Құрған кезеңінің Alectrosaurus динозавры

Extinct genus of dinosaurs

Alectrosaurus (/ə/,/l//ɛ//k//t//r//oʊ /'/s//ɔːr//ə//s/; meaning "alone lizard") is a genus of tyrannosauroid theropod dinosaur that lived in Asia during the Late Cretaceous period, about some 96 million years ago in what is now the Iren Dabasu Formation. It was a medium sized, moderately built, ground dwelling, bipedal carnivore, estimated at with a body shape similar to its much larger advanced relative, Tyrannosaurus. Alectrosaurus was a very fast running tyrannosauroid as indicated by the elongated hindlimbs that likely filled the niche of a pursuit predator, a trait that seems to be lost by the advanced and robust tyrannosaurids, in adulthood.

In 1923, the Third Asiatic Expedition of the American Museum of Natural History, led by chief paleontologist Walter W. Granger, was hunting for dinosaur fossils in Mongolia. On April 25 in the gobi desert, assistant paleontologist George Olsen excavated and recovered the holotype AMNH FARB 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 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 bad preservation. These discoveries were made at the Iren Dabasu Formation in what is now the Inner Mongolia Autonomous Region (Nei Mongol Zizhiqu) of China. Both genus and species were formally described and named by the American paleontologist Charles Gilmore in 1933. 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. Iren Dabasu and Bayan Shireh dinosaur faunas are very similar, so it is not surprising that a species of Alectrosaurus would be found there. However, it is possible that these specimens may not be referable to Alectrosaurus, because of some tyrannosaurid traits found in the Mongolian specimens which the holotype does not possess. Alexander Averianov and Hans Dieter Sues 2012 have estimated that the Iren Dabasu Formation is Santonian in age, correlating the Upper Bayan Shireh Formation. Van Itterbeeck et al. 2005 suggested that the Iren Dabasu Formation is probably Campanian Maastrichtian in age and possibly correlated with the Nemegt Formation. Near the holotype, the specimen AMNH FARB 6266 was found in the same strata but at different points also in 1923. It consists of 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. Its affinity to Alectrosaurus is somewhat unresolved since the specimen lacks hindlimb material, making direct comparisons with Alectrosaurus quite complicated. Overall, the hindlimbs were rather gracile, in contrast to the robust tyrannosaurids. The length of its tibia (shinbone) and femur (thighbone) are very close, in contrast to the majority of other tyrannosauroids, where the tibia is longer. The femur measures and the tibia The metetarsals are also closer in size to the tibia than in most other tyrannosauroids, where they are usually longer; the third is the largest, measuring long. The astragalus and calcaneum are nicely preserved, although the astragalus seems to be slightly damaged. They are strongly attached, but not fused. Some paleontologists have equivocally considered Alectrosaurus olseni to be a species of Albertosaurus. Alectrosaurus was originally characterized as a long armed theropod, but Perle 1977 and Mader & 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 incertae sedis. A 2013 study found it to be a sister taxa to Xiongguanlong while Carr 2005 found it to be a Dryptosaurid.

The hindlimb of the specimen 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. This interpretation is consistent with the results obtained in the limb proportion analysis performed by Scott Persons IV and Currie in 2016. By comparing the limbs of numerous theropods they noted that most tyrannosauroids were highly cursorial and leggy animals, with the exception of giant and stocky legged forms such as Tarbosaurus or Tyrannosaurus. Alectrosaurus was recovered with a relatively high CLP (Cursorial limb proportion) score at 16.5, higher than most carnosaurs.

Alectrosaurus was first recovered from the Iren Dabasu Formation, which has been dated to the Cenomanian stage, about 95.8 ± 6.2 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. Additional to this, a potential discovery was made on the Bayan Shireh Formation at the Bayshi Tsav locality. Here, Alectrosaurus lived alongside a diverse range of theropods such as Achillobator, Garudimimus or Segnosaurus. Ankylosaurs were represented by Talarurus and Tsagantegia, other herbivorous dinosaurs include Graciliceratops, Gobihadros and Erketu. 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.