Re: Sauropod Necks

["Matthew Bonnan" <mbonnan@hotmail.com>]

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.

Matt Bonnan
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