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Re: Sauropod Necks
Stanley Friesen said:
Elephants have been seen rearing up to get at foliage. This sort of
behavior is not that energy intensive with an animal like _Diplodocus_ with
a properly placed center of mass.
Elephants do, on occasion, rear up to reach foliage. For reasons I have
previously outlined, we don't know for sure that Diplodocus and other
sauropods could rear up on their hindlimbs. How do you know that tripodal
rearing in a sauropod is "not that energy intensive"?
I have heard this before [grazing sauropods with horizontal necks], and
continue to find it unconvincing. What other
group of land animals has developed an analogous feeding mechanism? I can
think of none. To my mind this makes its value doubtful, as I would think
it would have evolved more than once if it were truly useful.
All well-known animals with long necks use them for one purpose - access to
elevated food sources.
Careful, careful, careful. I can think of no other group of graviportal
(elephants, etc.) animals that: 1) achieved the sizes of many sauropods; 2)
had a U-shaped manus in which all the metacarpals formed a vertical, doric
column-like structure; 3) are graviportal and yet have large claws on the
hand and foot; 4) have no olecranon process (regained in titanosaurs, but
different); 5) have extremely hollow and pneumatic vertebrae; 6) have tiny
heads; 7) have a semi-plantigrade hindfoot; etc.
There really are no good extant analogs for sauropods. This is the danger
implicit in any kind of analogy with an elephant, giraffe, etc. Plus, the
animals with long necks you site are mammals. There is an oftentimes an
unconscious bias toward using mammals as a yardstick for evaluating
locomotion, behavior, physiology, etc., in dinosaurs, and this has lead, in
my opinion, to overlooking interesting aspects of all dinosaurs. Why do
sauropods have this really weird hand that no other graviportal mammals, let
alone other dinosaurs, have? (I gave a talk on this at SVP last year and am
trying to get a paper out on it ASAP) Does the absence of an ossified
patella and other sesamoids mean that dinosaurs were inferior to mammals in
their locomotor abilites, or were they just different?
For the long necks: if sauropods are descendants of animals like
prosauropods (a very sticky systematics issue still!), perhaps becoming
bigger gives you a longer neck that later becomes exapted for more efficient
feeding, but perhaps these weird dinosaurs didn't evolve a long neck
initially for feeding purposes. Evolution often works by producing
structures that are later adapted to new functions -- there is often the
temptation to assume that the evolution of various structures is guided
purely by "need."
Again, who says that mammals have to be a yardstick by which we judge
dinosaurs? Mammals are derived in many strange ways themselves. Suggesting
that because mammals with long necks (giraffes) use them for high browsing
and therefore possession of a long neck in sauropods implies the same
feeding strategy is the mammalian bias writ large. We're dealing with
archosaurs that lived in an ecosystem without flowering plants, where
gymnosperms were dominant, and where numerous other, different evolutionary
factors were at work. We must always keep this in mind and be judicious
about how we choose analogies (if we do at all), and what parts of those
analogies we choose.
Finally, grazing with a long neck in sauropods may not have evolved outside
of dinosaurs, or within other groups of dinosaurs, due to the historical and
evolutionary constraints of anatomy. All mammals have only 7 cervical
vertebrae, and while the range of neck length and motion in mammals is
amazing, it pales in comparison to what is available in dinosaurs, and by
extension, birds. Perhaps this evolutionary program of 7 neck verts in
mammals has prevented long-necked forms from evolving. Or, perhaps
something about the Mesozoic ecosystem combined with dinosaur ancestry had
something to do with the success of sauropods. Or, because mammals have to
chew their food and have larger skulls and teeth, sticking a heavy head on
the end of long neck is not feasible. The big heads and grinding teeth of
many ornithischians may have also been a factor working against them
evolving long necks for grazing. Perhaps accidental consequences of size,
ancestry, ecology, physiology, etc., all came together to select for animals
such as sauropods. Analogies are limiting.
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