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Re: Arboreal Theropods: The prize at the bottom of the cracker jack box



K Kripchak <saurierlagen1978@gmail.com> wrote:

> Using the existence of mobile shoulders, or the
> existence of perching toes, or whatever, as the linchpin selective
> advantage from which to build positive conclusions on "the moment of
> arboreality" may not actually be as strong a foundation as you think.
> Being small and having long, strong arms with big freakn' claws may
> have been pre-adaptive enough for first scratching out a niche in the
> trees. And unlike many other courses of reasoning, it actually
> provides an escape from that Catch 22.


Interesting point.  I fully agree that in the incipient or nascent
stages of arboreality, exaptation would be important: certain
predatory or terrestrial could be exapted toward a scansorial or an
arboreal function.


But thereafter... if an animal spends more of its time in trees, then
selection would be expected to favor the acquisition of full-blown
arboreal characters at the expense of terrestrial ones.  By "arboreal
characters" I mean those for quadrupedal locomotion in a
three-dimensional substrate (such as tree-crowns).  This is where the
mobile shoulders/ankles and grasping hallux come into play.  IMHO it's
a counsel of despair to argue that an animal can "get by" using
ancestral non-arboreal features in order to be truly arboreal.  Many
theropod features are actually maladaptive to quadrupedal climbing in
a canopy (especially the constrained mobility at the joints).  In
general, the locomotor adjustments and limb kinematics associated with
arboreal quadrupedality are very demanding.


Also Kris, the aforementioned arboreal scenario requires that small
theropods went from being terrestrial bipeds --> arboreal quadrupeds
--> arboreal bipeds.  This transitional phase of "arboreal quadruped"
is often glossed over.  But it entails a major ecomorphological shift.
 Tree-kangaroos (_Dendrolagus_) are often cited as examples of how a
terrestrial biped can become an arboreal quadruped.  Tree-kangaroos
are actually well-adapted for arboreality.  They've gained many
arboreal adaptations at the expense of terrestrial ones (e.g.,
http://dml.cmnh.org/2011Aug/msg00196.html).  So although I don't rule
out some small theropods being arboreal quadrupeds, this ecology would
be in spite of (not because of) their morphology.


Ronald Orenstein <ron.orenstein@rogers.com> wrote:

> Not to mention that there are many stages between fully terrestrial and fully 
> arboreal, and that trees are not
> the only things to climb on even for terrestrial animals.


Indeed.  Many small mammals are scansorial, because of the need to
negotiate uneven terrain.  This has facilitated the shift to full
arboreality in many lineages.  This (as well as a discussion of the
definitions of arboreal/scansorial/terrestrial) is covered in Chen &
Luo (2012; DOI: 10.1007/s10914-012-9199-9
http://www.springerlink.com/content/23252m78k072p834/)


>  Perhaps the first step towards arboreality came with
> roosting (as many herons and Galliformes, etc, do today), for which the main 
> advantage is probably predator
> avoidance; arboreal roosting is driven by conditions on the ground (i.e. 
> predator abundance) and requires no
> more than the ability to get out of reach of ground-based hunters.  The kind 
> of agility required for arboreal prey
> capture would not be required.


Yes, indeed.  Opportunistically seeking the safety of trees as a
refuge is one avenue for promoting scansorial or arboreal behavior in
otherwise terrestrial theropods.  From an ecomorphological
perspective, one pitfall (as Mike Habib and myself pointed out) it is
very difficult to delineate "partially arboreal" (i.e., only
occasionally venturing into trees) from "fully arboreal" based on
morphology alone.  So unless one has a compelling reason to put
non-avialan theropods in trees, why do it?







Cheers

Tim