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

Re: Birds and mosasaurs [Rather long and theoretical]

>     I wasn't trying to make a personal comment, but Fedduccia DID 
>say that, and you seem to me to be following a similar line of
>reasoning.  Fedducia says the tendancy of coelurosaurs to evolve
>bird-like features repeatedly implies that they were not ancestral to
>theropods, and you seem to be implying the same.

I'm going to cut off my part of this thread about now because I think we've
reached something of an impasse, and because we've certrainly reached (or
exceded) the bounds of my knowledge.

Here I frankly think I need more information, more brains, or both.  Suppose
species A is close to the ancestral form of a lineage.  Species B and C are
derived forms.  A+ means it has a trait, A- means that it doesn't.
Cladistic analysis deals very well with the case: A+ B+ C+.  What do we
conclude for A- B+ C+?  That there is an ancestral form A'+ which we've
missed and that A is not part of the clade?  That's fair enough.  But what
about the case here where we have birds (Z+++), and Maniraptorans with odd
assemblages like B-+-, C+--, D--+ , but A---.  (This grossly oversimplifies,
but bear with me).  We can declare, for example, B the winner based on
statistics, but we know that this means C and D independently developed
characteristics that are found in Z but not in B.  Alternatively, we can
assert that we've missed A'+++ as before.  Neither is very satisfactory.  

Frankly, I was trying to develop some explanation based on a notion similar
to phylogenetic space.  (All of this is ripped off from  various ideas
described in Rudolph Raff's book *The Shape of Life*)  Some genetic changes
are much more likely than others because they are at least not lethal, the
transitional forms may be beneficial, or they simply manifest themselves so
late or (if you agree with Raff) so early in development that their effect
is likely to be minor.  This raises the possibility of giving real meaning
to terms like "evolutionary potential."  That is, it may be possible for an
organism to have a suite of *genetic* characteristics which are "bird-like",
in the absence of  an avian phenotype -- simply because they can mutate in
that direction through viable intermediates.  

If you buy this, then the A--- species may be closer to Z+++ than any of A's
descendants, because each of the genotypes giving rise to the "-" phenotype
can evolve in the direction of "+" through developmentally and ecologically
viable intermediates.  How would we test for such a case?  Precisely by
finding, with non-random frequency, species derived from A which look like
B+--, C-+-, and D--+, i.e. independently derived characters in descendants
which look like a mosaic of the Z+++ phenotype.  

Phylogenetic distances can't really be measured directly, because they are a
complex product of genotypic constraints, developmental constraints, and
ecological constraints.  However, it is just such constraints that make the
idea of phylogenetic space useful, albeit at the risk of reintroducing
concepts like "evolutionary trajectory".  Minus any teleological component,
such terms simply describe a case in which genetic, developmental and
ecological constraints together permit evolution in one direction more
easily than in another, i.e. proximity in phylogenetic space.

The catch is this.  Since we're talking about hypothetical ancestor species
which are close to birds in phylogenetic space, but not in phenotype, we are
left somewhat uncertain about how far back that genotype existed.  We can't
test directly for genotype, and might not recognize the similarity (or
rather the potential similarity) even if we could. 

If this scenario makes any sense and if it can fairly apply to the case
under discussion -- two very big ifs -- it pushes the range of possible bird
ancestors back down the theropod line.  Its pretty hypothetical, but I'm
attracted to the idea because it explains the strange, mosaic evolution of
avian characteristics in the coelosaurian radiation AND explains why we see
these characteristics popping up anew, in different combinations, and in
different lineages, right into the latest Cretaceous, all without requiring
very much in the way of missing links or introducing biostratigraphic anomalies.

  --Toby White