[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: giant birds

In a message dated 11/9/99 4:27:11 PM EST, k.clements@auckland.ac.nz writes:

<< The above is a summary of what is referred to on the list as BCF. The 
 recent systematic literature (e.g. Sereno 1999, Xu et al. 1999) seems 
 (to me anyway) to contradict BCF.>>

Read more deeply into the systematics and the contradiction vanishes. Don't 
forget that the birdlike theropods of the Cretaceous are many millions of 
years removed from their Late Triassic through Late Jurassic common ancestors 
with birds, and that they themselves have undergone considerable evolution 
and adaptation into a lifestyle as terrestrial bipedal cursors. Lots of 
convergence, too.

<< It is clear to me how phylogenetic schemes are constructed, but I don't 
understand the reasoning behind BCF. What is the rationale for BCF?>>

I reject the "ground-up" hypothesis for the origin of avian flight on 
physical grounds, because it is much, much less likely (astronomically less 
likely) than a progressive "trees-down" origin. In the "ground-up" 
hypothesis, ancestral birds do not acquire an arboreal lifestyle until 
>after< they have evolve flight (otherwise "ground-up" and BCF differ only as 
to the timing of the arboreality), and they evolve flight as cursorial 
animals through the random accumulation of features that have nothing to do 
with flying but, in some miraculous fashion, at some point in time become 
exapted for flight. In BCF, birds go up into the trees, develop flight in at 
least one lineage (and likely more) as a series of incremental improvements 
to a progressively more efficient arboreal lifestyle. At various times--as 
has happened repeatedly throughout avian evolution through the modern 
era--some lineages abandoned the arboreal lifestyle and, if they had already 
developed some kind of flying ability, became secondarily flightless, 
cursorial animals. In the Mesozoic, we call such animals theropods; in the 
Cenozoic, we call them phorusrachids, diatrymids, ostriches, and so forth.

We can use the theropod cladogram to track some of the evolutionary 
developments of dinobirds as they became birds: bipedal posture was acquired, 
the tail stiffened, the fifth and then fourth manual digits were lost to make 
a rudimentary wing, the hallux retroverted, and so forth. BCF provides a 
functional scenario for the evolution of bird flight that follows directly 
from theropod phylogeny as it is currently understood.

<< The above statement from Dinogeorge seems to suggest a functional 
hypothesis based on the acquisition of flight. Functional hypotheses have 
been widely accepted yet proved wrong in the past (e.g. the evolution of the 
tetrapod limb - Coates and  Clack 1991), so to make sure I was being fair to 
BCF I thought I'd raise the question. >>

"Proved wrong"? There's only one way to "prove" something in evolutionary 
theory, and that is with a time machine. Tomorrow a newer hypothesis may 
"prove" the "disproof" wrong. Cladistics tells us nothing about the nature of 
the common ancestral forms, except that some of their features are apomorphic 
for their descendant groups. It tells us nothing about which features may 
have been present in the ancestral forms but were lost for functional reasons 
in some of the descendant forms, when the only thing we have in the fossil 
record are a few specimens of those descendant forms. BCF rejects the 
obviously wrong parts of the "ground-up" and "trees-down" hypotheses and 
synthesizes the remainder into a unified, parsimonious scenario for the 
origin of avian flight. That's the best we can do for now, given the sporadic 
(that is, nil) fossil record of Jurassic pre-Archaeopteryx dinobirds (for 
which, by the way, BCF also provides a tentative explanation).