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MORE ON SAUROPOD FEEDING HIGH & LOW



1 - There is a common belief that the long necks of sauropods evolved
to make it easy for them to eat large amounts of low plants without
expending the energy needed to move their vast bulk. This view is
incorrect because the cost of locomotion is remarkably cheap in giant
animals (as confirmed by recent measurements on walking
elephants). Sauropod necks were 3-10 meters long. The cost of moving
animals of 10-100 tonnes on land is just 1.5-10 kcal per meter, and
the amount of calories available in just one kilogram of fresh browse
is about 2000 kcal. Therefore, the energy expended by a giant
herbivore to move a few meters to reach a bush is incidental compared
to the energy to be gained from that bush.  (Factoid - One kcal = one
human dietary calorie, so next time you wolf down that diet yogart
realize that you are consuming 250,000 calories!)
       The necks of sauropods were extraordinary in length and
complexity.  Extraordinary selective pressures were necessary for
evolution to produce such necks. Avoiding burning a few kcals to move
a few meters is not an adequate explanation. The only logical
explanation for the extraordinary length of sauropod necks is to give
them the ability to reach beyond where short necks could, which is
only upwards. Apatosaur necks were by no means short by normal
standards. Thet were amazingly stout, indicating great strength.
        Also, the only other big land herbivores with very long necks,
giraffes and some extinct ungulates, were or appear to have been high
browsers. 

2 - If diplodocids were mainly low browsers, then they should have
worn down their teeth with lots of the grit that covers short, near
ground plants. However, electron scanning study of diplodocid teeth by
Fiorillo found that diplodocid teeth show little wear from grit, less
than that seen in shorter necked camarasaurs. This suggests
diplodocids fed mainly in the crowns of trees.

3 - It is difficult to see why the sled chevrons of diplodocids were involved
in supporting the tail when they are on the underside of the tail. The
primary support strutures should be dorsally located where they work under
tension, not ventrally where they are under compression. Nor does tail
support explain the presence of sled chevrons in stump tailed ground slothes.
A better explanation for sled chevrons in sauropods and slothes is as
protection for the underside of the tail during habitual rearing. 

4 - Brachiosaurs were strictly high browsers. Their rarity in the
Morrison Formation may be due to the absence of a large scale forest
canopy (their abundance in Tendaguru suggests that tall trees were
more abundant).  Camarasaurs were more common in the Morrison. They
appear ill suited for ground cover feeding because of their erect
necks, but their relatively short stature suggests they fed on an
abundance of medium sized riverine trees and taller shrubs. They could
also high browse by standing and even walking bipedally. The
diplodocids were the most trophically flexible sauropods in the
Morrison. Their ability to stand (but not walk) tripodally, and long
necks suggest they got most of their food from the medium to taller
riverine trees (8-20 m), which were the most dependable source of food
because they tapped into ground water available year round. However,
during the wet season flush of ferns on the extensive prairies,
diplodocids were able to feed on the abundant food source. This may
explain why diplodocids were the most abundant sauropods on the
Morrison. An absence of 100 m tall trees in the Morrison is not
important because no sauropod was that tall.  Likewise, giraffes often
live on grass plains with scattered medium sized (3-8 m) trees. In
fact, giraffes and elephants inhabit the Namib desert, were the only
browse is riverine trees.

GSPaul