Dear Mr. Brown,
Your confusion over the relationships between dinosaurs and
birds and between elephants and ourselves seems to be due to the wide
misuse of the terms "knees" and "elbows" to describe the joints of
animals other than humans. (Unfortunately, since I am a
microbiologist and bacteria don't have legs (!), I cannot provide you
with the correct terms but allow me to continue...).
ALL limbed higher animals have the same basic bone and joint
structure - a ball and socket joint at the join to the body (our
shoulder), a single bone connection to a hinge joint (our elbow)
followed by a two-bone connection to a joint capable of limited
rotation and bending in all directions (our wrist) with a final set of bones
splaying out in different directions (our fingers).
Now, in some species, the relative sizes of each of these parts can
be very different from that of humans, so giving the animal a very
different appearance. In the case of birds, what you might call their
feet are, in fact, just their toes and their apparent backward 'knee' is
actually their equivalent of an ankle joint. They do have a real 'knee'
further up the leg but this is usually hidden by feathers - you
probably would recognise it as the 'drumstick'! (Check out
a picture of an ostrich to see what I mean).
The same is true for most mammals. I think the development of the
'standing-on-their-toes' limb may have something to do with speed -
humans and elephants are relatively slow-moving for their respective
sizes. So the peoplr at Trivial Pursuit are wrong - all mammals have
knees. Presumably they, like you, were considering a leg that looked
very like a human's in form but even then I think they may have
over-looked hippos and rhinos! (Of course, in Trivial Pursuit,
whatever's on the card is the only right answer.)
From the evolution point of view, then, you can see that the
development of 'backward-pointing' legs is achieved by the simple
elongation/shortening of the leg components through generations and
not by the unbelievable, as you pointed out, mehtod of 'flipping' the
entire leg anatomy.
I think that the NYC natural history museum need to reconsider the
effect of their display if it fails to explain such relatively simple
evolutionary changes.
Hope this clears things up,
Cormac Shaw
On Thu, 12 Sep 1996 19:25:22 -0500 (EST)
"BROWN,DAVID,MR" <BKJI000 at MUSICB.MCGILL.CA> wrote:
> On a recent trip to the NYC museum of natural history, I was
struck
> by a puzzling feature of the dinosaur display: the theme of the
exibit
> was one of evolution and how scientists use common physical traits
in
> different species to help determine common ancestry. My problem is
> as follows: I have always been told, not only by watching Jurassic
Park
> , that birds are decendent of certain dinosaurs and that most
mammals
> are decendent of small, scurriing rodent-like creatures that
appeared
> just before the dinos.dissapeared. Now almost all of the dinosuars
I
> saw at the museum walked on hind legs with joints pointing forward,
> i.e. knees, whereas birds walk on legs with joints that point
backwars,
> i.e. elbows. When did the first dinosaur that walked on elbows
appear?
> Who were its ancestors? Also on the mamalian front, elephants, or
so
> I have been told by the good people at trivial pursuit, and the
only
> animals with four knees. How then can we and the elephants have
any
> common ancestor? No slow, evolutionary change could have brough
about
> a transition from knees to elbows or vice versa. If we are related
> would the link have to be some creature without limbs? Is it
possible
> that several mammals appeared around the world without any relation
> to one another?
>> Any light that could be shed on the subject would be much
appreciated.
> Thank you very much.
>> David Brown,
> Montreal, Quebec
>>>>________________________________________________________________________
Cormac Shaw, BSc, |
Dept. of Industrial Microbiology, | e-mail: cshaw at acadamh.ucd.ie
University College, Dublin 4, | phone: +353 1 706 1796
Ireland. | fax: +353 1 706 1183
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"Under the most rigorously controlled conditions of pressure,
temperature, volume, humidity, nutrients and other variables,
the organism will do as it pleases."
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