Re: dinosaur vs. bird locomotion

["Mickey Mortimer" <mickey_mortimer@email.msn.com>]

Chris Noto wrote-

>Recently in my Intro to Paleo course my prof gave an example of theoretical
>morphological analysis using the "did dinosaurs walk more like birds or
>like mammals?" comparison.  What struck me is that it seems intrinsically
>flawed because birds are a highly derived form of one branch of the
>therepods and comparing them to, say, a ceretopsian would be like using a
>kangaroo rat as a comparison for human locomotion just because we're both
>mammals.

The question is not intrinsically flawed, since any given species of
dinosaur had to walk more like a mammal or more like a bird (okay, I guess
you could argue that a hypothetical dinosaur could walk so uniquely as to be
utterly unlike either of them and therefore equally distant, but I don't
think an example like this exist in this case).  The answer, of course, is
both.  Some dinosaurs (theropods, basal ornithischians, small ornithopods,
pachycephalosaurs, psittacosaurs, etc.- the bipedal ones) walked more like
large terrestrial birds than mammals due to the fact that they were
medium-sized to large, bipedal and digitigrade.  All bipedal mammals are
either plantigrade (humans) or very small and tend to jump (rodents).  Then
there's kangaroos, that are just plain weird when it comes to locomotion.
Other dinosaurs, such as stegosaurs and sauropods, walked more like
elephants than any bird due to their quadrupedality, size and graviportal
limbs.  As for the leftovers (prosauropods, ankylosaurs, large iguanodonts,
hadrosaurs and ceratopsians), I'd say they were more mammalian due to their
quadrupedality alone, but strict analogs are hard to find.

Furthermore, even using it as a comparison to non-avian therepods
>seemes a little off to me.  Aren't ground dwelling birds (like chickens)
>secondarily flightless and have a whole evolutionary history, setting them
>apart from their non-avian ancestors?  Then there's the size difference.
>No bird ever got that BIG, the scaling could throw it all off then.  You
>need to take into account the limits of the material used here (ie, bone).
>Could that affect certain proportions in limb length and throw off the
>analysis?  I've been thinking about this quite a bit and it just doesn't
>seem quite kosher, the differences seem to great.  Isn't that why birds are
>set apart from dinosaurs in the first place?  Any thoughts or comments
>would be appreciated.

To answer your questions: Yes, chickens are secondarily flightless and have
had millions of years to evolve new locomotive tactics, but their
fundamental limb design is still the same (and very theropod-like).
Although scaling does have quite an affect on limb proportions and possible
use (I'd like to see a tyrannosaur hop!), there are many theropods known
within in the size range of birds.  These also differ from birds in the same
ways that larger theropods do however, just to a smaller degree.  I doubt
that being limited to bone affects proportions much, but I'm sure muscles
were different in theropods than they are in birds, which affected
locomotion.  For instance, theropods had propubic pelves (most did anyway),
and that long tail with muscles connected to the pelvis.  Finally, birds are
set apart from dinosaurs because (1) they have several features other
dinosaurs don't (although just as many features separate birds from other
dinosaurs as separate maniraptorans from other dinosaurs, etc, so it's not a
fundamental difference but rather chosen as a dividing point based on
reasons 2 and 3), (2) they are alive, and (3) they fly.  There was actually
a study done on limb proportions in birds and non-avian theropods to
determine any fundamental differences.  It was discovered that birds occupy
a much larger array of limb proportions than theropods did.  Basal birds,
however (such as Archaeopteryx, Confuciusornis and enantiornithines), plot
near theropods instead of other birds.