David Marjanovic wrote:
Well... veins have valves, so perhaps a little ectopic expression of
a few
genes...? But then perhaps valves are not necessary (of course the
maths on
this could be done pretty fast). Perhaps peristaltic arteries --
relaxing at
the base of the neck in each systole, then contracting in the
diastole,
starting a wave of contraction that propagated upwards -- would have
helped,
too. No more of a heart than the lateral hearts of an earthworm. :-)
Sauropods were quite a bit larger than earthworms, and it is important
to realise that the pressure at the base of a fluid column is in
proportion to the absolute vertical height above it. If a peristaltic
carotid artery were to function, it would have to interrupt this column
of blood and prevent the column from exerting its pressure at the heart
(i.e. the arterial walls would have to close completely, as in a
mechanical peristaltic pump). Like the appearance of valves in the
arteries, it is difficult to see how such an arrangement could evolve
from an artery with no peristaltic action. Intermediates (less than
full closure) would have no value.
On the size of giraffes:
Where does the commonly cited figure of 7 m come from? Was this an
overestimate, or a measurement of a distorted specimen? Or was it
real, like
the occasional elephant that is heavier than 12 t?
Some giraffes might get up just above 5 m and and would be considered
to
be within the normal range, but the average giraffe is less than that.
Maybe there is an odd giant for the record books, but you can bet that
its arterial blood pressure and the size of its heart would be
predictably larger. Compared to the length of sauropod necks, the
giraffe neck is quite short.
Compared to elephants, they did take the long-legged approach.
Compared to
giraffes they didn't, but then giraffes can and do run, while
sauropods were
graviportal like elephants. Perhaps it's significant that the
sauropods
already most commonly thought to have had vertical necks --
Brachiosauridae
(in the old sense at least) -- also had the longest forelimbs?
Having long forelimbs would help raise the heart of brachiosaurids, but
would not do much to reduce the arterial blood pressure of a long
vertical neck. If these animals held their necks horizontally, longer
forelimbs would increase the height of the browsing area in front of
them.