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Re: Ah ha! That's where therizinosaurs came from



 

> What the cladistics would
> have us believe is that all these predatory, omnivorous and herbivorous early 
> fliers
> were flitting about in the later half of the Mesozoic yet for some magical 
> reason were
> never spinning off reflightless forms that show up in the fossil record. 
> Really, that's what the 
> cladograms want us to take seriously.


Mr. Paul, I admit that the evolution of birds certainly has left a bewildering 
diversity of flightless forms in the fossil record, and you argue your case 
persuasively, as always. I take your hypotheses seriously and I hope I won't 
antagonize you when I note that I also take the other possible hypotheses 
seriously.

I have two questions for you, to help me be better informed in understanding 
your views.

Question 1:

 Flightless families of crown group birds seem different from non - avian 
maniraptorans in their systematics. Namely, they don't seem as diverse, 
cosmopolitan, and geologically persistent as families of non-avian 
maniraptorans.

As you touched on in Dinosaurs of the Air, there are two modes of flightless 
birds today, the island types and the continental types.

The most diverse flightless crown group birds like rails evolved independently 
of one another on islands and other restricted, far - flung, habitats. 
Continental flightless birds such as the ostrich, kiwi, or Rhea, tend to have 
very few species, very low diversity within each family, and to be restricted 
to single continents. No lineage of flightless crown group birds seems to have 
gone on to form a broad, global, radiation of new forms over a hundred million 
years, as did several non - avian maniraptoran groups.

Dromaeosaurs, as one example, achieved great diversity of body size, limb 
proportion, and foot morphology, and they also enjoyed global distribution and 
invaded habitats from aeolian deserts to polar forests. Do you see them all as 
spinning off in separate convergences on flightlessness, from flying 
archaeopterygian ancestors all around the world, and thus are dromaeosaurs a 
paraphyletic group? Or did they evolve flightlessness once and then spread 
around the world and diversify, unlike any family of crown group flightless 
birds?


Question 2: 

Your hypotheses rely on the logical inference that rules of thumb found in one 
group of animals "should", as you say, hold true in analog groups as well. 

For example, in your formulation, since many (though not all or even most) 
families of crown group birds gave rise to flightless species then so should 
basal and stem group birds. Another example is that, if we have no fossils that 
show the transitional ancestors of pterosaurs and bats, then we shouldn't find 
them for birds, either, and everything that looks like an ancestor of birds 
must be a secondarily flightless throwback.

You note in Dinosaurs of the Air that neither bats nor pterosaurs seem to have 
produced secondarily flightless species, and you attribute this to their 
quadrupedalism.  But perhaps they lacked the brain or metabolic development, or 
perhaps ground animal niches were dominated by other groups in the Mesozoic, 
and there are dozens of other possible reasosns that birds could re-evolve 
flightlessness so many times when other groups did not.

Do you concede that there are exceptions to such rules of thumb in Biology? For 
example, we could say that gliding animals evolve in arboreal habitats, but 
this rule of thumb would not be true of at least two exceptions: flying fish 
and flying squid.

Is it possible that such rules of thumb are prevailing patterns, but that they 
can have exceptions, and that they cannot be relied on as if they were 
inviolable natural laws?









Jason Brougham
Senior Principal Preparator
American Museum of Natural History
jaseb@amnh.org
(212) 496 3544