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Re: flights of fancy (or "I'm brave, but I'm chicken****")



In a message dated 95-12-07 08:30:33 EST, longrich@phoenix.Princeton.EDU
(Nicholas R. Longrich) writes:

>       Some weird stuff about Dromaeosaurs:    
>      --Reversed pubes. Why do they have these? One possibility is that 
>they evolved among flying animals to offset balance problems caused by 
>wings and large arms, or that they allowed the chest to become 
>shorter and more compact (as it tends to in flying animals). They evolved
>in herbivores, but to allow for a bigger gut. Dromaeosaurs did not evolve
>them for this reason.

In the context of BCF, this is a reasonable explanation. The BADD explanation
is, as far as I know, simply ad hoc: It happened, and it makes for a nice
possible synapomorphy uniting _Archaeopteryx_, dromaeosaurids, and birds.

>      --Stiffened tail. Not uncommon among dinosaurs, but closely 
>resembles that of Archies, and less so, Rhamphorhynchs. It's possible 
>that this was evolved to serve as a feathered elevator/rudder.

Although the tail of both _Archaeopteryx_ and dromaeosaurids is nice and
stiff distally and very flexible proximally, the tail of dromaeosaurids is
stiffened additionally by long, extended zygapophyses, which as far as I know
are not present in Archy. I've insisted from the beginning that the tail
probably provided the first real airfoil in dino-birds: stability (elevator)
and direction (rudder). It would be neat if dromaeosaurids retained the tail
with the horizontal fringe of feathers a la Archy (I've seen at least one
sculpture of _Deinonychus_ bearing such a tail), but there's no hard evidence
of this yet.

>      --Those funny bones that lie in rows on the dorsal side of the rib 
>cage- "ulcinaries" Paul calls them. These might have served to stiffen the
>rib cage, against the contraction of powerful arm and shoulder muscles.
> Well developed on modern birds, these are absent in Archaeopterygians.

Uncinate processes seem to have occurred too late in avian evolution to be
covered by BCF--they start occurring in birds well above the level of
dino-birds. Thus I haven't speculated on their appearance, but your
explanation seems reasonable.

>      --the folding arms. Are we seriously supposed to buy that folding arms

>were more efficient for some running theropods, and coincidentally,
> a few million years later, they happen to be useful for birds trying 
>to keep their flight feathers out of the way and out of harms way? Yeah, 
>right.

Some people think this is a viable alternative explanation
.
>      --big breastplate. I'll bet you the furcula is unusually large as 
>well among the Dromaeosaurs. (George?)

The furcula is something of a problem. It occurs in dino-birds _Longisquama_,
_Protoavis_, and _Archaeopteryx_, but mysteriously vanishes in most
theropods. This may be because (1) it is lost or becomes cartilaginous when
the wing/shoulder complex vestigializes in cursorial theropods, or (2) it is
easily lost (along with the abdominal ribs, for example) during
fossilization. Furculae are definitely present in _Oviraptor_, _Allosaurus_,
and an undescribed gracile new theropod from the Morrison, however. The
ceratosaurian _Segisaurus_ apparently had clavicles: maybe a reversal of the
furcula.

But furculae remain unknown in dromaeosaurids, even though we seem to have
nearly complete skeletons of _Velciraptor_ and _Deinonychus_ in which
furculae should have been preserved if they were present. High on my wish
list for future dromey discoveries.

>      --big arms. Theropods in general, from Tyrannosaurs to Carnotaurs, 
>have evolved smaller arms. Why the big reversal in Dromaeosaurs? 

Because, of course, it's not a reversal...!

>      --There hasn't been any good evidence argued against it. This 
>doesn't really count, but has anybody shown any reasons why this can't 
>work? I can understand how some of these might work as arboreal 
>adaptations, but others... Some of these have plenty of other 
>interpretations, I admit.

Nobody has shown why BCF can't work. They have provided numerous alternative
explanations for many of the features that support BCF, but these
explanations themselves are ad hoc. They don't hang together in a coherent
theory themselves. This is why I consider BADD--which is more or less the sum
of these explanations--less viable than BCF based on such current evidence as
we have.

The situation is similar to dinosaur endothermy. If you assert that dinosaurs
had some kind of endothermy--not necessarily the full-blown endothermy seen
in modern birds, but some means of maintaining a constant body temperature
and a constant metabolic level, then a whole host of dinosaur features
acquire fairly simple, straightforward explanations. (You know what they are;
I won't spend time repeating them here.) If dinosaurs were lizard-like
ectotherms, these features can still be explained, but the explanations are
often a real stretch.

>       The point is not that Archaeopterygians and Dromaeosaurs share a 
>lot of features. That only proves that they were related, not how. The 
>point is a lot of Dromaeosaur features seem to make little sense 
>outside the context of a flying ancestor. 

Amen.