Алиорамус: новый род тираннозавридов из позднего мелового периода Азии.

Tyrannosaurid theropod dinosaur genus from the Late Cretaceous period

Alioramus (,/æ//l//i//oʊ /'/r//eɪ//m//ə//s/; meaning 'different branch') is a genus of tyrannosaurid theropod dinosaurs from the Late Cretaceous period of Asia. It currently contains two species. The type species, A. remotus is known from a partial skull and three foot bones recovered from the Mongolian Nemegt Formation, which was deposited in a humid floodplain about 70 million years ago. These remains were named and described by Soviet paleontologist Sergei Kurzanov in 1976. A second species, A. altai, known from a much more complete skeleton also from the Nemegt Formation, was named and described by Stephen L. Brusatte and colleagues in 2009. Its relationships to other tyrannosaurid genera were at first unclear, with some evidence supporting a hypothesis that Alioramus was closely related to the contemporary species Tarbosaurus bataar. However, the discovery of Qianzhousaurus indicates that it belongs to a distinct branch of tyrannosaurs, namely the tribe Alioramini. Alioramus were bipedal like all known theropods, and their sharp teeth indicate that they were carnivores. Known specimens were smaller than other tyrannosaurids like Tarbosaurus bataar and Tyrannosaurus rex, but their adult size is difficult to estimate since both Alioramus species are known only from juvenile or sub adult remains. The genus Alioramus is characterized by a row of five bony crests along the top of the snout, a greater number of teeth than any other genus of tyrannosaurid, and a lower skull than most other tyrannosaurids.

The holotype (PIN 3141/1) of Alioramus is a partial skull associated with three metatarsals. A joint Soviet Mongolian expedition to the Gobi Desert in the early 1970s found these remains at a locality known as Nogon Tsav in the Mongolian province of Bayankhongor, Nemegt Formation. Alioramus was named and described by Russian paleontologist Sergei Kurzanov in 1976. Its crests and low skull profile looked so different from other tyrannosaurids that Kurzanov believed his find was far removed from other members of the family. Accordingly, he gave it the generic name Alioramus, derived from the Latin alius ('other') and ramus ('branch'), and the specific name A. remotus, which means 'removed' in Latin. In 1988 Paul gave a similar length of and a weight of In 2016 Molina Pérez and Larramendi estimated A. remotus at and , and A. altai at and Kurzanov, however, did not correct for lengthening of the skull by deformation during fossilization, which may indicate a shorter overall body length for this individual. If this specimen is a juvenile, then adult Alioramus would have reached greater lengths, but no confirmed adult specimens are known.

The skull of A. remotus was approximately long. In general, it is long and low, a shape typical of more basal tyrannosauroids and juveniles of larger tyrannosaurids. The premaxillary bones at the tip of the snout in Alioramus remotus have not been found, but are taller than wide in all tyrannosauroids for which they are known. Other tyrannosaurids had four premaxillary teeth, D shaped in cross section, on each side. Including 16 or 17 in each maxilla, and 18 in each dentary, Alioramus had 76 or 78 teeth, more than any other tyrannosaurid. The braincase of A. altai was intermediate between the basal theropod and avialan conditions.

Paleontologists have long classified Alioramus within the superfamily Tyrannosauroidea, but because its remains were for many years poorly known, a more precise classification had remained elusive until the discovery of A. altai. A 2004 study supported this result but suggested it was equally probable that Alioramus belonged outside the family Tyrannosauridae entirely, with its supposed juvenile characters actually reflecting a more basal position within Tyrannosauroidea. The description of A. altai in 2009 confirmed the placement of the genus within the Tyrannosaurinae. The description of Qianzhousaurus in 2014 erected a new branch of the tyrannosaur family named Alioramini; consisting of the long snouted Q. sinensis and the two known species of Alioramus. This clade had an uncertain placement relative to other members of the tyrannosaur branch in the initial analysis that discovered it. The primary phylogenetic analysis found Alioramini to be closer to Tyrannosaurus than to Albertosaurus, and therefore a member of the group Tyrannosaurinae. However, a second analysis in the same paper found it to be located outside of the clade including Albertosaurinae and Tyrannosaurinae, and therefore the sister group of Tyrannosauridae. Below is the first analysis found by the authors:

Brusatte and colleagues in 2009 indicated that Alioramus lacks many of the robust and brute skull traits (such as a deep maxilla, robust lower jaws, or peg like teeth) that are necessary to employ a "puncture pull" feeding characteristic of large tyrannosaurids. They suggested that Alioramus may have exploited a different feeding style, such as focusing small sized prey. This would also suggest that both Alioramus and Tarbosaurus—whose remains have also been collected at the Tsagan Khushu locality, making them sympatric—used different feeding strategies, avoiding competition.

The Beds of Nogon Tsav are considered to be the same age as the Nemegt Formation. The Maastrichtian stage in Mongolia, as preserved in the Nemegt Formation and at Nogon Tsav, was characterized by a wetter and more humid climate compared with the semi arid environment preserved in the earlier, underlying Barun Goyot and Djadochta Formations. Nemegt sediments preserve floodplains, large river channels and soil deposits, but caliche deposits indicate periodic droughts. This environment supported a more diverse and generally larger dinosaur fauna than in earlier times. Kurzanov reported that other theropods, including Tarbosaurus, ornithomimosaurs and therizinosaurs were discovered at the same locality, but these remains have never been reported in detail. If the Nogon Tsav fauna was similar to that of the Nemegt Formation, troodontid theropods, as well as pachycephalosaurs, ankylosaurids and hadrosaurs would also have been present. Titanosaurian sauropods were also potential prey for predators in the Nemegt.