| Alioramus Temporal range: Late Cretaceous, 70 Ma |
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| A. remotus skull diagram, known portions in white | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Dinosauria |
| Order: | Saurischia |
| Suborder: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Family: | †Tyrannosauridae |
| Subfamily: | †Tyrannosaurinae |
| Genus: | †Alioramus Kurzanov, 1976 |
| Species | |
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Alioramus ( /ˌæli.ɵˈreɪməs/; meaning 'different branch') is a genus of tyrannosaurid theropod dinosaur from the Late Cretaceous period of Asia. The type A. remotus is known from a partial skull and three metatarsals recovered from Mongolian sediments which were deposited in a humid floodplain between 70 and 65 million years ago. These remains were named and described by Russian paleontologist Sergei Kurzanov in 1976. A second species, A. altai, known from a much more complete skeleton, was named and described by Stephen L. Brusatte and colleagues in 2009. Its relationships to other tyrannosaurid genera are unclear, with some experts believing that Alioramus is closely related to the contemporaneous Tarbosaurus, or is a juvenile of that genus. However, the discovery of A. altai made it clear that the latter hypothesis was incorrect.
Alioramus was bipedal like most theropods, and its sharp teeth indicate that it was a carnivore. It was smaller than tyrannosaurids like Tarbosaurus and Tyrannosaurus, but its adult size is difficult to estimate since both species are known from juvenile or sub-adult remains. Alioramus is characterized by the row of five bony crests along the top of its snout, a greater number of teeth than any other tyrannosaurid, and a lower skull than those of other tyrannosaurids.
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Alioramus was estimated at 5 to 6 metres (16 to 20 ft) in length when originally described by Sergei Kurzanov in 1976.[1] 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.[2]
The skull of A. remotus was approximately 45 centimetres (18 in) long.[3] 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.[2] 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 1 centimetre (0.39 in) tall.[1]
At the back of the skull there is a transversely-oriented 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.[2] 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.[4] 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.[5] The braincase of A. altai was intermediate between the basal theropod and avialan conditions.[6]
The postcranial skeleton of Alioramus remotus is completely unknown except for three metatarsals, but the discovery of A. altai, which is known from substantially complete remains, has shed light on the morphology of the genus.[7] By inference from other tyrannosaurids, Alioramus moved about on two legs, which had proportions similar to those of ornithomimosaurs, with long tibiae and metatarsals in comparison to the femora. Like other tyrannosaurids, the forelimbs were very small and bore only two digits, although some tyrannosaurid specimens retained a vestigial third digit. A long tail would have balanced out the head and torso, putting the center of mass over the hips.[2]
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Cladograms of Tyrannosauridae highlighting the position of Alioramus
Note: Holtz finds two equally probable positions for Alioramus. |
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.[2] 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.[8] 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.[2] Another study omitted Alioramus altogether due to the only specimen's fragmentary nature.[9] The discovery of A. altai in 2009 confirmed the placement of the genus within the Tyrannosaurinae.[7]
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.[4][8] 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,[5] confirmed by the discovery of A. altai.[7] Specimens identified as immature Tarbosaurus have the same tooth count as adults.[10][11]
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. 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.[1] Alioramus is known from the holotypes of A. remotus[2] and A. altai.[7]
The Beds of Nogon-Tsav are considered to be contemporaneous with the Nemegt Formation.[1] This geologic formation has never been dated radiometrically, but the fauna present in the fossil record indicate it was probably deposited during the Maastrichtian stage, at the very end of the Late Cretaceous.[12]
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.[13] 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,[1] 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.[12] Titanosaurian sauropods were also potential prey for predators in the Nemegt.[4]
