surface of pedal phalanx II-2 is pointed, reduced pedal digit III, The lateral condyle of pedal phalanx III-1 is significantly deeper than the medial condyle, when in distal view, stocky pedal phalanx IV-2, when examined in proximal view, the dorsal half of the joint surface for metatarsal IV on metatarsal III is dilated anteriorly, and many others.[11]


In 1933, Charles Gilmore examined the available material and concluded that AMNH 6554 and AMNH 6368 were syntypes belonging to the same genus. He based this on his observation that the manual unguals from both specimens were morphologically similar. Observing similarities with the hindlimbs of specimen AMNH 5664 Gorgosaurus sternbergi, he classified this new genus as a "Deinodont", a term that is now considered equivalent to tyrannosaurid.[1] Due to its fragmentary nature, there is presently very little confidence in restoring its relationships with other tyrannosauroids and many recent cladistic analyses have omitted it altogether. One study recovered Alectrosaurus at no less than eight equally parsimonious positions in a tyrannosauroid cladogram.[12] Some paleontologists have considered Alectrosaurus olseni to be a species of Albertosaurus.[13]

The Bayan Shireh material may or may not belong to this genus, and needs further study. One cladistic analysis showed that the two sets of specimens group together exclusive of any other taxa, so they are probably at least closely related, if not the same species.[14]

In 2018, Rafael Delcourt and Orlando Nelson Grillo published an extensive phylogenetic analysis of the Tyrannosauroidea, classifying Gondwanan and Laurasian tyrannosaurs. Below is the cladogram showing the phylogenetic position of Alectrosaurus according to their analysis:[15]


Guanlong wucaii

Proceratosaurus bradleyi

Kileskus aristotocus

Sinotyrannus kazuoensis

Yutyrannus huali


Aviatyrannis jurassica

Dilong paradoxus

Santanaraptor placidus

Timimus hermani

Stokesosaurus clevelandi

Juratyrant langhami

Eotyrannus lengi

Xiongguanlong baimoensis

NMV P186046

Alectrosaurus olseni

Timurlengia euotica


Dryptosaurus aquilunguis

Appalachiosaurus montgomeriensis

Bistahieversor sealeyi


Gorgosaurus libratus

Albertosaurus sarcophagus


Qianzhousaurus sinensis

Alioramus remotus

Alioramus altai

Nanuqsaurus hoglundi

Teratophoneus curriei

Lythronax argestes

Daspletosaurus torosus

Daspletosaurus horneri

Zhuchengtyrannus magnus

Tarbosaurus bataar

Tyrannosaurus rex


Life restoration of the head

The hindlimb of the holotype AMNH 6554 is notable for the particular elongated digits and metatarsals, differing from other tyrannosauroids. These traits are found in terrestrial runner birds, suggesting that Alectrosaurus was suited as a fast-running tyrannosauroid dinosaur with well developed hindlimbs, probably a pursuit predator[8] In a 2001 study conducted by Bruce Rothschild and colleagues, 23 foot bones referred to Alectrosaurus were examined for signs of stress fractures, but none were found.[16]


Alectrosaurus was first recovered from the Iren Dabasu Formation.[1] It lived between 95 million and 80 million years ago.[9] During the Late Cretaceous, there was a large floodplain with braided fluvial environments in the formation. The floodplain environments had extensive vegetation, evidenced in the palaeosol development and the numerous herbivorous dinosaurs that were found in both the river channel and the floodplain sediments.[5] Contemporaneous paleofauna from this formation included another Theropods: Archaeornithomimus, Avimimus, Caenagnathasia, Erliansaurus, Gigantoraptor, Mononykus, Neimongosaurus, Saurornithoides and Velociraptor; the sauropod Sonidosaurus and the two hadrosaurids: Bactrosaurus and Gilmoreosaurus.[17][18]

Additional to this, a potential discovery was made on the Bayan Shireh Formation at the Bayshi Tsav locality.[3] The Bayan Shireh Formation is estimated to be Late Cretaceous in age, during the Cenomanian and Santonian stages.[19][4] Here, Alectrosaurus lived alongside other dinosaurs; diverse theropods such as Achillobator[20], Garudimimus[21] or Segnosaurus.[22] Ankylosaurs were represented by Talarurus and Tsagantegia[23], other herbivorous dinosaurs include Graciliceratops[24], Gobihadros[25] and Erketu.[26]

Alexander Averianov and Hans-Dieter Sues in 2012 estimated that the Upper Bayan Shireh correlated the Iren Dabasu Formation and both had similar environments. This correlation seems to be supported by a large number of ostracods reported in these formations.[7] Further support can be evidenced on the similarities between the dinosaur taxa: as previously mentioned, Alectrosaurus appears to occur in both Iren Dabasu and Bayan Shireh Formations, although this referral to the genus has not been fully confirmed, the tentative specimens show striking resemblance to the Iren Dabasu material. In 2015, a giant caenagnathid was reported from the Bayan Shireh Formation, preserving a partial lower rostrum (beak) that is extremely similar to that of Gigantotaptor. If confirmed by future studies, this represents the first documented occurrence of a taxon in both formations.[3][27]

See also


  1. ^ a b c d e f g h i Gilmore, C. W. (1933). "On the dinosaurian fauna of the Iren Dabasu Formation". Bulletin of the American Museum of Natural History. 67 (2): 23–78.
  2. ^ a b c d e Mader, B. J.; Bradley, R. L. (1989). "A redescription and revised diagnosis of the syntypes of the Mongolian tyrannosaur Alectrosaurus olseni". Journal of Vertebrate Paleontology. 9 (1): 41–55. doi:10.1080/02724634.1989.10011737.
  3. ^ a b c d Perle, A. (1977). "O pervoy nakhodke Alektrozavra (Tyrannosauridae, Theropoda) iz pozdnego Mela Mongolii" [On the first discovery of Alectrosaurus (Tyrannosauridae, Theropoda) in the Late Cretaceous of Mongolia]. Shinzhlekh Ukhaany Akademi Geologiin Khureelen (in Russian). 3 (3): 104–113.
  4. ^ a b Kurumada, Y.; Aoki, S.; Aoki, K.; Kato, D.; Saneyoshi, M.; Tsogtbaatar, K.; Windley, B. F.; Ishigaki, S. (2020). "Calcite U–Pb age of the Cretaceous vertebrate‐bearing Bayn Shire Formation in the Eastern Gobi Desert of Mongolia: usefulness of caliche for age determination". Terra Nova. doi:10.1111/ter.12456.
  5. ^ a b Van Itterbeeck, J.; Horne, D. J.; Bultynck, P.; Vandenberghe, N. (2005). "Stratigraphy and palaeoenvironment of the dinosaur-bearing Upper Cretaceous Iren Dabasu Formation, Inner Mongolia, People's Republic of China". Cretaceous Research. 26 (4): 699–725. doi:10.1016/j.cretres.2005.03.004.
  6. ^ Benton, M. J.; Shishkin, M. A.; Unwin, D. M.; Kurochkin, E. N. (2003). The Age of Dinosaurs in Russia and Mongolia. Cambridge University Press. pp. 434–455. ISBN 0-521-54582-X.
  7. ^ a b Averianov, A.; Sues, H. (2012). "Correlation of Late Cretaceous continental vertebrate assemblages in Middle and Central Asia" (PDF). Journal of Stratigraphy. 36 (2): 462–485.
  8. ^ a b Carr, T. D.; Williamson, T. E. (2005). "A reappraisal of tyrannosauroids from Iren Dabasu, Inner Mongolia, People's Republic of China". Journal of Vertebrate Paleontology. 25 (3).
  9. ^ a b Holtz, T. R.; Rey, L. V. (2007). Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. Random House. Genus List for Holtz 2012 Weight Information
  10. ^ Brett-Surman, M. K.; Holtz, T. R.; Farlow, J. O. (2012). The Complete Dinosaur. Indiana University Press. p. 360. ISBN 0-2533-5701-2.
  11. ^ Carr, T. D. (2005). Phylogeny of Tyrannosauroidea (Dinosauria: Coelurosauria) with Special Reference to North American Forms. University of Toronto. p. 1170. ISBN 0-494-02932-3. OCLC 75087116.
  12. ^ Weishampel, D. B.; Dodson, P.; Osmolska, H. (2007). The Dinosauria, Second Edition. University of California Press. pp. 111–136. ISBN 978-0-520-24209-8.
  13. ^ Dodson, P.; Britt, B.; Carpenter, K.; Forster, C. A.; Gillete, D. D.; Norell, M. A.; Olshevsky, G.; Parrish, J. M.; Weishampel, D. B. (1994). "Albertosaurus". The Age of Dinosaurs. Publications International, Ltd. pp. 106–107. ISBN 0-7853-0443-6.
  14. ^ Holtz, T. R. (2001). "The phylogeny and taxonomy of the Tyrannosauridae". Mesozoic Vertebrate Life. Indiana University Press. pp. 64–83. ISBN 0-253-33907-3.
  15. ^ Delcourt, R.; Grillo, O. N. (2018). "Tyrannosauroids from the Southern Hemisphere: Implications for biogeography, evolution, and taxonomy". Palaeogeography, Palaeoclimatology, Palaeoecology. 511: 379–387. Bibcode:2018PPP...511..379D. doi:10.1016/j.palaeo.2018.09.003.
  16. ^ Rothschild, B.; Tanke, D.; Ford, T. (2001). "Theropod stress fractures and tendon avulsions as a clue to activity". Mesozoic Vertebrate Life. Indiana University Press. pp. 331–336.
  17. ^ Weishampel, D. B.; Dodson, P.; Osmolska, H. (2007). The Dinosauria, Second Edition. University of California Press. p. 598.
  18. ^ Xi, Y.; Xiao-Li, W.; Sullivan, C.; Shuo, W.; Stidham, T.; Xing, X. (2015). "Caenagnathasia sp. (Theropoda: Oviraptorosauria) from the Iren Dabasu Formation (Upper Cretaceous: Campanian) of Erenhot, Nei Mongol, China" (PDF). Vertebrata PalAsiatica. 53 (4): 291 298.
  19. ^ Hicks, J. F.; Brinkman, D. L.; Nichols, D. J.; Watabe, M. (1999). "Paleomagnetic and palynologic analyses of Albian to Santonian strata at Bayn Shireh, Burkhant, and Khuren Dukh, eastern Gobi Desert, Mongolia". Cretaceous Research. 20 (6): 829–850. doi:10.1006/cres.1999.0188.
  20. ^ Perle, A.; Norell, M.A.; Clark, J. (1999). "A new maniraptoran Theropod−Achillobator giganticus (Dromaeosauridae)−from the Upper Cretaceous of Burkhant, Mongolia". Contributions from the Geology and Mineralogy Chair, National Museum of Mongolia (101): 1–105. OCLC 69865262.
  21. ^ Barsbold, R. (1981). "Toothless carnivorous dinosaurs of Mongolia" (PDF). Transactions, Joint Soviet–Mongolian Palaeontological Expedition. 15: 28–39.
  22. ^ Barsbold, R.; Perle, A. (1980). "Segnosauria, a new suborder of carnivorous dinosaurs" (PDF). Acta Palaeontologica Polonica. 25 (2): 190–192.
  23. ^ Park, J. Y.; Lee, Y. N.; Currie, P. J.; Kobayashi, Y.; Koppelhus, E.; Barsbold, R.; Mateus, O.; Lee, S.; Kim, S. H. (2019). "Additional skulls of Talarurus plicatospineus (Dinosauria: Ankylosauridae) and implications for paleobiogeography and paleoecology of armored dinosaurs". Cretaceous Research. doi:10.1016/j.cretres.2019.104340.
  24. ^ Sereno, P.C. (2000). "The fossil record, systematics and evolution of pachycephalosaurs and ceratopsians from Asia" (PDF). The Age of Dinosaurs in Russia and Mongolia. Cambridge University Press. pp. 489–491.
  25. ^ Khishigjav Tsogtbaatar; David B. Weishampel; David C. Evans; Mahito Watabe (2019). "A new hadrosauroid (Dinosauria: Ornithopoda) from the Late Cretaceous Baynshire Formation of the Gobi Desert (Mongolia)". PLoS ONE. 14 (4): e0208480. Bibcode:2019PLoSO..1408480T. doi:10.1371/journal.pone.0208480. PMC 6469754. PMID 30995236.
  26. ^ Ksepka, D. T.; Norell, M. A. (2006). "Erketu ellisoni, a long-necked sauropod from Bor Guvé (Dornogov Aimag, Mongolia)" (PDF). American Museum Novitates (3508): 1–16.
  27. ^ Tsuihiji, T.; Watabe, M.; Barsbold, R.; Tsogtbaatar, K. (2015). "A gigantic caenagnathid oviraptorosaurian (Dinosauria: Theropoda) from the Upper Cretaceous of the Gobi Desert, Mongolia". Cretaceous Research. 56: 60–65. doi:10.1016/j.cretres.2015.03.007.
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