Allosaurus (AL-oh-sore-us)
was a big carnivorous dinosaur with a length of up to 12 m
(39 ft).It was the most common huge predator in North America, 140
million years ago, in the Jurassic period.
Allosaurus is the
official state fossil of Utah, in the United States.
Allosaurus is a
classic big theropod: a big skull on a short neck, a long
tail, and abridged forelimbs.
Its most distinctive feature is a pair of blunt horns just
above and in front of the eyes. Although short in assessment
to the hindlimbs, the forelimbs are massive and bear large,
eagle-like claws. The skull shows evidence of being self-possessed
of separate modules, which could be moved in relation to one
another, allowing big pieces of meat to be swallowed. The
skeleton of Allosaurus, like other theropods, shows birdlike
features similar to a wishbone and neck vertebrae hollowed
by air sacs. It is thought that Allosaurus might have necessary
in packs, allowing it to bring down the huge sauropods of
the time. |
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Findings
Allosaurus is the
most general theropod in the huge section of dinosaur-bearing
rock in the
American
Southwest known as the Morrison Formation. Remains have been
improved in Montana, Wyoming, South Dakota, Colorado, Oklahoma,
New Mexico, and Utah in the United States; and in Portugal.
Curiously, Allosaurus shared the Jurassic landscape with several
other theropods, including Ceratosaurus and the massive Torvosaurus.
A famous fossil
bed can be found in the Cleveland Lloyd Quarry in Utah. This
fossil bed
contains
over 10,000 bones, mostly of Allosaurus, with other dinosaurs
like Stegosaurus and Ceratosaurus thrown in. It is still a
mystery how the bits and pieces of so many animals can be
found in one place: normally the ratio of fossils of carnivorous
animals over fossils of plant eaters is very small. Conclusion
like these can be explained by pack hunting, although this
is difficult to prove.
One of the more
important finds was the 1991 discovery of "Big Al"
(MOR 593), a 95%
comprehensive,
partially articulated, juvenile specimen that measured 8 meters
(26 feet) in length. Nineteen bones were broken down or showed
signs of infection, which probably contributed to Big Al's
death. It was featured in the BBC's Walking with Dinosaurs
series in the "Ballad of Big Al". The Museum of
the Rockies and the University of Wyoming Geological Museum
excavated the fossils near Shell, Wyoming. This skeleton was
at first discovered by a Swiss team led by Kirby Siber, which
later excavated a second Allosaurus "Big Al Two",
which is the best preserved skeleton of it's kind to date.
Classification
and history
The first Allosaurus
fossil to be described was a "petrified horse hoof"
given to Ferdinand
Vanpeer
Hayden in 1869 by the populace of Middle Park, near Granby,
Colorado. It was really a caudal vertebra (a tail bone), which
Joseph Leidy tentatively assigned first to the Poicilopleuron
genus, and later to a new genus, Antrodemus. However, it was
Othniel Charles Marsh who gave the formal name Allosaurus
fragilis to the genus and type species in 1877, based on much
better material counting a partial skeleton, from Garden Park,
north of Canon City, Colorado.
The name Allosaurus
comes from the Greek allos, meaning "strange" or
"different"; and
sauros,
meaning "lizard" or "reptile". The species
epithet fragilis is Latin for "fragile". Both refer
to lightening skin in the vertebrae.
It is unclear how
many species of Allosaurus there were. The fabric from the
Cleveland-Lloyd
Allosaurus
is much smaller and more lightly built than the huge, robust
Allosaurus from Brigham Young University's Dry Mesa Quarry.
Fossils like Allosaurus have been described from Portugal.
Allosaurus's closest
relative is probably the Lower Cretaceous Acrocanthosaurus.
