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Re: BADD BCF



In a message dated 95-12-06 02:32:04 EST, longrich@ponyexpress.Princeton.EDU
(Nicholas R. Longrich) writes:

>On Tue, 5 Dec 1995 Dinogeorge@aol.com wrote:
>> The main problem I've always had with the BADD/ground-up scenario is
getting
>> a 20-50 kg theropod with relatively small forelimbs to evolve into a 2-5
kg
>> dino-bird with grasping wings. It _could_ happen, but in my opinion it is
>> _far_ less likely than having a 2-5 kg dino-bird with grasping wings to
>> evolve into a 20-50 kg theropod.

>       I'm skeptical. You say that theropods can't be bird ancestors 
>because they're too big, but how do you know that?

I didn't say "can't," I said "far less likely."

>There could have been 
>many small theropods. Yes, there is little evidence of this. There is 
>little evidence of protobirds, either, however, so that is hardly a 
>legitimate reason to prefer protobirds over small theropods as bird 
>ancestors.

Yes, there _certainly_ were small theropods, lots more than we know about,
for that matter. (When you say "theropods," you mean, of course, the variety
with short forelimbs, not the variety with long forelimbs that would have
been closer to a proto-bird.) I didn't say small theropods couldn't be
ancestral to birds; I said I didn't think it likely that 20-50 kg theropods
would evolve into 2-5 kg dino-birds.

> Compsognathus, though not especially close to birds, shows 
>that relatively small theropods existed. And I've seen footprints that 
>would fit on a quarter, maybe a nickel, from theropods. They're mind 
>blowing. I suppose there's no real way of knowing they were adults, in 
>the same way there's no real way of knowing they weren't. 
>       Personally, I think arboreality is an interesting and plausible 
>way to explain why the first dinosaurs were bipedal. And I like the idea 
>of Velociraptor and kin as flightless birds, but I question all 
>the stuff in between which seems much more based on speculation (and some 
>of you will say- as if the other ideas AREN'T?- so yeah, it is all very 
>speculative).

BCF basically says archosaurs on the lineage toward modern birds started out
small and stayed small--small here meaning, say, anything from canary-size to
pigeon-size to crow-size--and that among these small archosaurs were
acronomic forms that became progressively better at living in high places.
Their forelimbs became suited to grasping, their claws enlarged and became
sharply curved, some of their bones became hollow, they acquired a furcula
and pre-feathers, they developed grasping hind limbs, pneumatic skeletons,
and so forth. Among them evolved various kinds of gliding and passively
flying animals, and among them also evolved ground-dwelling cursorial forms
which, having abandoned the arboreal, acronomic lifestyle, shrank their
forelimbs, lost their pre-feathers or feathers, and developed into the much
larger, graviportal ceratosaurs, carnosaurs, and so forth. Finally, among the
lineages of small, acronomic forms powered flight developed, and we got
birds. That's it in a nutshell. There's plenty of speculation to be sure, but
it's not as "blue sky" as something like "maybe folding forelimbs evolved to
help them run better."