| Alioramus | |
|---|---|
 | |
| Skeleton mount at Texas A&M University-Commerce | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Tyrannosauridae |
| Tribe: | †Alioramini |
| Genus: | †Alioramus Kurzanov, 1976 |
| Type species | |
| †Alioramus remotus Kurzanov, 1976 | |
| Other species | |
| Synonyms | |
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Alioramus (/ˌælioʊˈreɪməs/; meaning 'different branch') is agenus oftyrannosauridtheropoddinosaurs from theLate Cretaceous period ofAsia. It currently contains two species. Thetype species,A. remotus is known from a partialskull and threefoot bones recovered from theMongolianNemegt Formation, which was deposited in a humidfloodplain about 70 million years ago. These remains were named and described bySoviet paleontologistSergei Kurzanov in 1976. A second species,A. altai, known from a much more complete skeleton also from the Nemegt Formation, was named and described byStephen L. Brusatte and colleagues in 2009. Its relationships to other tyrannosaurid genera were at first unclear, with some evidence supporting a hypothesis thatAlioramus was closely related to the contemporary speciesTarbosaurus bataar. However, the discovery ofQianzhousaurus indicates that it belongs to a distinct branch of tyrannosaurs, namely the tribeAlioramini.
Alioramus werebipedal like all known theropods, and their sharp teeth indicate that they werecarnivores. Known specimens were smaller than other tyrannosaurids likeTarbosaurus bataar andTyrannosaurus rex, but their adult size is difficult to estimate since bothAlioramus species are known only from juvenile or sub-adult remains. The genusAlioramus 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.
Theholotype (PIN 3141/1) ofAlioramus is a partial skull associated with threemetatarsals. A jointSoviet-Mongolian expedition to theGobi Desert in the early 1970s found these remains at a locality known as Nogon-Tsav in the Mongolian province ofBayankhongor,Nemegt Formation.Alioramus was named and described byRussian 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 nameAlioramus, derived from theLatinalius ('other') andramus ('branch'), and the specific nameA. remotus, which means 'removed' in Latin.[2] A second species,A. altai, was discovered back in 2009 at the Tsagan Khushu locality also from the Nemegt Formation. However, several faunal differences may suggest that the respective locations ofA. remotus andA. altai differ in age. The holotype IGM 100/1844 is a partial skeleton that includes a very complete skull—more so thanA. remotus—with partialvertebrae,pelvic girdle and hindlimbs. The name for this species,altai, is in reference to theAltai Mountains.[3]
Alioramus remotus was estimated at 5 to 6 m (16 to 20 ft) in length when originally described by Sergei Kurzanov in 1976.[2] In 1988Paul gave a similar length of 6 m (20 ft) and a weight of 700 kg (1,500 lb).[4] In 2016 Molina-Pérez and Larramendi estimatedA. remotus at 5.5 m (18 ft) and 500 kg (1,100 lb), andA. altai at 5 m (16 ft) and 385 kg (849 lb).[5] Kurzanov, however, did not correct for lengthening of the skull by deformation duringfossilization, which may indicate a shorter overall body length for this individual. If this specimen is a juvenile, then adultAlioramus would have reached greater lengths, but no confirmed adult specimens are known.[6]
The skull ofA. remotus was approximately 45 cm (1.48 ft) long.[7] In general, it is long and low, a shape typical of morebasaltyrannosauroids and juveniles of larger tyrannosaurids. Thepremaxillary bones at the tip of the snout inAlioramus remotus have not been found, but are taller than wide in all tyrannosauroids for which they are known.[6] Thenasal bones are fused and ornamented with a row of five irregular bony crests that protrude upwards from the midline, where the nasal bones aresutured together. These crests all measure more than 1 cm (0.39 in) tall.[2][8]
At the back of the skull there is a protrusion, called thenuchal crest, arising from the fusedparietal bones, a feature shared with all tyrannosaurids. InAlioramus, the nuchal crest is greatly thickened, similarly toTarbosaurus andTyrannosaurus. Like the rest of the skull, thelower jaw ofAlioramus was long and slender, another possible juvenile characteristic.[6] As inTarbosaurus, a ridge on the outer surface of theangular bone of the lower jaw articulated with the rear of thedentary bone, locking the two bones together and removing much of the flexibility seen in other tyrannosaurids.[9] Other tyrannosaurids had four premaxillary teeth,D-shaped incross section, on each side. Including 16 or 17 in eachmaxilla, and 18 in eachdentary,Alioramus had 76 or 78 teeth, more than any other tyrannosaurid.[10] The braincase ofA. altai was intermediate between the basal theropod andavialan conditions.[11][8]
The rest of the skeleton ofAlioramus remotus is completely unknown except for threemetatarsals (bones of the upper foot), but the discovery ofA. altai, which is known from substantially more complete remains, has shed light on the anatomy of the genus.[3][8]
Paleontologists have longclassifiedAlioramus within thesuperfamily Tyrannosauroidea, but because its remains were for many years poorly known, a more precise classification had remained elusive until the discovery ofA. altai.[6] Acladistic analysis published in 2003 foundAlioramus could be further classified into the family Tyrannosauridae and thesubfamily Tyrannosaurinae, alongsideTyrannosaurus,Tarbosaurus andDaspletosaurus.[12] A 2004 study supported this result but suggested it was equally probable thatAlioramus belonged outside the family Tyrannosauridae entirely, with its supposed juvenile characters actually reflecting a more basal position within Tyrannosauroidea.[6] Another study omittedAlioramus altogether due to the only specimen's fragmentary nature.[13] The description ofA. altai in 2009 confirmed the placement of the genus within the Tyrannosaurinae.[3]
Tarbosaurus andAlioramus 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 thelacrimal bones in all other tyrannosaurids except adultDaspletosaurus. The two genera may be closely related, representing an Asian branch of the Tyrannosauridae.[9][12] Some specimens ofTarbosaurus have a row of bumps on the nasal bones like those ofAlioramus, although much lower. The long and low shape of the only knownAlioramus remotus skull indicated that it was immature when it died and might even have been a juvenileTarbosaurus, which lived in the same time and place. The more prominent nasal crests and much higher tooth count ofAlioramus, however, suggested it was a separate taxon, even if it is known only from juvenile remains,[10] confirmed by the discovery ofA. altai.[3] Specimens identified as immatureTarbosaurus have the same tooth count as adults.[14][15]
The description ofQianzhousaurus in 2014 erected a new branch of the tyrannosaur family named Alioramini; consisting of the long-snoutedQ. sinensis and the two known species ofAlioramus. 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 toTyrannosaurus than toAlbertosaurus, 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:[16]
Brusatte and colleagues in 2009 indicated thatAlioramus 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 thatAlioramus may have exploited a different feeding style, such as focusing small-sized prey. This would also suggest that bothAlioramus andTarbosaurus—whose remains have also been collected at the Tsagan Khushu locality, making themsympatric—useddifferent feeding strategies, avoiding competition.[3]
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 likeAlioramus andQianzhousaurus may have also prevented them from killing the same prey species that juvenile and adult tyrannosaurids of tyrannosaurids likeTarbosaurus 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.[17]
Examinations of the skulls of various genera of tyrannosauroids suggest thatAlioramus experienced lower stresses to its skull when feeding. Additionally, the same study suggests it and its relativeQianzhousaurus did not use the "puncture-and-pull" feeding method used by larger genera such asTarbosaurus orTyrannosaurus.[18]
Histological analyses performed on the holotype ofA. 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 growingTyrannosaurus. The team however, noted that in terms of body size this individual is closer to a seven/eight year-oldAlbertosaurus orGorgosaurus, and a five/six year-oldDaspletosaurus orTyrannosaurus, which may suggest thatAlioramus attained a comparably smaller adult size. Lastly, Brusatte and team argued against the skull shape and cranial ornamentation ofAlioramus being juvenile traits, given that: IGM 100/1844 is smaller and slender than comparably agedTyrannosaurus and has a longer snout than any known juvenile of large tyrannosaurids (Albertosaurus orTarbosaurus); and several well-documentedontogenic (growth) series of other dinosaurs evidence that ornamentation increases throughout growth. The latter may suggest that adultAlioramus possessed a rather elaborate cranial ornamentation.[3]
Examinations ofQianzhousaurus and its comparisons with both species ofAlioramus published in 2022 suggests that both Alioramus species are known from juvenile specimens in different growth stages, and thatQianzhousaurus 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 asTarbosaurus andTyrannosaurus did, but maintained a unique physiology better suited to pursuit of fast, small prey.[17]
The Beds of Nogon-Tsav are considered to be the same age as theNemegt Formation.[2] Thisgeologic formation has never beendated radiometrically, but thefauna present in the fossil record indicate it was probably deposited during theMaastrichtianstage, at the end of theLate Cretaceous.[19]
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, underlyingBarun Goyot andDjadochta Formations. Nemegt sediments preservefloodplains, largeriver channels andsoil deposits, butcaliche deposits indicate periodic droughts.[20] This environment supported a more diverse and generally larger dinosaur fauna than in earlier times. Kurzanov reported that other theropods, includingTarbosaurus,ornithomimosaurs andtherizinosaurs were discovered at the same locality,[2] but these remains have never been reported in detail. If the Nogon Tsavfauna was similar to that of the Nemegt Formation,troodontid theropods, as well aspachycephalosaurs,ankylosaurids andhadrosaurs would also have been present.[19]Titanosauriansauropods were also potential prey for predators in the Nemegt.[9]
Media related toAlioramus at Wikimedia Commons
Data related toAlioramus at Wikispecies
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