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Re: Origin of Birds
Ralph Miller wrote:
>As I understand it, cladistics are based on the synapomorphies (shared
>derived traits) between animals, inherited from common ancestors. Cladists
>don't necessarily concern themselves with precisely who begat whom.
That's kind of right and kind of wrong. There are two types of cladistic
taxonomy, the ordinary kind which most taxonomists simply call 'cladistics',
and a variant with the same methods in practice but a slightly different
philosophy usually known as 'pattern cladistics' or 'transformed cladistics',
but its practitioners simply call it 'cladistics'. Clear?
Ordinary cladists are very much concerned on precisely who begat whom. Their
aim is to reconstruct phylogeny - the evolutionary sequence of diverging
lineages. Synapomorphies are used to group pairs (occasionally larger sets)
of species together, on the grounds that they both descend from a particular
ancestor. The synapomorphies for the group can be used to reconstruct some
characteristics of the 'hypothetical common ancestor'. This is the organism
which is inferred from the analysis to have been the ancestor - the
'hypothetical' indicates that it is what is predicted from the analysis,
rather than (necessarily) a known species.
Once you've got an HCA for two species, you can group them together, replace
them with the HCA and go on to calculate what it's relationships are. This
is all done automatically by computers. However, cladists don't ask
palaeontologists to go out looking specifically for the ancestors they've
predicted. The reason for this is the sparseness of the fossil record: most
species that have existed are unknown to science and probably always will be.
The predicted ancestor may well really have lived, but it might have been
restricted to a small area, or to a short time, or might have been too rare to
have much chance of preservation.
Cladists don't seem to mention HCAs very much now, perhaps because they are
unlikely to be found. And even if one species is descended from another
known species, this is almost impossible to prove, so the hypothesis is very
hard to test. It takes a good fossil record and a lot of very careful study
before we can be sure that one species descends from another. It's often
said that one hominid species is an ancestor of another, but in most cases
there is a good chance that another unknown species existed which is the real
The older literature often suggests that one species is ancestral to another -
this would be frowned on nowadays by cladists and, I think, by evolutionary
taxonomists as both unlikely and very hard to test. Evolutionary taxonomists,
who allow paraphyletic taxa, have equal difficulty identifying ancestral
species. However, they can often specify a paraphyletic group, and state that
an organism had ancestors in this group. Cladists can say something very
similar by saying one clade is a subclade of another.
Now pattern cladists. Like ordinary cladists, they base their taxonomy on
phylogeny, as reconstructed from synapomorphies. But they use the word
'phylogeny' in a special sense. I have never been able to determine what it
is they mean by it. My taxonomy lecturer was a pattern cladist, and all we
got out of him was that "of course" phylogeny wasn't anything to do with
evolutionary history. Can anyone enlighten me?
So, to summarise, ordinary cladists are concerned with ancestors, as are
evolutionary taxonomists. Pattern cladists probably aren't. However, the
vocabulary of evolutionary taxonomy allows assertions like "dinosaurs are
descended from thecodonts", whereas the cladistic equivalent would be
"dinosaurs are archosaurs".
>could we determine whether some theropods descended directly from other
>non-avian theropods or were secondarily flightless descendants of flying
>forms? In other words, how might we determine the likely anatomy or
>identity of the aforementioned common ancestor? I suppose the jury will be
>out until more fossil evidence comes in, but, in that event, precisely what
>evidence would we need to settle the matter (assuming that we could set
>aside our biases)?
As mentioned above, we can reconstruct the characteristics of the hypothetical
common ancestor of two theropods A & B. If it turns out that A has all of the
characteristics of the HCA, it is possible that A is in fact the ancestor of
B, but this is very speculative. The fossil record would have to be much,
much better than it is before we could be sure there wasn't a C which was the
ancestor of A and B. If the fossil record was that good then stratigraphy
would also be very useful here - as it is, it isn't.
As for theropods being secondarily flightless, it would be very convincing
evidence if a theropod was found with asymmetrical feathers. Finding them in
distantly related theropod groups would show that most or all were descended
from birds. Such a discovery would be unlikely, though, even if theropods are
in fact birds.
George Olshevsky will doubtless tell you why he thinks Birds Came First, and
what future evidence could support his theory.
All the best,