Alioramus

Taxobox
name = "Alioramus"

---

fossil_range = Late Cretaceous
regnum = Animalia
phylum = Chordata
classis = Sauropsida
superordo = Dinosauria
ordo = Saurischia
subordo = Theropoda
familia = Tyrannosauridae
genus = "Alioramus"
genus_authority= Kurzanov, 1976
subdivision_ranks = Species
subdivision =
*"A. remotus" Kurzanov, 1976 (type)

"Alioramus" (pronEng|ˌæliːoʊˈreɪməs; meaning 'different branch') is a genus of tyrannosaurid theropod dinosaur from the Late Cretaceous Period of Asia. The type and only species, "A. remotus", is known from a partial skull and three metatarsals recovered from Mongolian sediments which were deposited in a humid floodplain between 70 to 65 million years ago. These remains were named and described by Russian paleontologist Sergei Kurzanov in 1976. Its relationships to other tyrannosaurid genera are unknown, but some experts believe "Alioramus" is closely related to the contemporaneous "Tarbosaurus", or is a juvenile of that genus.

While very little skeletal material is known, "Alioramus" was probably 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 it is known mainly from a skull, which may belong to a juvenile. "Alioramus" is characterized by the row of five bony crests along the top of its snout, and also has more teeth than any other tyrannosaurid. Its skull is lower than those of other tyrannosaurids, but this might be a juvenile feature.

Description

"Alioramus" was estimated at between 5 and 6 meters (16.5 to 20 ft) in length when originally described by Sergei Kurzanov in 1976.cite_journal |last=Kurzanov |first=Sergei M. |authorlink=Sergei Kurzanov |title=A new carnosaur from the Late Cretaceous of Nogon-Tsav, Mongolia |language=Russian |journal=The Joint Soviet-Mongolian Paleontological Expedition Transactions |volume=3 |pages=93–104] 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.cite_book |last=Holtz |first=Thomas R. |authorlink=Thomas R. Holtz, Jr. |year=2004 |chapter=Tyrannosauroidea |editor= Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka (eds.) |title=The Dinosauria |edition=Second Edition |publisher=University of California Press |location=Berkeley |pages=111–136 |isbn=0-520-24209-2] The skull was approximately 45 centimeters (18 in) long.cite_book |last=Currie |first=Philip J. |authorlink=Phil Currie |year=2000 |chapter=Theropods from the Cretaceous of Mongolia |title=The Age of Dinosaurs in Russia and Mongolia |location=Cambridge |publisher=Cambridge University Press |pages=434-455 |isbn=978-0521545822] 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" 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 1 centimeter (0.4 in) tall. 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. 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.cite_journal |last=Hurum |first= Jørn H. |coauthors=& Sabath, Karol. |year=2003 |title=Giant theropod dinosaurs from Asia and North America: Skulls of "Tarbosaurus bataar" and "Tyrannosaurus rex" compared |journal=Acta Palaeontologica Polonica |volume=48 |issue=2 |pages=161–190 |url=http://www.app.pan.pl/acta48/app48-161.pdf] 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.cite_journal |last=Currie |first=Philip J. |authorlink=Phil Currie |year=2003 |title=Cranial anatomy of tyrannosaurids from the Late Cretaceous of Alberta |journal=Acta Palaeontologica Polonica |volume=48 |issue=2 |pages=191–226 |url=http://app.pan.pl/acta48/app48-191.pdf] The postcranial skeleton of "Alioramus" is completely unknown except for three metatarsals, but tyrannosaurids had essentially conservative morphology. "Alioramus" moved about on two legs, which most likely had proportions similar to those of ornithomimosaurs, with long tibiae and metatarsals in comparison to the femora. Like other tyrannosaurids, the forelimbs were likely very small and probably 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.

Classification and systematics

Currie "et al." 2003
clade| style=font-size:80%;line-height:80%
label1= Tyrannosauridae
1=clade
label1=void
1=Albertosaurinae
label2= Tyrannosaurinae
2=clade
label1=void
1=clade
label1=void
1="Daspletosaurus"
2=clade
label1= void
1="Tarbosaurus"
2="Alioramus"
2=clade
label1=void
1="Nanotyrannus"
2="Tyrannosaurus"
Holtz 2004
clade| style=font-size:80%;line-height:80%
label1=Tyrannosauroidea
1=clade
1="Alioramus"?
label2= Tyrannosauridae
2=clade
1=Albertosaurinae
2=clade
label1= Tyrannosaurinae
1=clade
label1=void
1="Daspletosaurus"
2=clade
label1= void
1="Alioramus"?
2=clade
1="Tarbosaurus"
2="Tyrannosaurus" Note: Holtz finds two equally probable positions for "Alioramus".
Paleontologists classify "Alioramus" within the superfamily Tyrannosauroidea, but because its remains are poorly known, a more precise classification remains elusive. 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".cite_journal |last=Currie |first=Philip J. |coauthors=Hurum, Jørn H; & Sabath, Karol. |authorlink=Phil Currie |year=2003 |title=Skull structure and evolution in tyrannosaurid phylogeny |journal=Acta Palaeontologica Polonica | volume=48 |issue=2 |pages=227–234 |url=http://app.pan.pl/acta48/app48-227.pdf] 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.cite_journal |last=Carr |first=Thomas D. |coauthors=Williamson, Thomas E.; & Schwimmer, David R. |year=2005 |title=A new genus and species of tyrannosauroid from the Late Cretaceous (middle Campanian) Demopolis Formation of Alabama |journal=Journal of Vertebrate Paleontology |volume=25 |issue=1 |pages=119–143 |url=http://www.bioone.org/perlserv/?request=get-abstract&doi=10.1671%2F0272-4634(2005)025%5B0119%3AANGASO%5D2.0.CO%3B2 |doi=10.1671/0272-4634(2005)025 [0119:ANGASO] 2.0.CO;2 |doilabel=10.1671/0272-4634(2005)025[0119:ANGASO]2.0.CO;2]

"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 like those of "Alioramus", although much lower. The long and low shape of the known "Alioramus" skull indicates that it was immature when it died and might even be a juvenile "Tarbosaurus", which lived in the same time and place. The more prominent nasal crests and much higher tooth count of "Alioramus", however, suggest it is a separate taxon, even if it is juvenile. Specimens identified as immature "Tarbosaurus" have the same tooth count as adults.cite_journal |last=Maleev |first=Evgeny A. |authorlink=Evgeny Maleev |year=1955 |title=New carnivorous dinosaurs from the Upper Cretaceous of Mongolia. |journal=Doklady, Academy of Sciences USSR |volume=104 |issue=5 |pages=779–783 |language=Russian] cite_journal |last=Currie |first=Philip J. |authorlink=Phil Currie |year=2003 |title=Allometric growth in tyrannosaurids (Dinosauria: Theropoda) from the Upper Cretaceous of North America and Asia |journal=Canadian Journal of Earth Sciences |volume=40 |issue=4 |pages=651–665 |doi=10.1139/e02-083]

Discovery and naming

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. No other remains besides the holotype are known.

Paleoecology

The Beds of Nogon-Tsav are considered to be contemporaneous with 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 stage, at the very end of the Late Cretaceous.cite_journal |last=Jerzykiewicz |first=Tomasz |coauthors=& Russell, Dale A. |year=1991 |title=Late Mesozoic stratigraphy and vertebrates of the Gobi Basin |journal=Cretaceous Research |volume=12 |issue=4 |pages=345–377 |doi=10.1016/0195-6671(91)90015-5] The Maastrichtian lasted from 70 to 65 million years ago.cite_book |last=Gradstein |first=Felix M. |coauthors=Ogg, James G.; & Smith, Alan G. |year=2005 |title=A Geologic Time Scale 2004 |location=Cambridge |publisher=Cambridge University Press |pages=500pp. |isbn=978-0521781428]

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.cite_book |last=Osmólska |first=Halszka |authorlink=Halszka Osmólska |year=1997 |chapter=Nemegt Formation |title=The Encyclopedia of Dinosaurs |editor=Currie, Philip J.; & Kevin Padian (eds.) |location=San Diego |publisher=Academic Press |pages-471-472 |isbn=0-12-226810-5] 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, 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. Titanosaurian sauropods were also potential prey for predators in the Nemegt.

References

External links

*" [http://www.dinodata.org/index.php?option=com_content&task=view&id=6031&Itemid=67 Alioramus remotus] " at [http://www.dinodata.org/index.php DinoData.org]
*" [http://home.comcast.net/~eoraptor/Tyrannosauroidea.html#Alioramusremotus Alioramus remotus] " at The Theropod Database
* [http://web.archive.org/web/20070110004450/http://www.paleograveyard.com/alioramus.html Skull image of "Alioramus"] at [http://web.archive.org/web/20070326170159/http://paleograveyard.com/ The Grave Yard]