>>>look at the "neck" topic.
By PZ Myers
Imagine a long-necked animal.
Most people will, I suspect, picture a giraffe. Other likely candidates are
swans or long-necked dinosaurs or plesiosaurs; many vertebrates have evolved
long, relatively flexible necks, the better to reach food that is otherwise out
of reach, or is more easily captured with a mobile head on a flexible stalk.
Now picture the anatomical features that produce that long neck. That's often
more difficult if you haven't poked around in comparative anatomy, but
basically, the neck has a core structure of bony vertebrae stacked like spools
along its length, each one separated from the other by a joint.
This series of joints is what gives the neck its flexibility and, as you might
guess, the more numerous the joints, the more flexible the structure would be.
One simple question to ask is how many vertebrae are present in the neck of
these various animals? It's easy to count; just tally up the vertebrae from the
base of the skull to the first vertebrae that bear ribs. The ones that lack
ribs are the cervical (a fancy word for "neck") vertebrae, and the first ones
that have ribs are the thoracic ("chest") vertebrae. Easy, but we get one
surprising result.
Plesiosaurs, those aquatic reptiles of the Mesozoic, are impressive: Some
species had about 40 cervical vertebrae. Modern birds also have
representatives with numerous cervical vertebrae, up to 25 in swans.
Diplodocids, the characteristic long-necked herbivorous dinosaurs, had 12 to 13.
Giraffes have seven.
Even stranger, imagine any short-necked mammal-a dolphin, a mole, a cow, a
human being-and you get exactly the same answer: Typically, they all have just
seven vertebrae, no matter how many millimeters or how many meters long their
neck might be. (There are some obscure exceptions: Manatees and two-toed sloths
have six cervical vertebrae, and three-toed sloths have nine. Otherwise, seven
is the rule for mammals.)
_______________
Judy
http://icehorses.net
http://clickryder.com
