Alioramus
Alioramus 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.
History of discovery
The holotype 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 and ramus, and the specific name A. remotus, which means 'removed' in Latin. The fossil material of the second species named in 2009, A. altai, was discovered back in 2001 at the Tsagan Khushu locality also from the Nemegt Formation. However, several faunal differences may suggest that the respective locations of A. remotus and A. altai differ in age. The holotype IGM 100/1844 is a partial skeleton that includes a very complete skull—more so than A. remotus—with partial vertebrae, pelvic girdle and hindlimbs. The name for this species, altai, is in reference to the Altai Mountains.Description
Alioramus remotus was estimated at in length when originally described by Sergei Kurzanov in 1976. 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.Skull
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. The nasal bones are fused and ornamented with a row of five irregular bony crests that protrude upwards from the midline, where the nasal bones are sutured together. These crests all measure more than tall.At the back of the skull there is a protrusion, called the nuchal crest, arising from the fused parietal bones, 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 lower jaw 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.
Postcranial skeleton
The rest of the skeleton of Alioramus remotus is completely unknown except for three metatarsals, but the discovery of A. altai, which is known from substantially more complete remains, has shed light on the anatomy of the genus.Classification
s 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 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.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 lacrimal bones 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:
Paleobiology
Feeding
Brusatte and colleagues in 2009 indicated that Alioramus lacks many of the robust and brute skull traits 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.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.''