Pack Hunting Dinosaurs?
When
the first carnivorous dinosaur remains were discovered they were found
as isolated individuals, occasionally in close proximity to
herbivorous dinosaurs that may have been their prey. This gave rise
to the notion that the dinosaurs were solitary hunters in a similar
fashion to some of the predators today such as tigers and bears.
However new fossil discoveries around the world combined with modern
study of these ancient predators has steadily eroded the acceptance of
this notion.
Slowly,
more fossil sites are being discovered which show the presence of more
than one predator in that location. In 1931 Barnum Brown was
recovering the remains of the dinosaur Tenontosaurus
when another
fossil specimen of a carnivorous dinosaur which would later be named
Deinonychus
was also discovered. But here it wasn't the discovery of
a new predatory dinosaur that was important but the fact that a total
of five individuals of the same species were discovered around the same
Tenontosaurus skeleton.
It
was not until the late 1960s that the potential importance of this
site was realised when the fossil material was studied by a team led by
John Ostrom. Furthermore, the site would prove not to be unique,
with other Tenontosaurus and Deinonychus
remains (usually the teeth
of Deinonychus) being discovered together. It
is certain that
Deinonychus fed upon Tenontosaurus
but the questions that beg asking
are did Deinonychus make the kill? and if so how
could such a small
predator kill such a large herbivore?
The
quick answer is that as an individual hunter Deinonychus
couldn't.
But as a pack? Immediately the tantalising possibility that some
dinosaurs were pack hunters was too much to resist, and the notion
quickly spread throughout the world. However this idea was not and
still is not universally accepted by all. One answer is that
Deinonychus was not involved in the killing but
merely scavenged the
kill of another predator. This has also given rise to the comparison
of a feeding hierarchy, as seen in modern day Komodo Dragons (Varanus
komodoensis).
Komodo
Dragons do not form groups and remain solitary creatures throughout
their lives. As well as being powerful hunters they are also known to
search out and eat carrion, something that they may be able to smell
from over five miles away. This means that several Komodo dragons can
and often do approach the same carcass, and in this situation a
hierarchy of the largest and strongest eat first while the smaller ones
wait for them to eat their fill and leave the carcass. However, if
another Komodo Dragon challenges the other then a fight ensues for
dominance of the meal. Although not always fatal, the loser can be
so badly injured that it is killed and actually ends up getting eaten
by other Komodo Dragons.
In
this analogy, The Deinonychus remains discovered
with the
Tenontosaurus represent the killed losers that
challenged for a right
to feed at the remains. The problem with this kind of comparison is
that you are applying animal behaviour that has been studied as it
happened, in a species that is well known to us, to an animal that
died out 108 million years ago and is only known to us from bones.
While a good theory and a plausible one, it is not an absolute
answer that can be applied to either Deinonychus or
other predatory
dinosaurs as a whole.
Another
possible answer is that of a predator trap. There are many kinds of
predator trap but they essentially work upon the same principal; An
animal dies or becomes trapped in a hazardous area such as a tar pit.
Predators then come to feed upon it but end up getting trapped
themselves. A third answer is that they were all killed by some event
such as a flood or landslide, resulting in their fossilisation
together.
The
larger Theropods
Deinonychus
is not the only carnivorous dinosaur that has several of the same
species preserved together, in fact it is just one of a growing
number of carnivorous dinosaurs that have been preserved in what appear
to be groups. A mixed bone bed consisting of five hadrosaurs and
three Daspletosaurus
individuals of different ages were found together
at the Two Medicine Formation in the US state of Montana. As evidence
that Daspletosaurus hunted together, it is not
great, in fact it's
a weaker argument for the support of dinosaur pack hunting than that
provided by the multiple Deinonychus remains.
This is mainly because
the remains of herbivorous dinosaurs outnumber the remains of the
predators making it unlikely that they were killed by the Daspletosaurs.
Geological
study of the site indicates that the dinosaurs were not washed there by
a river system, and teeth marks on the hadrosaur remains akin to
those made by Daspletosaurus teeth proves that the
hadrosaurs died
first. Further, the hadrosaur bones are well scattered indicating
that their bodies had been thoroughly pulled apart. It is unknown for
certain how the hadrosaurs died but they did die together, and if
they did not drown, they may have been killed by something else they
could not have run from.
The
late Cretaceous was a time of high geological activity with the
continental plates breaking up and sliding into one another. While
everyone knows about volcanoes blowing their tops and spewing lava all
over the place, many fail to realise that this is but a final outcome
of the geological stresses that take place deep underground. Just as
dangerous, if not more so than the eruption itself is the release of
toxic gasses such as carbon dioxide among others.
The
five hadrosaurs may have been in a geologically active area when they
were caught in a sudden release of carbon dioxide. This could have
deprived the hadrosaurs of oxygen for anything from a few minutes to
even hours causing them to suffocate. Afterwards, once the carbon
dioxide had cleared and the air content returned to normal, roaming
Daspletosaurus may have found the carcasses and
began eating them only
to find themselves caught in a second discharge of carbon dioxide.
A
possible second explanation that relies upon the conjecture that they
were indeed hunting together is that the Daspletosaurs
had killed the
five hadrosaurs in similar manner to a modern day fox that will kill
all of the chickens in a coop even though it can't possibly eat all of
them. This is done so that the fox knows where to find an easy next
meal, and is why foxes usually return to areas where they have killed
but not eaten all of the birds, it simply prevents their prey from
moving on.
In
all, the remains of the Daspletosaurs in this
instance do not greatly
support the theory for pack hunting in dinosaurs. But there is
another bit of interesting evidence to be found in Daspletosaurus,
and other tyrannosuarids such as a Gorgosaurus
and Tyrannosaurus
itself, and these are bite marks that sometimes appear on the faces.
Intraspecific (as in members of the same species) biting of the
face is often seen in other animals, especially when carnivores are
establishing dominance over one another. The teeth marks found in
some of these skulls have been well enough preserved to indicate that
they have healed after they were inflicted indicating that they were
not necessarily meant to be fatal.
Predator
only bone beds
The
re-discovery of a bone bed in 1997 would lead to much better
evidence suggesting that the larger theropods could form packs.
Originally discovered in 1910 by Barnum Brown, the site was not
too well documented save for a mention of several tyrannosaur remains
found together. After its mention in 'The National Geographic
Magazine', the site along with its location faded from memory. It
would not be until Dr Philip Currie, who was looking for evidence to
support the idea of pack behaviour in carnivorous dinosaurs that the
site was remembered. The problem was that the best that Dr Currie and
his team could hope for was that the bone bed was somewhere along the
Red Deer river in Alberta, Canada. Luckily, a few photographs
remained of the site and after painstakingly searching the area, the
tyrannosaur remains were discovered.
Now
quite commonly known as the Dry Island Bone bed, the exact dinosaur
species was Albertosaurus,
a tyrannosaurid of the late Cretaceous.
Most importantly it was the only species to be recovered from this
bone bed with up to twenty-two individuals of all ages being discovered
together. This was the first (albeit for the second time) major
discovery to represent possible pack behaviour in dinosaurs.
Critics
of the site have cited the concentration as evidence of a predator
trap, or the 'Komodo Dragon' theory. Another idea is that
environmental factors forced the predators together, such as
gathering around a watering hole during a drought. Regardless of the
interpretation of the fossil material, it still remains good evidence
to support the predator pack theory, with multiple individuals, all
different ages, only one species, just like you get with pack
hunting predators today.
Albertosaurus
would prove to be not the only predatory dinosaur to contribute to the
pack theory. Down in Argentina, soon after the rediscovery of the
Dry Island bone bed, a second bed was discovered. Excavated by Dr
Rodolfo Coria and Dr Philip Currie, they not only identified a new
carnivore, Mapusaurus,
they found remains that belonged to up to
seven individuals in the same spot. Again, there was only one type
of dinosaur, and all the individuals in the group were all different
ages when they died.
Footprints
and Track ways
Those
who hold firm to the belief that dinosaurs did not hunt in packs often
point to the existence of fossilised track ways. These track ways
exist all over the world, sometimes left by individuals, there are
also good imprints that show entire herds of herbivorous dinosaurs not
only walking together but changing direction together as well,
proving the herbivorous dinosaurs at least did live in groups. But
tracks left by predatory dinosaurs have always been known to be
solitary.
However,
new discoveries are even casting doubt onto this method of
interpreting dinosaur behaviour. In 2007 a track way was
discovered in China that shows what appear to be several dinosaurs
similar to Deinonychus moving together as a group.
Although not
evidence of pack hunting, it could prove precious evidence that it
was not only the herbivores that moved in groups.
Definition
of pack hunting
Going
on the assumption that at least some of the predatory dinosaurs did
hunt in packs, it would then bring the question, to what extent?
One possibility is that there were no defined groups as such, more a
collection of predators that stalked the fringes of a herd. Driven by
hunger some might have charged at a herd of dinosaurs trying to break
an individual off from the rest. As the prey dealt with one
predator, a second predator might have moved in from the side to
deliver a fatal or crippling wound. This could be called a form of
accidental cooperation as the predator that delivered the decisive blow
did so out of opportunity rather than planning. How the kill is then
divided amongst others could be a case of largest and strongest first,
much like the aforementioned Komodo Dragons.
Going
further than this would bring in the concept of advanced pack behaviour
including cooperative hunting and social structure within the pack.
This could mean a dominant individual or pair within the group with
the other members being subordinate to them. It could also mean
hunting strategy, perhaps with the younger pack members breaking up a
herd and driving an individual towards an older and larger pack member
that was better equipped to make a kill.
This
behaviour is seen today in packs of wolves or prides of lions, and
while they are mammals not reptiles like the dinosaurs were, mammals
are the current dominant life forms on this planet, that fill the
same roles that would have once been taken by dinosaurs. While it is
easy to draw an analogy between the dinosaurs and modern lizards on the
basis that they are both reptiles, lizards today do not have to take
down big game like the reptiles of the Mesozoic. It is worth
remembering that assuming the dinosaurs were just like lizards led to
grossly incorrect physical reconstructions that were not even close to
how they really were. Assuming they were like modern lizards, when
many of the dinosaurs filled ecological niches that modern lizards do
not, can be equally misleading when reconstructing the past.