Alioramus
Name: Alioramus
(Different Branch).
Phonetic: Ah-lee-oh-ray-mus.
Named By: Sergei Kurzanov - 1976.
Classification: Chordata, Reptilia, Dinosauria,
Saurischia, Theropoda, Tyrannosauroidea, Alioramini.
Species: A. remotus (type),
A.
altai.
Diet: Carnivore.
Size: Up to around 6 meters long.
Known locations: Mongolia.
Time period: Maastrichtian of the Cretaceous.
Fossil representation: Remains from two individuals
including partial skull and post cranial material. However all
current remains are of either juveniles or sub-adults.
Alioramus
has stirred a lot of interest and controversy since its discovery,
and even though Alioramus has earned worldwide
popularity it was for a
long time only known from a partial skull and three metatarsals
(foot bones). Another problem was the fact that when
reconstructed, the skull was narrow and low like in some juvenile
forms of other better known tyrannosauroids.
This led to Alioramus
being considered for a long time to be a juvenile of the larger
Tarbosaurus
by many palaeontologists until the
discovery of the second
species A. altai in 2009. This specimen is
thought to represent a
sub-adult because the skull bones had begun fusing together,
indicating that the individual was approaching maximum size but still
not even close to Tarbosaurus. Further support
for keeping Alioramus
its own separate genus comes from the fact that Alioramus
had many more
teeth in its mouth than Tarbosaurus individuals
that were both juvenile
and fully grown. Also Alioramus has a series of
five crests on the
top of its snout which so far remain unknown in Tarbosaurus.
There
has in the past been a proposed synonymy between Alioramus
and the
genus Qianzhousaurus. However later study of Qianzhousaurus
fossils
have cast doubt upon this idea, and the two genera are generally
regarded as very similar but different, The two genera are now more
often placed in their own group within the Tyrannosauroidea, known as
the Alioramini..
Hunting and potential prey
specialisation
The
legs of Alioramus had proportionately longer lower
limb segments, an
arrangement that is reminiscent of the fast running ornithomimid
dinosaurs like Gallimimus.
Bearing in mind that
the Alioramus
specimen in question was a sub adult, the longer legs do fit in with
the tyrannosauroid body morphs for younger individuals. Tyranosauroid
genera where the juvenile as well as adult individuals are known,
such as Albertosaurus,
consistently shows that tyrannosauroids had
proportionately longer legs when young that proportionately shortened
as the individual grew to adult size. This is a simple adaptation
that allowed small tyrannosauroids to hunt small and fast prey until
they grew larger when they would need to hunt larger but slower prey to
maintain an adequate food intake. However, the only currently known
limb remains for Alioramus come from the second
species A. altai,
and this individual is thought to be a subadult.
Without
further remains attributable to a younger juvenile or even better a
full grown adult, it is impossible to say how short the legs of
Alioramus became. But the fact that the legs still
retain fast
running proportions at a stage when its overall growth would be slowing
down suggests that Alioramus was relatively quick
and nimble when
compared to other medium to large members of the Tyrannosauroidea.
Combined with its more gracile build and it may well be that Alioramus
was a niche specialist that hunted dinosaurs that were too swift for
the larger Tarbosaurus to catch.
The
large number of moderately sized and relatively evenly spaced and
arranged teeth also suggests a specialisation for fast but unarmoured
prey as these would provide a greater area for seizing prey. The
narrow skull also meant proportionately weaker biting muscles than
other tyrannosauroids suggesting that Alioramus
went to work on softer
and unarmoured flesh. Large tyrannosauroids like Daspletosaurus
and
Tyrannosaurus
by comparison have fewer but larger teeth, and deep
snouts to house powerful biting muscles to drive those teeth through
bone, feeding behaviour which is also confirmed by fossil evidence.
The
fact that Alioramus was present in the same
location as Tarbosaurus
also follows a trend of a robust and gracile morph split between two
tyrannosauroid genera per area that seems to have repeated itself.
Such a niche split can also be seen in North America during the
Campanian with Daspletosaurus (robust) and
Gorgosaurus
(gracile)
crossing territories, and during the Maastrichtian with
Tyrannosaurus (robust) and Albertosaurus
(gracile) also having
similar approximate areas.
Body shape and form
The
name Alioramus translates as ‘different branch’
and is in reference
to Alioramus’s different head and body shape to
other tyrannosauroids.
The head shape of Alioramus is more like the
earlier basal members of
the Tyrannosauroidea that were generally smaller theropods that focused
on smaller and less powerful prey. The fact that Alioramus
still has
this overall shape suggests that while Alioramus
was related to other
tyrannosauroids it was actually passing along a different evolutionary
path that allowed the retention of this more basal feature.
The
most distinctive features of Alioramus are five one
centimetre high
ridges that run across the top of its snout. Head ornamentation is
virtually unknown in the later and larger tyrannosauroids, although
some earlier members such as Guanlong
did actually have quite elaborate
head crests. It could be that the head ornamentation of Alioramus
is
a throwback to these earlier forms and its retention is actually a
sexually selected characteristic with the most developed ridges being
the most attractive to others of their kind. It’s also possible that
the ridges may have allowed Alioramus to
differentiate between members
of its own genera and juvenile Tarbosaurus that may
have looked roughly
similar.
Another
area of contention is the estimated size of Alioramus.
When the first
specimen was discovered the individuals size had to be estimated by
comparison of the incomplete skull of the holotype to other more
complete examples. This yielded an estimated length of between five
and six meters, however the skull material was slightly deformed
during the fossilisation process, a result of the extreme pressure
the bones were subjected to with increasing layers of sediment being
deposited on top of the bones. This led to the original length being
considered an overestimate of the actual living animal, however the
discovery of A. altai has since confirmed that
six meters was
probably easily possible for a full grown adult Alioramus,
if not a
conservative estimate. Unfortunately without fossil material from a
confirmed fully grown adult Alioramus, it remains
impossible to say
for certain.
Further reading
- A new carnosaur from the Late Cretaceous of Nogon-Tsav, Mongolia -
The Joint Soviet-Mongolian Paleontological Expedition Transactions (in
Russian) 3: 93–104. - Sergei M. Kurzanov - 1976.
- Theropods from the Cretaceous of Mongolia - The Age of Dinosaurs in
Russia and Mongolia. Cambridge: Cambridge University Press. pp.
434–455. ISBN 978-0-521-54582-2. - Philip J. Currie - 2000.
- A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of
Mongolia - Proceedings of the National Academy of Sciences of the
United States of America. online preprint (41): 17261–6. - Stephen L.
Bursatte, Thomas D. Carr, Gregory M. Erickson, Gabe S. Bever &
Mark A. Norell - 2009.
- Variation, Variability, and the Origin of the Avian Endocranium:
Insights from the Anatomy of Alioramus altai
(Theropoda:
Tyrannosauroidea) - PLOS Collections. - Gabe S. Bever, Stephen L.
Brusatte, Amy M. Balanoff & Mark A. Norell - 2011.
- The osteology of Alioramus, a gracile and
long-snouted tyrannosaurid
(Dinosauria, Theropoda) from the late Cretaceous of Mongolia. -
American Museum Novitates (366): 1−197. - Stephen L. Brusatte, Thomas
D. Carr & Mark A. Norell - 2012.
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