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More Bird origins: ignore if you wish...
>I'm dealing in likelihoods here--and in the unified picture of theropod/avian
>evolution that BCF presents. If theropods started out as cursorial animals, I
>would expect to see evolutionary modifications that improved their
>cursoriality, or their ability to use their forelimbs better for grasping and
>holding prey, and so forth.
Quite so. You know, every time this particular theropod worker looks at his
favorite fossils, he sees abundant evidence of enhanced cursorial ability
(relative to other archosaurs) and derived grasping functions in the jaws
and/or forelimb. I don't understand how others do not see these clearly
>Beyond the fact that my lame mind cannot imagine how the appearance of any of
>these characters would have conferred any selective advantages on a cursorial
>animal that used its forelimbs for snatching prey
I think it might be best not to add to that line...
>>These are cautions, not fatal objections. But the fact remains that the
>>only way to settle the matter is by discovery of further transitional forms.
>>For example, the absolute clincher for showing that, say, Velociraptor was
>>secondarily flightless would be an analysis showing that it lay WITHIN a
>>clade of known winged forms. We do not have the taxa to show this.
>Actually, it does--Archaeopterygiformes. _Archaeopteryx_, _Deinonychus_,
>_Velociraptor_, and other dromaeosaurids share enough apomorphies that even
>cladists will soon admit that they belong to a single clade.
We do, but we also find that Archaeopteryx shares a few derived characters
with later birds that are not shared with dromaeosaurids. So, the clade
that contains Archie and the dromaeosaurids also contains later birds.
(On the other hand, you finally provided the missing name for the node
comprised of dromaeosaurids+birds within Maniraptora!!)
> The big problem,
>of course, is that only one (maybe two or three, if _Archaeopteryx_ comprises
>more than one species) of the known archaeopterygiform taxa was volant. But
>soon the Korean archaeopterygiform will be described, and there will
>undoubtedly be others.
On this, we both agree.
>This will never happen, because the BADD cladists weasel out of presenting
>_any_ taxa as ancestral.
Not true!! Blatantly not true, and it is not polite to keep spreading this
falsehood, so that I have to keep on reposting the following statements:
Cladistics recognizes that, due to the vagaries of the fossil record,
finding fossils of the members of the ancestral population of any taxon is
an unlikely occurrance. However, there are certain criteria by which we can
recognize if a fossil species is potentially ancestral to a later taxon:
A) The potential ancestor shares derived characters with the later taxon.
B) The potential ancestor lacks any autapomorphies (derived features of its
own). [Any fossil containing features A & B is called a metataxon].
C) The potential ancestor occurs lower in the stratigraphic record than its
descendants (and it would be nice if it is found within the known geographic
range of the descedants).
The basal hominine Ardipithecus ramidus shares derived characters with later
hominines, lacks any autapomorphies, and occurs earlier than
Australopithecus, Paranthropus, or Homo. Thus, it may be the ancestor of
Archaeopteryx occurs earlier than any other confirmed birds, it possess a
few derived features shared with later birds but lacking in other theropods,
and lacks any derived features of its own. Despite invalid arguments about
rates of evolution, it could still be directly ancestral than later birds.
The evidence to reject this is to find an earlier fossil which
unquestionably shares derived features with later birds not found in Archie.
Thomas R. Holtz, Jr.
Dept. of Geology
University of Maryland
College Park, MD 20742