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Alioramus (; meaning 'different branch') is a genus of tyrannosaurid theropod 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 metatarsal recovered from the 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. However, the discovery of Qianzhousaurus indicates that it belongs to a distinct branch of tyrannosaurs, namely the tribe Alioramini. Alioramus were like all known theropods, and their sharp teeth indicate that they were . Known specimens were smaller than other tyrannosaurids like Tarbosaurus bataar and Tyrannosaurus, 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.
At the back of the skull there is a protrusion, called the nuchal crest, arising from the fused , a feature shared with all tyrannosaurids. In Alioramus, the nuchal crest is greatly thickened, similarly to Tarbosaurus and Tyrannosaurus. Like the rest of the skull, the mandible of Alioramus was long and slender, another possible juvenile characteristic. As in Tarbosaurus, a ridge on the outer surface of the angular bone of the lower jaw articulated with the rear of the dentary bone, locking the two bones together and removing much of the flexibility seen in other tyrannosaurids. 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.
Tarbosaurus and Alioramus shared several skull features, including a locking mechanism in the lower jaw between the dentary and angular bones, and both lacked the prong of the nasal bones which connected to the in all other tyrannosaurids except adult Daspletosaurus. The two genera may be closely related, representing an Asian branch of the Tyrannosauridae. Some specimens of Tarbosaurus have a row of bumps on the nasal bones like those of Alioramus, although much lower. The long and low shape of the only known Alioramus remotus skull indicated that it was immature when it died and might even have been a juvenile Tarbosaurus, which lived in the same time and place. The more prominent nasal crests and much higher tooth count of Alioramus, however, suggested it was a separate taxon, even if it is known only from juvenile remains, confirmed by the discovery of A. altai. Specimens identified as immature Tarbosaurus have the same tooth count as adults.
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:
Foster with team in 2022 hypothesized that due to their slim and gracile build, Alioramin genera may have been hunters of small, particularly fast and nimble prey, which would have allowed alioramins to avoid competition with larger tyrannosaurs that specialized in killing larger animals. The long and delicate snouts of alioramins like Alioramus and Qianzhousaurus may have also prevented them from killing the same prey species that juvenile and adult tyrannosaurids of tyrannosaurids like Tarbosaurus hunted, though these larger tyrannosaurs themselves may have hunted alioramins as prey on occasion. Alioramins may also have had a different feeding strategy than other tyrannosaurids, as their jaws seem to have been weaker than those of the larger genera, and even juveniles of larger species have proportionately higher bite forces than alioramins of equivalent size. Furthermore, Alioramins seemingly remained confined to Asia, suggesting some factor prevented them from colonizing the better-sampled fossil deposits from North America. Why this may be remains a mystery until more evidence is discovered.
Examinations of the skulls of various genera of tyrannosauroids suggest that Alioramus experienced lower stresses to its skull when feeding. Additionally, the same study suggests it and its relative Qianzhousaurus did not use the "puncture-and-pull" feeding method used by larger genera such as Tarbosaurus or Tyrannosaurus.
Examinations of Qianzhousaurus and its comparisons with both species of Alioramus published in 2022 suggests that both Alioramus species are known from juvenile specimens in different growth stages, and that Qianzhousaurus represents an adult example of the alioramini. The examinations also suggest that the variation seen between the various species is consistent with the growth trends seen in other tyrannosaurid genera, though specimens that could constitute a full growth series from infant to adult for each species have not been recovered for any of these tyrannosaurs. One part of the growth series across all specimens in this study was discovered to remain unique to alioramin tyrannosaurs; the rugose process of the jugal starts small and conical in early life, but becomes massive and indistinct as the animals grow. This same study also suggests Alioramins did not undergo a secondary metamorphosis from slender juveniles to robust adults like other tyrannosaurs such as Tarbosaurus and Tyrannosaurus did, but maintained a unique physiology better suited to pursuit of fast, small prey.
Postcranial skeleton
The rest of the skeleton of Alioramus remotus is completely unknown except for three (bones of the upper foot), but the discovery of A. altai, which is known from substantially more complete remains, has shed light on the anatomy of the genus.
Classification
have long classified Alioramus within the Taxonomic rank 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 cladistic analysis published in 2003 found Alioramus could be further classified into the family Tyrannosauridae and the subfamily Tyrannosaurinae, alongside Tyrannosaurus, Tarbosaurus and Daspletosaurus. 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. Another study omitted Alioramus altogether due to the only specimen's fragmentary nature. The description of A. altai in 2009 confirmed the placement of the genus within the Tyrannosaurinae.
Paleobiology
Feeding
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.
Ontogeny
Histology performed on the holotype of A. altai (IGM 100/1844) by Brusatte and colleagues in 2009 determined that this individual had an internal bone structure corresponding to a nine year-old and actively growing Tyrannosaurus. The team however, noted that in terms of body size this individual is closer to a seven/eight year-old Albertosaurus or Gorgosaurus, and a five/six year-old Daspletosaurus or Tyrannosaurus, which may suggest that Alioramus attained a comparably smaller adult size. Lastly, Brusatte and team argued against the skull shape and cranial ornamentation of Alioramus being juvenile traits, given that: IGM 100/1844 is smaller and slender than comparably aged Tyrannosaurus and has a longer snout than any known juvenile of large tyrannosaurids ( Albertosaurus or Tarbosaurus); and several well-documented ontogenic (growth) series of other dinosaurs evidence that ornamentation increases throughout growth. The latter may suggest that adult Alioramus possessed a rather elaborate cranial ornamentation.
Paleoenvironment
The Beds of Nogon-Tsav are considered to be the same age as the Nemegt Formation. This geologic formation has never been dated radiometrically, but the fauna present in the fossil record indicate it was probably deposited during the Maastrichtian faunal stage, at the end of the Late Cretaceous.
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 , large river channels and Paleosol, 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, and 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 , and would also have been present. were also potential prey for predators in the Nemegt.
See also
External links
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