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Testing for arboreality (was RE: On science (was Re: a bunch of other stuff))



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Dinogeorge@aol.com
>
> I should also mention that I didn't say it was impossible to do empirical
> science in a field such as paleontology. For example, you could
> construct a
> model of a dinosaur behavior such as walking and compare what
> your model says
> about dinosaur trackways with actual dinosaur trackways. This
> would allow you
> to falsify your model. Your model would be supported if actual trackways
> correspond to the predictions of your model, and would be
> falsified if they
> do not.

And here's the clincher:

> But this says something about how dinosaurs did >not<
> walk (behave)
> rather than about how they did walk, although things do advance a
> bit. Enough
> instances like this and your model will look pretty good.

By George, I think he got it!!

And that is indeed how it works: it is possible to construct tests which
eliminate possibilites.  The statement "theropod X was arboreal" is
impossible to prove; however, we can get much further by restating the
hypothesis "theropod X could not be arboreal" and testing this statement.

However, how to test?  Therein lies the difficulty.  Below I reveal a
potential research program to do this study.  It will be long; it will
involve a lot of measurements; I suspect that it might make a good Masters
Thesis for those in search of one.

The heart of the problem is that the questions about theropod arboreality
(or scansoriality) have been extremely poorly thought out.  First of all, we
have to recognize that there are different methods of getting into and
clambering around trees.  Off the top of my head, I can think of some of the
following:
*Clambering up the sides a la squirrels, cats, etc.: claws are used, but no
grasping with the hands & feet as such;
*Shimmying up the sides a la sifakas and some other primates: long arms and
hands are used to grasp the trunk;
*Creeping up and down the trunk a la woodpeckers and other tree climbing
birds;
*Walking along the tops of branches;
*Brachiating underneath the branches;
to name a few.

So, the first step will be the categorization of different modes of movement
in trees.  Among the groups to examine would be birds, primates and other
archontans, various carnivorans, some rodents, edentates, various
marsupials, and a lotta lizards.

The second step is the search for some osteological correlates with these
behaviors.  Here's where the fun begins...

You see, while there are some studies (primarily on primates and birds, so
far) on the osteological correlates of tree-climbing, I don't know of any
really extensive comprehensive literature on the subject.  This is
relatively virgin territory.  Neontologists don't really need to do these
kind of studies: they can drive out to Costa Rica and watch their lizards
climb.  That's why a lot of the best literature on the subject of general
principles of biomechanics, locomotion, etc. have been done by
paleontologists, who are trying to address the problems across species
(including forms who haven't been doing too much of anything lately).

Some obvious aspects of anatomy to examine would be: claw form; phalangeal
proportions; phalangeal articulation angles; carpal/tarsal structure;
intramembranal proportions; limb articulation morphology.  There are others,
of course: size would be a good determinant for scansoriality, in so far as
I can fairly easily reject this hypothesis for sauropods or adult
tyrannosaurs or the like.

However, there are some difficulties with this approach.  There is no
guarantee that any particular mode of scansorial ability is uniquely
associated with a particular morphological condition.  For example, Hopson's
recent work (shown at SVP and the Ostrom Symposium) on the relative
propotions of the phalangeal digits in bird feet and non-avian hands and
feet can distinguish between graspers and non-graspers.  In volant birds,
these match up well with tree-dwellers (graspers) and ground birds
(non-graspers).  However, all non-avian theropods other than _Eoraptor_ that
he examined had grasping hands and all had non-grasping feet.  Does this
demonstrate that they could not climb?  No.  Does it demonstrate that they
did climb?  No.  However, it does strongly suggest that their feet were not
used as grasping organs for climbing.  We could thus eliminate particular
modes of arboreality requiring foot-grasping from the repetoire of those
particular theropods.

What about claw angle?  Wait for SVP '00 for some recent results on that
front.

I forsee the most effective way of approaching this problem is the breaking
down of arboreality/scansoriality into several particular modes of tree
moving; finding character conditions associated with these functional
categories, if any (no guarentee there, but definitely worth searching for);
and testing different theropod specimens against each of these categories.

We could find that particular theropods (e.g., tyrannosaurids) may not match
any of the studied climbing categories, and are thus provisionally rejected
as climbers.  Other taxa may  be rejected from some categories, but not one
or more of the others.  While this does not demonstrate climbing ability, it
means that we cannot at present reject the hypothesis that theropod X was a
climber.  Furthermore, if the particular functional complex found is
otherwise only known in climbers of that mode (and even better, if different
clades independantly acquired both that climbing mode and those functional
complexes), the data would even more strongly support that condition.

As you can see, there is a LOT of work required in it.  The questions of
theropod scansoriality are a long way from being addressed.  However, I
think it is well worth the effort.

Now, researchers young and old: go forth and do this work so I can see your
results!

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843