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Re: [Fwd: MEGALANCOSAUR notes from Ruben.



In a message dated 4/6/00 6:20:53 PM EST, cbrochu@fmnh.org writes:

<< I suspect John was actually trying to say "birds must have had an arboreal 
gliding ancestor, and maybe it looked something like Megalancosaurus." (John 
- if I'm misinterpreting you, I apologize.)  This would still be 
objectionable from a phylogenetic perspective, because it assumes a priori 
something about the evolutionary process that we should be recovering from 
the phylogenetic pattern. >>

The problem is that the phylogenetic pattern is still far too coarse to 
resolve whether birds had an arboreal gliding ancestor or some kind of 
cursorial theropod-like ancestor. One must therefore also weigh arguments of 
physical possibility and so-called common sense against scenarios based on 
phylogenies derived from a highly imperfect fossil record demonstrably 
strongly biased against small, arboreal vertebrates. Known theropod dinosaurs 
fit the patterns of evolution of secondary flightlessness observed in 
Cenozoic birds, and considering the ease and number of ways in which 
flightlessness coupled with large size has evolved in multiple modern bird 
lineages, I think it is extraordinarily likely that theropods are simply 
secondarily flightless descendants of pre-archaeopterygid birds (whatever 
those might have been: we have literally no fossil examples--yet). Birds (and 
other theropods) evolved more or less gradually and incrementally from 
arboreal forms, not miraculously from ground-dwelling runners without ever 
passing through an arboreal stage.