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Re: Parrish's neck work ...



Hi Ronald:

You said: "I am curious that no one has mentioned rearing here.  If
any sauropods
could rear, there would be no need for them to flex the neck upward
to any
degree as the rearing would do that for them - but a considerable
degree of
ventriflexion might be extremely useful to a rearing sauropod trying
to get
at some tasty foliage on the far side of a tree.  Of course, if
rearing was
part of their behaviour then Parrish's discovery does not mean that
they
could not feed in trees - only that they had to rear to do it."

You bring up a good point.  It's difficult to know whether or not
sauropods could rear.  And yes, Parrish's work does not eliminate the
possibility that some sauropods reared up on their hind limbs. 
However, based on what I've seen (I mostly study diplodocid and
camarasaurid sauropods), it's tough to imagine that if these animals
did rear how they did it.

Let me explain a little bit further what I mean.  My opinions aside,
here is what we know about diplodocids (the sauropods usually
considered to be rearing):

1. These animals have elongate, horse-side heads with small,
pencil-shaped teeth.

2. The forelimbs of diplodocids are significantly shorter than their
hindlimbs, so that the animal's  shoulders were below the hips.

3. There are tall neural spines on the vertebrae which are
concentrated at the hips, above the portion called the sacrum.

4. The femur, like most dinosaurs, has a landmark called the fourth
trochanter -- this a small ridge located about halfway down the femur
in sauropods.  In dinosaurs like T. rex, it is located closer to the
head of the femur.

5. The tail is elongate, has sled-like chevrons on its underside (in
case you don't know what chevrons are, they are the bony spines that
protrude below the tail vertebrae that protect the delicate arteries,
veins, and nerves), and, where complete tails are available, a
whiplash-like tail end composed of several, finger-sized vertebrae.

Arguments for or against rearing in sauropods center on these
characters.  Let's start with the sacral spines, as these are usually
cited as evidence for rearing.  The assumption is that a large nuchal
ligament ran off the sacral region, across the back, and inserted
itself on the neck or back of the skull.  If you're unfamiliar with
what a nuchal ligament is, it is a rubber-band like tissue which in
mammals runs from the shoulder vertebrae to the back of the skull.  It
is a passive tissue.  That is, like a rubber band, it can only store
energy and release it, but it cannot generate energy on its own.

Let me give you an example.  A buffalo has a very large head which
would be difficult to lift up if it dropped its head down to graze. 
When it does this, the nuchal ligament in its neck stretches and
stores the energy imparted to it by the neck and shoulder muscles. 
When the buffalo relaxes its neck muscles, the nuchal ligament
"springs" back, releasing the stored energy and helps to bring the
buffalo's head back up.

How a passive tissue could generate enough energy to lift the entire
front end of a sauropod off the ground has never been investigated. 
There is a lot of speculation on how or why it might work, but so far
there have been no quantitative models or studies of this.  Why else
would you have tall neural spines over the sacrum?

Perhaps (but this has not been investigated either) to hold up that
long tail.  From the footprint evidence, we have never found a tail
drag mark from sauropods.  The use of nuchal ligament in the tail
would seem to make sense, and the height of the sacral spines would
give it a good angle of insertion on the tail.  BUT this is still
speculative -- someone has yet to do a model or test or whatever.  I
provide it only as another alternative to the height of the neural
spines.

The forelimbs are shorter than the hind limbs.  This does not
necessarily guarantee rearing either, because it can be argued (with
an equal amount of speculative energy) that the lower forelimbs got
the head closer to the ground, where grazing on ferns was possible. 
Either way, the shorter forelimbs also have ties to the evolutionary
past of sauropods.  Early saurischian dinosaurs have shorter forelimbs
than hindlimbs, and the condition seen in diplodocids may just reflect
this and have little to do with evolving towards a rearing condition.

The elongate heads with their pencil-like teeth are very weird. 
These animals did not chew, but swallowed whatever they ate whole,
because unlike mammals, they have no molars, no canines, etc.  It's as
if your mouth were filled entirely with pencil-thin incisors.  The
best you could manage would be to snip off things and swallow them. 
These teeth don't instantly strike someone as being able to handle
tough, woody foods or even some of the tougher piney plants.  Again,
even if the sauropods get their heads up in those trees, how they were
used and what food was available for them to break off and swallow is
difficult to know.

The fourth trochanter is the site of attachment for a big tail muscle
called the Caudofemoralis longus, which was the prime mover of the
thigh when these animals walked (as it is in living alligators and
crocs today).  How this muscle would have been affected when the tail
it was attached to was bent away from the femur has never been
investigated.  Again, we need more information.  And, even if you
could rear up, the animal couldn't be very mobile and it would
unsteady, even with a "tripodal" tail pose.  And how many kilocalories
are available to it when it gets up in the trees?  Is it spending a
lot of energy for very little foliage?  Why couldn't a sauropod just
knock down a tree and eat it that way?  There are no good answers to
these questions right now, but speculation abounds.

The tail and its sled-like chevrons have been argued to have
supported these animals, with the chevrons providing a "base" for the
tail during rearing.  Again, no models or quantitative study has been
conducted along these lines.  Plus, many of the Chinese sauropods have
sled-like chevrons, like Mamenchisaurus and Shunosaurus.  Not too many
people have argued that these animals reared up, probably because some
of them have club-like tails which may have served as weapons. 
Perhaps the sled-like chevrons protected the tail blood vessels and
nerves when the tail was violently thrust side to side.  Could the
whip-like tails of sauropods have chevrons for the same reasons? 
Again, speculation abounds but these are great questions waiting for a
modeling and functional morphology study.

But, didn't sauropods rear to mate?  Not all vertebrates rear up on
their hind legs to mate.  This assumes that male sauropods had a large
single or hemi-penis which could only be inserted into the female by
mammal-like mating rituals.  Some birds just touch for a second to
transfer sperm.  In some mammals, like certain cammels, the female
lies sideways on the ground while the male mounts her from the side. 
Again, another great area for investigation.

These are weird animals.  The temptation is great to compare them to
elephants and giraffes (because that is all we have alive today), but
we must be extremely careful not to let our generalizations go too
far.  Saruopods are huge, strange, saurischians.  Even their
relatives, the birds and crocs, are quite derived.  This is why we
desperately need good functional studies that go beyond the vast
generalizations and analogies to living vertebrates.

I hope this helped you and some others.  The rearing question is not
settled, and it will take many more years of research to gain a better
understanding of just what those weird and loveable sauropods were
doing.

Sincerely,
Matt Bonnan
Dept. Biological Sciences
Northern Illinois University