Name:
Allosaurus
(Different lizard).
Phonetic: Al-loh-sore-us.
Named By: Othniel Charles Marsh - 1877.
Synonyms: Antrodemus, Creosaurus,
Labrosaurus.
Classification: Chordata, Reptilia, Dinosauria,
Saurischia, Theropoda, Allosauridae.
Species: A. fragilis (type),
A. europaeus, A. jimmadseni.
Type: Carnivore.
Size: 8.5 meter average body length, with some
individuals approaching 9.7 meters long. Some fragmentory
specimens have been interpreted as coming from animals 12 meters
long.
Known locations: USA, Portugal, possibly
Tanzania.
Time period: Kimmeridgian to Tithonian of the
Jurassic.
Fossil representation: There are probably more
Allosaurus fossils than any other of the large
theropods. Its
reconstruction is without doubt.
Along with Tyrannosaurus, Allosaurus is probably the most often represented large theropod dinosaur in popular culture. This is because it was one of the earliest discovered large predators and is known from more remains than any other large predatory dinosaur. One myth about Allosaurus that does need to be dispelled is that it was the ancestor of Tyrannosaurus. Although both share a similar morphology and Allosaurus is much older, the tyrannosaurids are thought to have evolved from the coelurosaurid group independent of Allosaurus. This means that as the tyrannosaurids evolved, they replaced dinosaurs like Allosaurus as the dominant theropods of North America.
Many
names but just one dinosaur?
The
discovery of Allosaurus was a result of the 'bone
wars', a
time of large scale fossil discoveries across North America.
Unfortunately due to the nature of what was going on during the bone
wars, different sets of Allosaurus were named
under a few different
species. This has resulted in other palaeontologists having to
re-evaluate the original fossil material and put the remains in their
correct places, resulting in the reduction of species names and the
synonymization of other dinosaurs into Allosaurus.
Although
existing synonyms for Allosaurus include Antrodemus,
Creosaurus and Labrosaurus,
two more dinosaurs named Epanterias and
Saurophaganax
may also prove to be additional
examples of Allosaurus.
Both Epanterias and Saurophaganax
are only known from partial
remains, yet both are significantly larger than Allosaurus
with size
estimates approaching (and occasionally exceeding) eleven metres
long. While some researchers already consider them to be larger
individuals of Allosaurus, others are more
inclined to wait upon new
material and further study.
The
Allosaurus genus has had a lot of additional species
attributed to it,
and the list of confirmed species has been changing sporadically
throughout most of the history of the genus. A. fragilis,
the
type species of Allosaurus has and always will be
valid. A.
europaeus and A. jimmadseni are often
also credited as valid.
Further species such as A. amplexus, A.
atrox, A. maximus
and A. tendagurensis are more controversial.
Further
study of Allosaurus is undoubtedly needed to
resolve the
species naming issues, but part of the problem is the holotype
specimen itself. When Othniel Charles Marsh first described
Allosaurus in 1877 he did not have the complete
dinosaur,
although somewhat ironically his rival Edward Drinker Cope acquired a
particularly well preserved specimen two years later, but failed to
realise the significance as the raw material remained in storage while
he worked on other fossils. The holotype specimen itself was composed
of a right humerus, a few vertebrae, teeth, toe bone and a
fragment of a rib. As such subsequent discoveries were studied
against this incomplete material, but not always against other
Allosaurus discoveries, resulting in numerous
'new' species that
under modern study do not appear all that different. This is why many
Palaeontologists have called for a neotype specimen to be established
from one of the more complete later discoveries, so that new and
existing findings can be measured against much more accurate data.
Allosaurus
Morphology
Allosaurus
had the typical theropod morphology of a bipedal stance on two large
legs and a long tail held out to counter-balance the head of an S
curved neck. The most distinctive feature of Allosaurus
are the small
horns above its eyes which are special growths of the lacrimal bone of
the skull. Theories as to their purpose include potential weapons or
sun shades to prevent glare in the eyes, to the more widely accepted
species recognition of its own kind, and possibly display for
attracting a mate.
The
placement of the eyes in the skull shows that while Allosaurus
did have
binocular vision, it was limited to twenty degrees. This means that
Allosaurus would need to make sure that its prey
stayed directly in
front of it, because if its prey turned sharply, it could quite
easily break this cone of vision. Although Allosaurus
would still see
the prey, it would only be able to use one eye and as such would not
have depth perception which could result in a missed strike. Although
it seems easy enough to point out that Allosaurus
would just have to
turn its head to regain depth perception, a successful strike
requires split second timing with only the smallest margin of error.
A loss of depth perception for only a fraction of a second may have
been enough for a prey item to avoid being bitten.
CT
scans of Allosaurus skulls have revealed a few
interesting glimpses
into the life of the living animal. The Brain is similar to those
found in crocodiles, and Allosaurus had very
large olfactory bulbs,
but had an underdeveloped area for assessing them. It could be that
Allosaurus relied upon only recognising a few
smells, such as prey
animals, carrion, or even others of its own kind. The inner ear
seems best suited to low frequency sounds, and analysis of the
vestibular system, the part that controls balance, shows that the
head was usually held horizontally level.
The
arms are very robust when compared to later large theropods, and the
hands end with three digits that have large claws. The retained
development of the arms suggests that they played a key role in the
lifestyle of Allosaurus, but the actual method of
use remains open to
strong debate among palaeontologists.
The
overall morphology of Allosaurus is dependent upon
the age of the
individual, with juvenile specimens consistently showing
proportionately longer legs than adults. This suggests that juvenile
Allosaurus were very quick, perhaps among the
fastest of the
dinosaurs of that time and location. This would make a whole lot of
sense as when small, the only prey items available to juvenile
Allosaurus were smaller and quicker animals.
As
Allosaurus grew older and increased in size is would
need more
sustenance that could only be provided by larger dinosaurs. Since
these are generally slower and heavier, Allosaurus
morphed with age
to have shorter and more robust legs to better deal with the stresses
of tackling them. For such prey items, high speed would be an
unnecessary luxury, and longer lighter legs would have only been more
susceptible to injury.
Interestingly,
although Allosaurus is not directly related to
them, this changing
morphology of longer legged and faster juveniles can also be seen in
tyrannosaurids
such as Albertosaurus.
This could possibly serve as a
case on convergent evolution in larger theropods, although the
genetic markers for such growth may herald from further back in
dinosaur evolution.
Feeding strategy
Although
it seems to be a foregone conclusion that Allosaurus
was a predatory
animal, it like Tyrannosaurus has been accused of
being just a
scavenger. This primarily comes from the head and dentition, as
although the head is very strongly built, the bite force of
Allosaurus was very weak for a large theropod with
some studies
suggesting that it was even less than a modern day big cat like a
leopard.
For
this reason you have to look at other areas of the body, particularly
the arms that were proportionately larger than much later large
theropods like Tyrannosaurus. There is also a
large number of
injuries to the arms including tendon avulsions, which is where the
tendon that connects the muscle to the bone is torn free, sometimes
with part of the bone still attached. Now a carcass doesn't struggle
when a carnivore scavenges it, so these kinds of injuries must be
caused by conflict with another dinosaur, although conversely it’s
fair to say that they may have been caused in a struggle with another
Allosaurus.
Even
if the other dinosaur was another Allosaurus, it
does still show that
Allosaurus could be extremely aggressive when it
needed to be. Had
the arms come into use when hunting, then Allosaurus
may have gripped
hold of an animal as it positioned its mouth to strike at a soft spot
like the throat. Such a target wouldn't necessarily require a lot of
bite force, just enough to do some damage to the veins or maybe close
off the wind pipe. This would mean that its mode of attack was not
particularly refined because whereas Tyrannosaurus
could easily use its
tremendous bite force to kill with a bite to the back of a neck, and
Deinonychus
could stab a jugular vein with its sickle claw,
Allosaurus had little choice but to get physical
with its prey.
While
the above is a very generalist approach to attacking and dispatching
prey, there is room for more specialised feeding. The jaws of
Allosaurus could open extremely wide, in part due
to the weaker bite
muscles not resisting its opening so much. By opening its jaws wide,
Allosaurus could in theory use its jaws as a kind
of rasp to strip off
thin strips of surface flesh from its prey. Several bites like this
would have the effective of subjecting its prey to tremendous blood
loss, possibly to the extent of causing it to collapse. This would
go some way to explaining the solid skull construction and quite small
teeth. It could also support a flesh grazing theory where Allosaurus
could attack and feed from a large sauropod without actually killing it.
Regardless
of the actual strategy, good evidence for Allosaurus
being an active
predator comes from a fossilised vertebra that shows an impact
inflicted by a thagomizer tail spike from a Stegosaurus.
As a
Herbivore, Stegosaurus would have had no reason
to lash out against
an already dead Allosaurus as only a living animal
could have posed a
threat. Another Stegosaurus fossil shows a bite
mark to one of its
neck plates that seems to have been inflicted from an Allosaurus.
These fossil remains indicate that Stegosaurus may
have formed an
important albeit heavily armoured part of the Allosaurus
diet.
Regardless
of how active a hunter Allosaurus was, it would
almost certainly not
pass up the chance of a free meal when one was presented. There are
also significant fossil finds that also indicate that Allosaurus
regularly scavenged the carcasses of dinosaurs, possibly even members
of its own species.
Significant discoveries
Allosaurus
had a broad expanse that included much of the prehistoric USA.
However discoveries that lie further afield may actually represent
similar but different species of dinosaur. The only really accepted
remains of a non-American Allosaurus are found in
Portugal and are
referred to as A. europaeus. Another common
non-American species
that is often cited as Allosaurus is A.
tendagurensis from Tanzania,
although this material is fragmentary and thought by many to represent
another kind of Theropod.
Part
of the confusion for potential remains comes from the fact that
Allosaurus had a successful body plan that was seen
in a large number
of other Jurassic Theropods. Afrovenator,
a carnivorous dinosaur
from North Africa is very similar to Allosaurus
except for its slightly
longer forearms. Other dinosaurs such as Fukuiraptor
from Japan and
Australovenator
from Australia are so similar that they frequently have
comparisons drawn between them and Allosaurus.
Most
of the major discoveries pertaining to Allosaurus
have been made in the
USA, and more specifically the Cleveland-Lloyd Dinosaur Quarry in
Utah. This is but part of the large Morrison Formation that has
yielded many of the most important dinosaur discoveries, but at this
quarry is a huge bone bed of dinosaurs, of which at least over forty
Allosaurus individuals have been identified with
many more likely
awaiting discovery.
A
very interesting fact about the Cleveland-Lloyd Dinosaur Quarry is that
the predators outnumber the herbivores by a ratio of about 3:1.
Although this has raised the notion of pack hunting in dinosaurs such
as Allosaurus, a study of the site geology
reveals deposits of
mudstone, formed by a river system that created mud flats. It is
thought that herbivores became trapped in the mud, which in turn
attracted predatory dinosaurs intent upon feeding on the trapped
dinosaurs only to find themselves trapped.
This
means that the Cleveland-Lloyd site most likely represents an ancient
predator trap, one that has managed to preserve, among others,
several juvenile specimens helping to reveal the growth of
Allosaurus. The large number of remains,
although not indicative of
pack hunting, may be interpreted that scavenging formed an important
part of Allosaurus feeding strategy. This is
evidenced by the fact
that at least forty-four Allosaurus individuals are
known, where as
there are only about seven other sets of remains of carnivorous
dinosaurs, and these are all different species such as Ceratosaurus
and Stokesosaurus.
Amongst
the vast Allosaurus remains at the Cleveland-Lloyd
quarry one bone in
particular proved to be of very special interest. This was a tibia
(shin bone) that belonged to what must have been a female Allosaurus
as it shows the presence of medullary tissue. Medullary tissue is
found in the females of modern birds and is used as a calcium reserve
for egg shell production during ovulation. Another insight is that
the individual is estimated to have only been ten years of age,
meaning that she was no way near fully grown when she reached
reproductive maturity. The discovery of medullary tissue is not
unique to Allosaurus with medullary tissue also
being found in
Tyrannosaurus and the herbivore Tenontosaurus,
discoveries that further
establish the link between dinosaurs and modern birds.
In
1991 an Allosaurus that would be named 'Big
Al' was discovered
in Wyoming. With ninety-five per cent of the individual recovered
it was at the time the best preserved Allosaurus,
but even more
significantly Big Al showed signs of a startling number of injuries and
infections. These include damage to the ribs, toe bones and
vertebra, not just breaks but also subsequent bone infection known as
Osteomyelitis. This
is where microorganisms infect a bone that has been damaged by some
kind of trauma that has resulted in a break, which in turn are
attacked by the body’s immune system. The results can include blood
vessels in the bone becoming blocked by pus, in turn resulting in
bone tissue dying (a process called necrosis) and the body then
attempting to grow new bone around the dead tissue.
The
results of the infection are misshapen bones that can as in the case of
the toe bones impair mobility reducing the chance to effectively hunt
other dinosaurs. This could explain why Big Al was a sub adult, and
estimated to have only been eighty-seven per cent fully grown at the
time of death. A possible life scenario for Big Al formed the basis
for the television documentary special The Ballad of Big Al.
Inter species interaction
Conflicts
among Allosaurus individuals were almost certainly
common as they often
are among predators today. As in other predators these conflicts
would have usually been for the rights to feed at a carcass, or
mating rights, perhaps even by a potential mate that was not
interested in the attentions of the wrong individual. These
encounters can be seen in the injuries that appear to have been
inflicted by other Allosaurus such as teeth marks
to jaws.
Because
Allosaurus was probably the apex predator of its
time, and evidence
shows that it did not shy away from creatures equally as big as
itself. However many of the potential prey items were much bigger,
and these were the massive sauropods that roamed the Jurassic of North
America. When juvenile, a sauropod would have been easy pickings
for an adult Allosaurus, but it’s quite
inconceivable that even an
adult Allosaurus would have been able to take down
a fully grown
sauropod
like Brachiosaurus.
This
has led to the suggestion of pack hunting, with multiple Allosaurus
individuals attacking a single herbivore that otherwise would have been
too big to attack. The idea of pack hunting is a controversial one as
many people will say that dinosaurs didn't hunt in packs because you
don't see lizards teaming up to take down larger prey today. This of
course depends on how much dinosaurs were like lizards, not just in
biology but behaviour as well.
A
possible 'middle ground' for the idea that several Allosaurus
could
gather to attack a large sauropod is what’s termed 'mobbing
behaviour'. This is where rather than working to a coherent plan,
several individuals relentlessly attack a creature that has become
separated from the group and wear it down by attrition. When the
animal can take no more it collapses and the predators move in to it.
This behaviour has been seen in some birds, and since they are
considered to be direct descendants of the dinosaurs, there behaviour
patterns are more easily accepted when transferred to dinosaurs.
One
study that can be mentioned is the reaction of African predators to the
sounds of animals in distress. This study is often cited as support
for potential pack hunting in the sabre toothed cat Smilodon,
and was
centred on playing the recordings of prey animals that were in distress
like they would be when stuck in the mud. The
quick revelation for this study found that predominantly social and
pack hunting predators approached the sounds, while the solitary
predators mostly stayed away. It was theorised that this occurrence
came about from the fact the solitary, and usually smaller predators
knew that the animals in distress would be attracting larger and more
numerous predators to that area, and as such stayed away in order to
avoid becoming food for the other predators themselves.
Remember
that at the Cleveland-Lloyd Quarry, Allosaurus
remains vastly
outnumber the remains of other predators. Also, study of the
geology reveals strong evidence for an ancient predator trap that would
attract predators looking for an easy meal. Now
it would be very careless to claim that this is absolute proof of pack
hunting in Allosaurus, but what should be
remembered is that this
study is less about actual animal biology and more about the behaviour
and interaction between known solitary and pack hunting animals. It
should also be remembered that big mammalian predators are the only
animals that currently fill the ecological niches that large predatory
dinosaurs once did.
It
may well be that we will never be able to say the extent if indeed any
social interaction that Allosaurus individuals had
with one another,
as proposed theories are always down to the individual interpretation
of fossil material. It is only when the interpretations begin to fit
in with one another across multiple examples of evidence can we begin
to say things with even a small degree of certainty.
Further reading
- Notice of new dinosaurian reptiles from the Jurassic formation. -
American Journal of Science and Arts. 14 (84): 514–516. - Othniel
Charles Marsh - 1877.
- Notice of new dinosaurian reptiles. - American Journal of Science and
Arts. 15 (87): 241–244. - Othniel Charles Marsh - 1878.
- A new opisthocoelous dinosaur. - American Naturalist. 12 (6):
406–408. - Edward Drinker Cope - 1878.
- American Jurassic dinosaurs. - Transactions of the Kansas Academy of
Science. 6: 42–46. - Samuel Wendell Williston - 1878.
- Principal characters of American Jurassic dinosaurs. Part II. -
American Journal of Science. Series 3. 17 (97): 86–92. - Othniel
Charles Marsh - 1879.
- The dinosaurian genus Creosaurus, Marsh. -
American Journal of
Science. Series 4. 11 (62): 111–114. - Samuel Wendell Williston - 1901.
- Osteology of the carnivorous dinosauria in the United States National
Museum, with special reference to the genera Antrodemus
(Allosaurus)
and Ceratosaurus. - Bulletin of the United States
National Museum. 110
(110). - Charles W. Gilmore - 1920.
- Allosaurus fragilis: A Revised Osteology. Utah Geological Survey
Bulletin 109 (2nd ed.). - Salt Lake City, Utah Geological Survey -
.James H. Madsen - 1993.
- A reassessment of the gigantic theropod Saurophagus maximus
from the
Morrison Formation (Upper Jurassic) of Oklahoma, USA. - Daniel J. Chure
- 1995.
- A discriminant analysis of Allosaurus population
using quarries as
the operational units. - Museum of Northern Arizona Bulletin. 60. -
David K. Smith - 1996.
- On the presence of furculae in some non-maniraptoran theropods. -
Journal of Vertebrate Paleontology. 16 (3). - Daniel J. Chure &
James Madsen - 1996.
- The discovery of a nearly complete Allosaurus
from the Jurassic
Morrison Formation, eastern Bighorn Basin, Wyoming. - Brent Breithaupt
- 1996.
- A morphometric analysis of Allosaurus. - Journal
of Vertebrate
Paleontology. 18. - David K. Smith - 1998.
- Skull and tooth morphology as indicators of niche partitioning in
sympatric Morrison Formation theropods. - Gaia. 15: 219–266. - Donald
M. Henderson - 1998.
- Patterns of size-related variation within Allosaurus.
- Journal of
Vertebrate Paleontology. 19 (2). - David K. Smith - 1999.
- On the presence of Allosaurus fragilis
(Theropoda: Carnosauria) in
the Upper Jurassic of Portugal: First evidence of an intercontinental
dinosaur species. - Journal of the Geological Society. 156 (3). - B. P.
P�rez-Moreno, D. J. Chure, C. Pires, C. Marques Da Silva, V. Dos
Santos, P. Dantas, L. Povoas, M. Cachao & J. L. Sanz - 1999.
- Allosaurus, crocodiles, and birds: Evolutionary
clues from spiral
computed tomography of an endocast. - The Anatomical Record. 257 (5). -
Scott W. Rodgers - 1999.
-Theropod forelimb design and evolution. - Zoological Journal of the
Linnean Society. 128 (2): 149–187. - Kevin M. Middleton. - 2000.
- Observations on the morphology and pathology of the gastral basket of
Allosaurus, based on a new specimen from Dinosaur
National Monument. -
Oryctos. 3. - Daniel J. Chure - 2000.
- Forelimb biomechanics of nonavian theropod dinosaurs in predation. -
Senckenbergiana Lethaea. 82 (1): 59–76.Kenneth Carpenter - 2002.
- Multiple injury and infection in a sub-adult theropod dinosaur
(Allosaurus fragilis) with comparisons to allosaur
pathology in the
Cleveland-Lloyd Dinosaur Quarry Collection. - Journal of Vertebrate
Paleontology. 22 (1). - Rebecca R. Hanna - 2002.
- Paleoecological Analysis of the Vertebrate Fauna of the Morrison
Formation (Upper Jurassic), Rocky Mountain Region, U.S.A. - New Mexico
Museum of Natural History and Science Bulletin 23. - John R. Foster -
2003.
- Morphology, taxonomy, and stratigraphy of Allosaurus
from the Upper
Jurassic Morrison Formation. - Journal of Vertebrate Paleontology. 23
(3). - Mark A. Loewen, Scott D. Sampson, Matthew T. Carrano &
Daniel J. Chure - 2003.
- Sizing the Jurassic theropod dinosaur Allosaurus:
Assessing growth
strategy and evolution of ontogenetic scaling of limbs. - Journal of
Morphology. 267 (3). - Paul J. Bybee, A. H. Lee & E. T. Lamm -
2006.
- The large theropod fauna of the Lourinha Formation (Portugal) and its
similarity to that of the Morrison Formation, with a description of a
new species of Allosaurus. - Oct�vio Mateus, Aart
Walen &
Miguel Telles Antunes - 2006.
- Hindlimb allometry in the Late Jurassic theropod dinosaur Allosaurus,
with comments on its abundance and distribution, by John R. Foster
& Daniel J. Chure. In Paleontology and Geology of the Upper
Jurassic Morrison Formation. New Mexico Museum of Natural History and
Science Bulletin, 36. - John. R. Foster & Spencer G. Lucas
(eds).
- Jurassic West: the Dinosaurs of the Morrison Formation and Their
World. - Bloomington, Indiana:Indiana University Press. - John Foster -
2007.
- The case of "Big Al" the Allosaurus: a study in
paleodetective
partnerships. - Brent H. Breithaupt - 2007.
- Nuevos restos de Allosaurus fragilis (Theropoda:
Carnosauria) del
yacimiento de Andr�s (Jur�sico Superior; centro-oeste de Portugal) [New
remains of Allosaurus fragilis (Theropoda:
Carnosauria) of the Andr�s
deposit (Upper Jurassic; central-west Portugal)]. Cantera
Paleontol�gica. - Elisabete Malafaia, Pedro Dantes, Francisco Ortega
& Fernando Escaso - 2007.
- How big was 'Big Al'? Quantifying the effect of soft tissue and
osteological unknowns on mass predictions for Allosaurus
(Dinosauria:Theropoda). - Palaeontologia Electronica. 12 (3). - Karl T.
Bates, Peter L. Falkingham, Brent H. Breithaupt, David Hodgetts,
William I. Sellers & Philip L. Manning - 2009.
- Allosaurus Marsh, 1877 (Dinosauria, Theropoda):
proposed conservation
of usage by designation of a neotype for its type species Allosaurus
fragilis Marsh, 1877. - Bulletin of Zoological Nomenclature.
67 (1). -
Gregory S. Paul & Kenneth Carpenter - 2010.
- Variation in a population of Theropoda (Dinosauria): Allosaurus
from
the Cleveland-Lloyd Quarry (Upper Jurassic), Utah, USA. -
Paleontological Research. 14 (4).Kenneth Carpenter - 2010.
- Allosaurus fed more like a falcon than a
crocodile: Engineering,
anatomy work reveals differences in dinosaur feeding styles. - Ohio
University - 2013.
- Osteology of a large allosauroid theropod from the Upper Jurassic
(Tithonian) Morrison Formation of Colorado, USA. - Volumina Jurassica.
12 (2). - Sebastian G. Dalman - 2014.
- New insights into the lifestyle of Allosaurus
(Dinosauria: Theropoda)
based on another specimen with multiple pathologies. - PeerJ PrePrints.
3. - C. Foth, S. Evers, B. Pabst, O. Mateus, A. Flisch, M. Patthey
& O. W. M. Rauhut - 2015.
- Cranial anatomy of Allosaurus jimmadseni, a new
species from the
lower part of the Morrison Formation (Upper Jurassic) of Western North
America. - PeerJ. 8: e7803. - D. J. Chure & M. A. Loewen. -
2020.
- Notes on the cheek region of the Late Jurassic theropod dinosaur
Allosaurus. - PeerJ. 8: e8493. - Serjoscha W. Evers,
Christian Foth,
& Oliver W.M. Rauhut - 2020.
- High frequencies of theropod bite marks provide evidence for feeding,
scavenging, and possible cannibalism in a stressed Late Jurassic
ecosystem. - PLOS ONE. 15 (5): e0233115. - Stephanie K. Drumheller,
Julia B. McHugh, Miriam Kane, Anja Riedel & Domenic C. D’Amore
- 2020.