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Alioramus

Alioramus, meaning “˜different branch’, is a genus of theropod dinosaur from the Late Cretaceous Period (70 to 65 million years ago) from what is now Asia. It is known from a partial skull and three metatarsals (foot bones). It was discovered in the Gobi Desert in the early 1970s by a Soviet-Mongolian expedition. The fossil was found at Nogon-Tsav in the Mongolian province of Bavankhongor in the Nemegt formation. Alioramus was named and described by Russian paleontologist Sergei Kurzanov in 1976.

Although the Nemegt formation has never been dated radiometrically, other fauna present in the record indicate it was most likely deposited during the Maastrichtian stage, near the end of the Late Cretaceous Period. The climate of the Maastrichtian stage around Nogon-Tsav, was wetter and more humid than in earlier formations. Nemegt sediments have preserved floodplains, large river channels and soil deposits, but sodium deposits also indicate periodic droughts.

The humid climate supported more diverse and larger dinosaurs than in earlier times. Other theropods, including Tarbosaurus were found in the same locality. If the Nogon-Tsav fauna was similar to that of the Nemegt Formation, then troodontid theropods, pachycephalosaurs, ankylosaurids and hadrosaurs would have been present as well.

Alioramus was believed to be far removed from other members of its family based on its crests and the low skull profile. Kurzanov named it Alioramus, which comes from the Latin alius (other) and ramus (branch). The species name A. remotus means “˜removed’ in Latin. A second species is also known, A. altai.

Although only a few fossil fragments have been found, it is believed Alioramus was bipedal like most other theropods. Its sharp teeth indicate that it was most likely a carnivore. It was smaller than Tarbosaurus and Tyrannosaurus. Its adult size is hard to estimate because the skull and metatarsals are believed to be from a juvenile specimen. It had 5 bony crests along the top of the snout, and had more teeth than any other tyrannosaurid. The skull, which sits lower than those of other tyrannosaurids, may be a juvenile feature only.

Alioramus was originally estimated to be 16 to 20 feet in length when described by Sergei Kurzanov in 1976, but based on skull lengthening and deformation during fossilization, it may be an overestimated size. If the fossil is that of a juvenile, however, then adult length could be much greater. The skull measured 18 inches long.

A nuchal crest, which is found in all tyrannosaurids, is situated at the back of the skull. In Alioramus, the nuchal crest is greatly thickened, similar to the Tarbosaurus and Tyrannosaurus. The lower jaw was long and slender, which is a possible juvenile characteristic. Alioramus also had more teeth than any other tyrannosaurid.

Alioramus most likely has small forelimbs that bore only two digits, although some tyrannosaurids retained a third digit. It would have had a long tail to help maintain balance. The center of its mass would have been over the hips.

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Alioramus


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