|Right pes of A. olseni, specimen 6554, American Museum of Natural History|
Alectrosaurus (/əˌlɛktroʊˈsɔːrəs/; 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.
Discovery and naming
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. These discoveries were made at the Iren Dabasu Formation in what is now the Inner Mongolia Autonomous Region (Nei Mongol Zizhiqu) of China.
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.
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, a formation which is also of uncertain age. Estimates suggest it was deposited from Cenomanian through Late Santonian stages, between 98.5 million and 83.5 million years ago. 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. Furthermore, several partial skeletons found in both Inner and Outer Mongolia might belong to Alectrosaurus. 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.
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.
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.
It was a medium-sized tyrannosauroid, reaching probably 5 m (16.4 ft) in length, and a weight between 454 to 907 kg (1,000.9 to 1,999.6 lb). 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 hind foot (and ankle) are also closer in size to the tibia than in most other tyrannosauroids, where the hind foot is usually longer.
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. The remaining material, AMNH 6554 represents the hind limb with characteristics of a true tyrannosauroid, and were assigned as the lectotype for Alectrosaurus olseni. 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.
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.
According to Carr 2005, Alectrosaurus can be distinguished based on the following characteristics:
- A spike-like process extends from the caudodorsal surface of the medial condyle of the femur.
- An oval scar is present on the posterior surface of the femur, which is lateral to the midline.
- The medial margin of the joint surface for the astragalus on the tibia is straight.
- A shallow muscular fossa extends posteriorly from the medial pocket of the fibula.
- The presence of an abrupt expansion in length of the anterior margin of the joint surface for the tibia on the fibula.
- The tendon pit adjacent to the ventrolateral buttress of the astragalus undercuts the medial surface of the buttress.
- The base of lateral flange of metatarsal I is triangular.
- Metatarsal I is anteroposteriorly narrow.
- The apex of distal joint surface of metatarsal I is situated medial to the midline of the bone.
- The lateral collateral ligament pit of metatarsal I does not extend anteroventrally adjacent to the distal joint surface.
- The lateral condyle of pedal phalanx I-1 extends above the dorsal surface of the bone.
- The ventral lateral condyle of pedal phalanx I-1 extends ventrolaterally.
- The medial ligament pit of pedal phalanx I-1 is small and circular.
- The dorsolateral condyle of metatarsal II is pediculate.
- The medial edge of the medial ventral condyle of metatarsal II extends below the shaft surface.
- A spur extends from the posterolateral edge of metatarsal II above the distal joint surface.
- The dorsal margin of the proximal surface of pedal phalanx II-2 is pointed.
- The lateral dorsal condyle of pedal phalanx II-2 reaches the midlength of the collateral ligament pit, when in dorsal view.
- A deep and narrow cleft separates the distal condyles of pedal phalanx II-2.
- The center of the flexor groove of pedal phalanx II-2 is convex.
- The flexor tubercle of pedal unguals II-IV are hypertrophied and reach the level of the proximal joint surface.
- The proximal joint surface of pedal digits II-IV bear a low vertical ridge on the midline.
- The dorsal lateral and ventral lateral condyles of metatarsal III are pediculate.
- The dorsal margin of the distal condyle of metatarsal III is horizontally oriented, when in anterior view.
- The medial edge of the distal joint surface of metatarsal III extends beyond the shaft margin.
- A shallow supracondylar pit is present on metatarsal III.
- The distal joint surface of metatarsal III is hyperextended onto the shaft, when in ventral view.
- The shaft of metatarsal III is elongate.
- Pedal digit III is short.
- The lateral condyle of pedal phalanx III-1 is significantly deeper than the medial condyle, when in distal view.
- The distal joint surface of pedal phalanx III-1 is deeply concave.
- The posterior margin of the distal condyle of pedal phalanx III-1 is convex, when in ventral view.
- The distal condyles of pedal phalanx III-2 are narrow and deep, when in distal view.
- The lateral ridge that bounds the flexor groove of pedal phalanx III-2 is a prominent keel, when in ventral view.
- Rugosities are absent above the collateral ligament pits of pedal phalanx III-3.
- The wide posterior region of the shaft of pedal phalanx III-3 is limited to the posterior third of the shaft, when in dorsal view.
- In pedal phalanx III-3, the scar that is posterodorsal to the collateral ligament pit is low, when examined in medial view.
- The dorsal ridge of pedal ungual III does not follow the midline, when in dorsal view.
- The distal joint surface of metatarsal IV is pediculate, except for the medial ventral condyle.
- The lateral distal condyle of metatarsal IV is hyperextended onto the ventral surface of the bone.
- The cleft that separates the condyles of metatarsal IV extends onto the distal end of the joint surface.
- The distal margin of the lateral distal condyle of pedal phalanx IV-1 is flattened, when examined in lateral view.
- Pedal phalanx IV-2 is narrow, when examined in proximal view.
- The lateral condyle of pedal phalanx IV-2 extends ventrolaterally, when in dorsal view.
- The joint surface of the lateral distal condyle of pedal phalanx IV-3 extends proximally, when in dorsal view.
- A narrow cleft separates the distal condyles of pedal phalanx IV-4.
- The medial collateral ligament pit of pedal phalanx IV-4 is situated close to the dorsal margin of the bone.
- A longitudinal groove excavates the distal third of the ventral surface of pedal phalanx IV-4.
- The dorsal half of the joint surface for metatarsal IV on metatarsal III is dilated anteriorly.
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. 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. Some paleontologists have considered Alectrosaurus olseni to be a species of Albertosaurus.
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.
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 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.
Alectrosaurus was first recovered from the Iren Dabasu Formation. It lived between 95 million and 80 million years ago. 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. 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.
It was also found in the Bayan Shireh Formation. The Bayan Shireh Formation is estimated to be Late Cretaceous in age, during the Cenomanian and Late Santonian stages. Alexander Averianov and Hans-Dieter Sues in 2012 estimated that the formation correlated the Iren Dabasu Formation and had fluvial environments. Here, Alectrosaurus lived alongside other dinosaurs, such as Theropods: Achillobator, Erlikosaurus, Garudimimus and Segnosaurus. Thyreophorans: Talarurus and Tsagantegia. Marginocephalians: Amtocephale and Graciliceratops. The hadrosauroid Gobihadros and the sauropod Erketu.
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