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On the Issue of Sprawling Dromaeosaurs (long)
It's simple really...
Since the issue was first brought up a year ago, nothing presented thus far has
really justified negating the apparent possibility of the lateral sprawl in the
basal dromaeosaurs hailing from Jehol. Though I do believe that the evidence
supports the case that these animals possessed the ability to sprawl, I haven't
been completely satisfied with the arguments presented in its favor. Yes Jaime,
I was well aware of the posts from 2003. I even participated in the discussion.
But thanks for the reminder. Since I was referred back to these posts, I
decided to reread all of them in hopes that my referral was justified. Maybe I
would find something that I missed which would actually answer some questions.
Unfortunately, that didn't happen. Even back then, the discussion was mired in
confusion and constantly plagued by silly and distracting bickering over such
things as the correct definition of a biplane. It looked like a terminal case
of issue dodging rather than an intellectual
examination of the possibilities. My latest posts aimed at reviving the
discussion didn't perform as well as I would have liked either. Answers
answering nothing and unsubstantiated claims of âtens of millions of yearsâ
were pretty much the end result.
It all left me sitting here shaking my head.
Therefore, this particular post is simply aimed at presenting some opinions,
observations, and questions geared toward exposing what I can only deem as a
fun filled cruise down the River Dogma. It's as a last ditch effort to get a
productive discussion going. Hopefully, no one diverts us up some tributary.
The best case presented against a sprawling gate in these basal dromaeosaurs
has merits resting upon arguments concerning anatomy. While reviewing these
arguments, a typical pattern unfolded exposing tactics that I see as having
been extremely misleading. Though several possible options on how these animals
may have been able to laterally extend their hind limbs were marginally dealt
with, it seems that only those that were easy to dismiss were actively
discussed. For lack of a better comparison, it was reminiscent of people who
have a set opinion and are never going to bend without direct written
permission from the almighty. In essence, premises were restricted to only
those facts that supported particular views.
Now... We all know that Greg Paul has made a claim or two about a few things
concerning what he has observed with the femoral heads of
microraptors/sinornithosaurus. In stead of being a stimulus of novel thinking
for a new situation, these claims mostly had the same effect as blood in the
water has on sharks. I'm not writing to support or deny Paul's ideas either
way. The frenzy is still in the stage where people are pointing fingers, making
claims that others are seeing things that are simply not there. As far as I am
concerned, until better clear-cut evidence is uncovered, we simply should not
form a rigid opinion at this time when it comes to femoral heads. Owing to the
fact that I myself cannot pass judgment due to what I see as being a lack of
sufficient evidence either way, I find it prudent to leave that one be. As
indicated from the past discussions, pursuing the issue any further along these
lines is like poking a dead horse with a stick. Weâll get about just as f!
time to move on.
Regardless, I feel that Paul has done a very good job at pointing out a
particular flaw in logic that is too often used by many in this field. It is
almost a paleontological equivalent of the tyranny of the majority, where new
finds being questioned are by default, defined in an extremely ridig way by
what is already known. Using such a method is of course what science is all
about. After all, in its most basic form, science is simply an expression of
statistics. But as Paul noted, problems in understanding rapidly materialize
when we worship the methods and fear the novel. And boy oh boy are we in big
trouble when the dreaded concept of âOutside of the known rangeâ rears its
smelly head. To paraphrase Paul with a bit of artistic license, if we were to
interpret human fossils based on other extant primates, you would have to
concluded that humans are not bipedal despite the evidence to the contrary.
Since we currently use the ability to walk in a fully erect bipedal mann!
er as a
defining characteristic of what makes a human a human, we end up blinking
ourselves out of existence by said logic used. I am quite sure that if we put
even a smidgen of time into it, we could think of numerous traits in numerous
species that are in the extreme minority relative to the whole (or norm).
On to that anatomy I mentioned... I agree with what Scott and Jaime have said
concerning trochanter issues and how they would collide with the ilium. It
seems a logical conclusion, that if the trochanters are as they say, and the
femoral head is close to the way they see it described, that it would be rather
dangerous for microraptors to pull their femora out to the side in the way
currently being envisioned. Reshaping of the femoral head as well as a
rearrangement of the trochanters would need to occur for this sort of lateral
extension to happen without dislocation. If a bare bone morphology exists that
shows a setup that would allow this type of lateral extension of the femur, I
agree, it is not clearly indicated by the current fossil evidence.
The perforated acetabulum is mentioned time and time again with all the
reverence of a magic bullet as evidence against the lateral sprawl. But I tell
ya, for the life of me, I cannot see nor has anyone explained to my
satisfaction why this is the case. If these animals lived an arboreal lifestyle
as suggested by some, the hip region wouldn't have had near the pressure
subjected upon it as compared to animals living a totally terrestrial
existence. They would have weighed extremely little and used both front and
hind limbs for locomotion. I find it terribly hard to believe that minor
modifications of soft tissues could not make up for whatever foreseen problems
supposedly exist relating to a perforated acetabulum.
There's another biomechanical possibility that hasn't been seriously
considered, even though I believe that it was actually mentioned once by Jaime.
Unfortunately, the idea was quickly whisked away to the dreary, dark hallway
closet of impossibility without any sort of viable explanation. (In my opinion
For your consideration, I give you the remix of the forward-tucked femora, and
it goes a little something like this...
Take the femora and move them forward as if the animal is sitting in a nest.
Then roll the femora so the lateral aspect of the shaft turns to point in a
dorsal direction. This would have the effect of orientating the distal condyles
so that they pointed in such a way as to allow the tibiotarsus to extended
outwards in a lateral manner. This motion also has the effect of orientating
the leg feathers in the direction of the airflow, which is precisely where we
want them if they are going to be involved in any sort of flight-related
If anyone really sees the need for a massive change in the shape of the femoral
head that would be required in order to allow this type of motion, please speak
up and explain. Is a radical manipulation of the soft tissues also required in
order to facilitate this movement? Again, if so, please explain. Do we really
need a massive change in the insertion points and in the arrangement of muscles
in the hind limb? Again, explain if this is the case. And most importantly,
based upon the preserved bones, what do we actually know when it comes to the
soft tissues of the hind limbs of these basal dromaeosaurs that would spell
doom to this idea?
Now... when it comes to modern birds, they have a range of motion that is
restricted to only a few degrees in a lateral direction, prohibiting the
extension of the lower portion of their legs in the manner I just described
(That being, they'd dislocate). But here's something to keep in mind... Modern
birds do not have fully developed wings on their hind limbs with asymmetrical
flight feathers set up in the same exact manner as their actual forewings.
Finding full fledged wings on the hind limbs was rather unexpected to say the
least. Owing to this little surprise, I think it would be a really smart idea
to think out side of the box. The newly discovered trait dictates that we ask,
âCould the range of motion in the hind limb have been selected enough so as
to send them out in a lateral direction in the manner that I mention above?â
Answer: Of freakn' course it could.
If anyone has any doubts as to if this was possible, I would love to know why.
We are talking only a few degrees more from what modern birds are capable of,
and selection sure can do amazing things with variations in a population over
âmillions of yearsâ. Check for yourself and look into the variations found
in humans when it comes to the range of motion of joints like knees, elbows,
fingers, etc. As you are checking, keep in mind the extreme low level genetic
diversity found in humans as compared to many (or most actually) other animals.
In doing this little exercise involving the natural world, I myself discovered
that in a comparison with the person with me as I write this (that is of the
same European descent), my fingers bend backwards almost 45 degrees less than
hers. I know others that can bend their fingers much more than that, coming
just shy of touching the back of their hand. Also important is that this girl
is not a dysfunctional mutant and neither am I (Opin!
vary of course). What about differing degrees of joint motion found in
so-called "double jointed" people? Just as an example, I know of one person who
can actually hyperextend their knees. This was such a shock to the doctor, that
he actually called in a colleague to see something that both of them had only
read about. Furthermore, in a past life, I was a competitive gymnast. All I can
say is that I've seen people bend in ways that they shouldn't be bending. Now
consider the probable availability of abundant genetic diversity within a
population of basal dromaeosaurs over âmillions of yearsâ.
The obvious question is whether or not we can see this type of range of motion
within any of these fossils. Unfortunately for us, there is a bit of a problem.
Dried bones are in no way a complete reflection of what restricts or even
facilitates the range of motion in joints. That's the job of ligaments,
tendons, and especially cartilage, working in conjunction with the actual bones
themselves. As John Hutchinson has said, soft tissue anatomy is oh so very
important when deriving the biomechanics of an organism. When we see the dry
bones of Archaeopteryx's wrist and try to imagine it being used in a power
stroke, you can't help but to ask yourself why the hand didn't just flap around
helplessly in the wind. Even the bony elements found in modern birds are far
from being fully developed when it comes to being able to brace the manus
during this maneuver on their own. Also, evidence for the muscles to fully
power the stroke in the first place were absent from the record in all !
bavarica*, owing to the fact that it is the only specimen preserved with an
ossified sternum. Do we really doubt that the sternum was present in the other
specimens in a cartilaginous form?
In other words, we infer what is not there based upon what is there.
Simply put, we need to interpret what is not preserved based upon what is
preserved in the fossil as a whole. Because we know that such long asymmetrical
primary feathers needed to be explained, it forced us to consider elements that
were not fossilized with the specimens. Same applies to the range of motion in
the manus. You can't run away from the fact that one needs to take into
consideration a way of stabilizing the manus relative to the distal portion of
the antebrachium even if these animals only glided (never mind if they utilized
an active power stroke). Even in modern birds, taking away the soft tissue
tosses our understanding of wing action into a state of total confusion.
Just as we needed to consider the soft tissues to explain the phenomenon of
full fledged aerodynamic wings associated with a hand having little in the way
of bracing indicated in the osteology, so we again need to explain full fledged
aerodynamic wings on the hind limbs of these basal dromaeosaurs. Avoiding this
issue because it contradicts established ideology screams dogma. I would also
like to say that using only dry elements without taking into consideration the
massive amount of variation in the range of motion that cartilaginous caps can
bring about is a complete disgard of the obvious and should have been
discussed, in detail, from the very beginning. Not only that, but acting as if
millions of years are required to develop the range of motion I talked about in
this post is just plain false, and so is the claim that such changes would have
thrown some sort of profound wrench into the ontogeny of these animals. The
changes our hips have gone through from Ape to Human a!
more vast than what I am asking you to consider here.
This leaves us with one last question... Can soft tissues that are not
preserved make up for what's indicated, or at the very least hinted at, when it
comes to accounting for the in sutu position of dried bones? If the answer is
yes, what exactly stops one from applying this reasoning to these basal
dromaeosaurs and their ability to laterally extend their tibiotarsus? I
seriously doubt that anyone can argue that these soft tissues and selection
wouldn't be able to solve the problem. But if so, please feel free to elaborate
as to why.
The reality of it all is that it's a far cry from being a huge evolutionary
jump to allow these animals the ability to laterally orientate their
tibiotarsus. Attaching the sprawl to flapping, thus increasing the height of
the hurdle for evolutionary change (not to mention causing confusion and a
distraction from the actual issue), is rather sad. Flapping may rely on the
ability to extend the hind limb out laterally, but lateral extension in no way
relies on the ability to flap. And besides, who says that the hind limbs were
flapping to begin with?
In conclusion, there are actually three camps here... Some say lateral motion
is true. Some say it is not. And a third says it is a likely possibility that
has to be considered. Given the facts in our possession at this very instant,
I'm a current resident of camp number three.
It's time to get off of the boat.