[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Fuel for another debate

Regarding the question: why were T.rex's arms so small?

swf@elsegundoca.ncr.com         sarima@netcom.com       wrote:
> There are several ideas, none of them much better than speculation.
> It has been suggested that they were used to brace T. rex when
> he got up from lying down, for instance.
> *My* personal speculation is that the short arms allowed T. rex
> to restrain potentially dangerous prey more rigidly (shorter
> arms means less freedom of movement).  In particular, I suggest
> that T. rex was specialized to eat Triceratops, which it hunted
> by gripping it at the base of the neck to prevent its using its
> horns while it was being killed.

Evolutionary tautology: tyrannosaurs had small arms because they
didn't need bigger ones!

Compare tyrannosaurs to dromaeosaurs (Deinonychus, Velociraptor,
Troodon) which had fairly long arms and 3-fingered hands, which are
fairly obviously useful as grappling hooks to hang onto the bodies of
larger prey while slashing with the scimitar claws on the hind feet.

Proto-tyrannosaurs mustn't have used their hand-claws that way.
They lost their multi-fingered grasping hands in favour of simple
2-pronged hooks, and lost the pulling, hugging arm articulation
in favour of one suited for pushing something away from the chest,
or down toward the feet.

The terror birds (Pharopharcos etc.) of the early Tertiary had no
arms at all, and did pretty well as bipedal predators.  A recent
SciAm article described how the closest living relatives of the
terror birds hunt: run the prey down, seize it in the beak,
thrash it around to snap the spine.

Perhaps regular-sized arms got in the way when thrashing an
Edmontosaurus by the neck.

On the other hand, tyrannosaur arms didn't dwindle away to nothing,
and analysis of the joints, tendon attachment points, and estimated bone
strength shows that they were very strong for their size, and therefore
must have been useful, even essential, for something important to the
animals' way of life.

Even the `smaller' tyrannosaurids (Albertosaurus, Gorgosaurus are big
enough for me!) had similarly proportioned arms to T.rex, so I doubt
it's a specialization to fending off Triceratops horns/shields, unless
a whole range of mini-ceratops had exactly the same defense mechanisms
and proportions.  And anyway, wouldn't the more numerous, non-armoured
duckbills be a more attractive prey?

One idea is that tyrannosaurs might have killed by tackling their prey
from behind, straddling and pinning the victim with its body while biting
its victim's neck and head (clean off!?).  Perhaps a pair of tiny
but strong arms could pin the prey under T.rex's body and keep it centred
while the killing bite was given.  Is that necessary and sufficient to
explain this aspect of their anatomy?

Don't forget `Deinocheirus' -- known only by its huge, razor-clawed
multi-dactyl hand and arm.  Is it likely to come from a T.rex - sized
dromeosaur?  How did it get so big, using the grasping-hand architecture,
if T.rex could be more successful/numerous using the tiny-hand architecture?

How about another, possibly related question: why were tyrannosaurs' mouths
so huge and toothy?  Of presently-living animals, few have jaws so large
in proportion to the skull and rest of the body.  Various deep sea fish,
frogs, crocodilians and baleen whales are about it, as compared to the
mammalian carnivores and even sharks's proportionally smaller jaws.
Why wouldn't tyrannosaurs have been able to kill using a smaller gape
and fewer teeth, and then bit off smaller chunks at their leisure?

Is the size of the jaw an indicator of some essential aspect of their
lifestyle?  Perhaps taken together, a big mouth and small arms are clues
to an important feature of their way of life.

Mike Bonham        bonham@jade.ab.ca      Jade Simulations International

``Organization is the enemy of improvisation.''-- Lord Beaverbrook