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Re: Arboreal Theropods: The prize at the bottom of the cracker jack box
I completely understand your points.
Consider that our theropods were able to scamper about semi-well in an
arboreal environment. Once that environment became more of a key
niche, selection refined those exapted traits. The thing is... We
like to only focus on those now-refined traits (again exapted, not
adaptive) as the hallmarks of being arboreal. Furthermore, these
animals are inconveniently extinct. We cannot observe their
behaviors. As such, it is extremely difficult to completely resolve
the niche they were filling until AFTER they become more highly
specialized for that niche.
We are trapped by circular reasoning; "Theropods in the trees would
probably not have been specialized for such a limited niche, and would
appear to be non-arboreal to those only using highly specialized (and
modern) traits to determine arborealism."
In other words, animals with a wide range of habitat can live in
places they are not highly specialized for... And... one would
naturally rule out fossil animals living in these wide ranges if only
highly specialized traits are used as the markers for an animal that
ventured into that particular aspect of its environment.
My bottomline is that traits exapted for a new environment will not be
specialized until after their niche is refined. The theropods at the
center of this debate would have had trees as part of their niche, but
not as their purely refined niche... And we are left with those
theropods being ruled out as arboreal with circular arguments based on
Tim Williams wrote:
> 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......<
> 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
Eh... I don't necessarily see a problem. In fact, the hodgepodge of
traits possessed by the beasties we've already found provide echoes of
those transitions... and this also means that arguing "this should
happen before this happens" doesn't hold much water for me. Just look
at all the freakn' variation in those beasties we already know about!
This is intrinsically linked to something else that I mention with
great trepidation... "There's the trait you are looking for! But I
don't think it means what you think it means because of where you
found it." I'm sorry, but a number of these animals straddle lines,
fitting either here, or there, depending on the meaning assigned to
certain traits, some of which also straddle lines. There is
ambiguity, plain and simple, as there well should be with animals "in
transition", settling into a new niche. But yes... for the sake of
being empirical, we must identify, classify, and assign meaning. Damn
you Science, and your need for measurable evidence from which to base
I liken the use of the fossil record to resolve such evolutionary fog
to a rated PG-13 strip tease: It offers a tantalizing glimpse, but it
never, ever gives a clear look at all the goods. Instead, you are
left frustrated, filling in the blanks with related information, never
knowing if you actually end up with the accurate picture.
And Ron... I 100% agree that there were certainly many, many different
"pressures" influencing behaviors that took them into the trees (and
even back down again), probably across many different lineages and at
many different times throughout their evolutionary histories.
On Thu, Jul 18, 2013 at 7:01 AM, Tim Williams <firstname.lastname@example.org> wrote:
> K Kripchak <email@example.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 <firstname.lastname@example.org> 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
>> 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?