[net.origins] Large animals and gravity

friesen@psivax.UUCP (Stanley Friesen) (09/03/85)

	I went to the UCLA library Saturday and traced down a few of
the references I have mentioned in the last few weeks.  Unfortunately,
one of the journals was at the bindery, so I couldn't find all the
articles I was looking for.  I have therefor decided to post the
references as a series of articles, with a mini-review of the more
important ones.

Alexander, R.M, 1985.  Mechanics of posture and gait of some
large dinosaurs.   Zoological Journal of the Linnean Society 83:1-25

	This article covers alot of ground. It gives estimates of the
mechanical factors influencing dinosaurs and how htese would effect
the dinosaurs in life.  Several of the factors bear on the issue of
whether or not suvh animals could have supported themselves on land
and moved at reasonable speeds. Dr Alexander himself does not actually
address these issues, he treats them as if they have already been
solved.  There are several references mentioned in his bibliography
which *do* address these issues.  In fact the bibliography is a very
good one, and is worth some attention.
	The first calculation on the support capacity of the dinosaur
skeleton is the calculation of the stress components on the vertebral
column. It is interesting to note that the estimated intervertebral
compression for the Sauropods("Brontosaurs") is *one* *third* of that
generated in the *human* backbone by our vertical posture! Thus the
backbone could *easily* support the weight required of it.
	In another section, where he is trying to get some idea of
dinosaurian gait(rate and phase of limb movement - not actual speed),
he calculates what I will call a limb load factor. This number is in
effect the amount of weight a limb *could* support divided by the
amount of the actual weight of the animal it *is* supporting.  Thus a
load factor of one would indicate that the limb in question could
support the required portion of the animals weight, but *nothing*
else.  A lower factor would indicate insufficient strength to support
the weight, and a higher value would indicate excess capacity.  Dr
Alexander uses this to estimate the maximum pace for some dinosaurs.
This works because  during locomotion excess stress is placed on the
limbs when they hit the ground on the down stroke &c.  This excess is
greater the faster the legs are moving, thus a galloping animal
requires a much larger load factor than a standing animal, and even
walking will require a factor somewhat larger than 1.0.  An Elephant
has a load factor of *aproximately* 10 and can go at a "slow",
shuffling gallop(just a touch faster than a walk).  The *lowest*
reported estimate for any dinosaur is *three*, which is plenty large
enough for walking on land, and Dr Alexander estimates the load factor
for the *same* form(Diplodocus if I remember correctly) at *five*.  It
is interesting to note that the load factor for the heavier Sauropod,
Apatosaurus(ex Brontosaurus), is essentially the same as that of an
Elephant! Thus there is no reasion to assume that the low load factor
for Diplodocus is due to weight, it appears to be due to its lightness
of build(that is assuming my memory is right).  The basic result is
that even the larger Sauropods have sufficient limb support to walk
about on dry land, and in many cases to even gallop a little.  The
load factor for Tyrannosaurus is also about the same as an Elephant,
thus the statement by someone on the net that it couldn't exceed some
ridiculously low speed without breaking its legs is also baseless. And
Triceratops had a sufficiently large load factor for full scale
galloping, despite being as heavy as an Elephant.  Of course, as Dr
Alexender points out these pace estimates assume that dinosaurs used
the same safety factor as a modern Elephant, which is certainlyy
reasonable in the case of Sauropods, but may well need to be revised
for things like Tyrannosaurus.

	Well, this seems to have come out longer than I intended! I
think I had better stop now.
-- 

				Sarima (Stanley Friesen)

UUCP: {ttidca|ihnp4|sdcrdcf|quad1|nrcvax|bellcore|logico}!psivax!friesen
ARPA: ttidca!psivax!friesen@rand-unix.arpa