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A loooong message regarding sauropods



Nick Longrich wrote:    

>       "Another very interesting feature, not absolutely
> unique to sauropods but unusual, is the evolution in the last half of the
> back of a bony projection on the center of the rear face of the arch that
> fits neatly into a groove on the front of the arch of the succeeding
> vertebra (hyposphene-hypantrum articulation). This virtually locks the
> vertebrae together and provides great stability". Just ran across that
> looking for the pleuocoel ref.in McIntosh's segment in "The Age of
> Dinosaurs: Short courses in Paleontology".
        
        Hyposphene/hypantra articulations have been noted for quite
awhile amongst sauropods (check out Cope quotes in Osborn and Mook
1921, in fact I believe Cope may have even coined the term!) and
appear in different positions in different taxa (and some not at all).
Clearly a device designed to inhibit torsion (which is a bad thing for
the spinal cord) the neat thing about these items is that, though they
appear many times in sauropods they are somewhat differently
constructed among taxa, leading me (and others) to use them as
possible phylogenetic fingerprints.  Check out the hyposphene/hypantra
on caudal dorsals of diplodocids, _Brachiosaurus_ and
_Argentinosaurus_ (the latter appears in Bonaparte and Coria (1993).
Bonaparte and Coria show their ideas of the evolution of this
character in South America.  What I find most fascinating is that
caudals 1 and sometimes even 2 of _Diplodocus_ and _Apatosaurus_ still
have what Gilmore (1936) called a "vestigial hyposphene".  Even cooler
is that _Supersaurus_ has well developed hyposphenes as far back as
approximately caudal 7!!!  Maybe Gilmore was going the wrong way?  :)
YPM 429 (holotype of _Barosaurus lentus_) also displays a hint of a
hyposphene on its most proximal 3 caudals (using the revised count
from my thesis) and _Brachiosaurus altithorax_ and _B.  brancai_ have
it on their first 2 caudals (look at the photos in Janensch, and check
out the written description of Riggs, heck even the new BYU
_Brachiosaurus_ material has this character on its first 2 caudals,
but not on its 3rd {that is if Ray hasn't broken them yet :) )).
Clearly they provide some functional benefits, but in _Brachiosaurus_,
with its wimpy tail (the wimpiest tail of ALL the sauropods) the
reason is beyond my ken.  I mention the value of the hyposphene in my
thesis, and in a manuscript (on hold) I detail much of these
structural components.  The paper is on hold because the folks at
Stony Brook have convinced me to test some of these ideas.  Sigh, more
work.
         
Nick also wrote:

>Another interesting feature is that although Diplodocus does
> have tail pleurocoels, they are on the VENTRAL side of the centrum! 
        
I must vehemently disagree here.  The pneumatic fossae (a term I
prefer over pleorocoels) of _Diplodocus_ are located on the lateral
sides of the centrum beneath the caudal ribs (the latter make up the
"roof" of the fossae).  *Ventral* excavation is something I have been
wrestling with for 2 years.  It is NOT pneumatic in origin (at least
not in the sense of an air-sac system like the pneumatic fossae on the
lateral surface of the centrum.  Check out Britt (1993) for a complete
and quite eloquent (though a bit long, like my email messages)
description of post-cranial pneumaticity) but what is I do not know.
I say the ventral excavation is not pneumatic because it does not a)
show the traditional bone reworking that accompanies sauropod
pneumaticity, b) there are no ducts or connecting passageways that are
present in all pneumatic sauropod cavities (ask Brooks Britt about his
_Apatosaurus_ neural spine that can be played like a flute), and c)
cross sections of _Diplodocus_ caudal vertebrae show that nothing was
actually connecting wtih the ventral excavation cavity.  However, your
idea is very intriguing.  I have seen similar exacavations on the
ventral surfaces of croc caudal vertebrae _and_ the caudal vertebrae
of Cathy Forster's new Madagascar bird look _exactly_ like those of
_Diplodocus_ (it scared me at first.  Really.).  I have some crocs "on
order" to see what is in these depressions.  Anybody know what's going
on in the birds?

Nick continues (with a fine ASCII drawing to boot):

>This appears to mean that they were were experiencing less compression
> ventrally than dorsally. Think about what that means. The only way I can
> figure out how it must have happened would be if it were in fact leaning
> back on the tail as a prop, which would tend to put compression on the top
> , tension on the bottom- and *that* just might help explain why there are
> those big chevrons, which as I believe Greg has noticed previously, should
> be bearing tensional stresses.
> 
>       
>           =o
>            |
>            |
>             \
>              \
>               \    
>                 +
>           \   ///+
>            \_o////+
>                 ///+
>                    O+ <--Top under compression
>                    | +
>                    |  ++   
>                    |    ++
>                   ^^      + + ======------------
>                       
>                       ^
>                       |
>                        bottom under tension
> 
>       The thing is, the tail curves "upward" as the diplodocus leans on
> it, and the top side is compressed, and the bottom side is pulled. So this
> might explain why there is ventral excavation, if diplodocus did this.
        
        Ventral excavation in _Diplodocus_ continues through the
caudal 25 or so (though substantially reduced at the end).  Commonly
used to conveniently identify _Diplodocus_ from _Barosaurus_, ventral
excavation depth is actually sooo variable as to be next to useless
other than to say, "oh, a diplodocid that is not _Apatosaurus_!".  In
articulated specimens (at least in those that nefarious museum
personel did not custom fit a rod iron "rainbow" to, removing any and
all hope of ever measuring ventral excavation depth) the depth can be
extraordinarily variable.  U of U has an articulated _Barosaurus_
caudal vertebral series where 2 caudals have a depth of over 40 mm,
while 4 others are nearly flat and the remaining 4 in the series are
~10 mm deep.  In _Diplodocus_ specimens from Dry Mesa and DINO, as
well as BCQ, the depth is variable and randomly spaced.  Just overall
trends can be noted.  (Some of this can be found in the Curtice and
Wilhite 1996 paper in the Paradox Basin Symposium Volume, the rest in
my thesis).  The chevrons of _Diplodocus_ are really cool (and
_Apatosaurus_ may have even neater ones depending on how much more of
the tail from Cactus park appears!).  They change in conjunction with
the neural spine's morphological shift from the "ice-cream scoop" look
created by the prominent supraprezygapohysial laminae to the laterally
compressed spines of caudal 16+.  The chevrons may very well have
acted as a protector of tail vessels while on the ground.  Or possibly
increased attachment for musculature.  Or who knows what.  I can
speculate all day, throw out all kinds of "maybes" and "what ifs" (and
don't get me wrong, I thoroughly enjoy reading and doing these) but
when it comes to sauropods I need to figure out a way to test these
suggestions using what tools are available to us today: engineering
principles, cutting edge bone stress modeling by Ben Xien, and clever
approaches.  Additionally, note that some folks have seen the
"diplodocid" style chevron in non diplodocid taxa.

        Nick further writes:
>       And I have a possible answer to one of Brian's questions. From
> McIntosh's segment in Dinosauria, on the subject of forked neural spines:
> "This unusual development apparently arose independently in the two
> families and provided a channel for a heavy ropelike muscle, which
> connected the skull with the spines of the cervical and dorsal vertebrae
> and was useful in raising the skull and neck. Parts of the calcified
> muscle are actually preserved in one specimen of Camarasaurus."
>       We could still probably argue whether it was a muscle or a
> ligament, but it sure sounds like _something_ was running through that
> fork! 
>       Yet again, if this muscle could help raise the parts of the spine
> that it passed through, it is interesting to note that in Camarasaurus, it
> is bifurcate to the shoulders, but in diplodocids, it is birfurcate all
> the way back to the hips. 

        The "calcified muscle" _Camarasaurus_ specimen is being
prepared at BYU as I type.  I am making arrangement to get a sauropod
histologist (Kristy Curry) to poke at this structure and tell us what
it is.  As a follow up to my note of humans having a bifurcate spine,
we have tinkered with some cadavers and think that the spine is being
pulled apart by muscles, rather than having the ligamentum nuchae or
some other such element actually running down the center of the spine.
But this does not answer what is going on in sauropods.  The
bifurcation of the _Camarasaurus_ neural spines is built differently
than those of diplodocids (but for a trip check out the _Camarasaurus
lewisi_ paper (McIntosh et. al, 1996 in BYU Geology Studies).  Extreme
bifurcation (_Amargasaurus_), if filled with fluid, would make for an
EXTREME investment of energy on the part of an animal (or so note my
comrades here.  They didn't buy the "reptilian camel" story I tried to
pawn off on them either...).  However if air filled the spinal
bifurcation then that would be fine they tell me.  But then that
doesn't match anything we know.  Which doesn't mean it could not have
been.  Maybe the high spines of _Amargasaurus_ had dual intra and
supraspinous musculature that helped it steer (kind of like the ropes
the stagecoach driver wiel) so the quadruped could "turn on a dime"?
:)
        Sauropods are so cool.

        BC