Re: Gallery and Commentary for Copenhagen Mamenchisaurus

[Mike Taylor <mike@indexdata.com>]

> Date: Tue, 22 Jun 2004 21:08:48 -0400
> From: "Dr. Darrin T. Milne B.Sc., D.C." <dr.milne@sympatico.ca>
> 
> Thanks for the link below. I signed up. I read the article and from
> what I understand, the authors concluded that the structure of the
> facet joints suggests these beings held there necks in the
> horizontal plane and hence, were essentailly bovine-like in feeding
> habits.

Yup, that's about the shape of it.

> I have a question: what is the evolutionary advantage of having such
> a long neck when a shorter one could do the same job for ground
> feeders?

The classic paper on this is Martin 1987.  His conclusion is that
long-necked sauropods benefitted from the ability to graze/browse wide
areas without walking, by moving the neck from side to side.  He
guessed (there are no calculations in the paper) that this is more
energetically efficient than walking from tree to tree as extant
herbivores do.  But Paul (1988) argued (p. 188):

        It seems improbable that necks meters in length
        evolved in order to save a few steps towards a plant.
        The cost for 10-100 tonne animals to move 1m would
        only be 1.5-10 kcal (Fedak and Seeherman, 1979;
        Langman and others, 1995), whereas 1kg of fresh browse
        contains ~2000 kcal (see below).  In fact, a long neck
        reduces over all energy efficiency because the large
        respiratory dead space of a long trachea increases the
        cost of breathing (Daniels and Pratt, 1992).  On the
        other hand, sauropods could fully exploit their energy
        expensive necks only if they regularly used them to
        reach plant energy found in tree crowns.

I find this persuasive; but nowhere near as persuasive as it would be
if the respiratory and other energetic costs of a long neck were
quantified and compared with locomotory costs.  Unfortunately, it's
hard to get figures -- even approximate figures -- for these costs.

So the basic problem of sauropod neck posture is that the morphology
seems to tell us one thing, whereas energetic and evolutionary
considerations tell us another.  If you can resolve this
contradiction, you will make me a happy man.  One possibility is just
that Stevens and Parrish's program is producing incorrect results (I
am computer programmer in my day-job, so I know how hard it is to
shake all the bugs out of a programme).  But Martin (1987) figured a
similar range of postures for _Cetiosaurus_ from first principles, so
that's not likely.

> I do not know of an analogous animal in present day that carriers a
> long neck horizontal to the ground.

Neither can I.  One of the great difficulties of working on sauropods
is the lack of extant analogues.  There is just nothing like them
around today.  The best you can do for envisaging sauropod necks is
probably a combination of an ostrich (which is somewhat related and
shares some anatomical features) and a giraffe (which is less
pathetically undersized).  But really, there are limits to how much a
2m giraffe neck can tell us about an 11m _Mamenchisaurus_ neck.

> I could only imagine a horizontal neck in a water dwelling
> creature. But, I think current research suggests there creatures
> were mainly land dwelling yes?

Yes, that's pretty much a done deal.  There are at least three lines
of evidence.  First, there's all the sauropod tracks that were clearly
made on land.  Then there's the fact that sauropod feet, while
absolutely large, are small relative to mass, so that they would have
sustained about twice the pressure of those of domestic cattle
(Alexander 1989), and would surely have become mired in mud.  And
Coombs (1975) makes a strong biomechanical argument (too complex to
reproduce here) that the deep, relatively narrow torsos of sauropods
are an adaptation for carrying weight in terrestrial locomotion.

> In any event, thanks so much for the article and for the
> information.

No problem -- it's always a pleasure to talk sauropods.


References
----------

Alexander, R. McN.  1989.  Dynamics of Dinosaurs and Other Extinct
Giants.  167 pp.  Columbia University Press, New York.

Coombs, W.  1975.  Sauropod Habits and Habitats.  Palaeogeography,
Palaeoclimatology, Palaeoecology 17: 1-33.

Daniels, C. B. and J. Pratt. 1992.  Breathing in long necked
dinosaurs; did the sauropods have bird lungs?  Comp. Biochem. Phys.,
101A, 43-46.

Fedak, M. A. and H. J. Seeherman.  1979.  Reappraisal of energetics of
locomotion shows identical costs in bipeds and quadrupeds including
ostrich and horse.  Nature, 282: 713-716.

Langman, A. and others.  1995.  Moving cheaply: energetics of walking
in the African elephant.  J. Exper. Biol., 198: 629-632.

MARTIN J. (1987): Mobility and feeding of Cetiosaurus (Saurischia:
Sauropoda) - why the long neck?.  Occasional Papers of the Tyrrell
Museum of Palaeontology #3 (Fourth Symposium on Mezozoic Terrestrial
Ecosystems, Drumheller). pp150-155

Paul, G.S. (1998). Terramegathermy and Cope's Rule in the land of
titans. Modern Geology 23:179-217.

--

P.S.  I don't have the Daniels and Pratt, Fedak and Seeherman or
Langman et al. papers -- I've just copied the references here from the
list in Paul 1988.  If anyone has spare copies of these that they'd
like to send my way, they would be very much appreciated.