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Birds, no mosasaurs [VERY long & theoretical]



At 08:57 AM 2/24/98 -0600, Toby White wrote:

>Out of curiosity, do you know of a case which *does* fit or at least better
>fits the model I described, i.e., mosaic evolution of Z-like traits in group
>A where Z and A's LCA was an older model without strong Z-like characters?

Hmmm...  Will think about that one for a while.  Anyone have any likely
candidates?

I suppose on a general level, you could use ornithomimosaurs and ratites:
their most recent common ancestor didn't look very ostrich-like, but both
lineages independantly developed extremely long necks, toothless beaks, and
a similar skull profile.

>>Not by the megaton.  All that needs to be there is a single lineage.  And,
>>of course, I do not advocate that dromaeosaurids are ANCESTRAL to birds,
>>only the sister taxon to them.
>
>Right you are.  However, that still means that the main lines of the
>Maniraptoriform radiation had to be essentially complete by the mid-Jurassic
>at the latest and perhaps as far back as the late Permian.

Well, there's no reason to go THAT far back!  There is still a lot of time
between the earliest confirmed dinosaurs (c. 230 Ma) and the Middle Jurassic
(c. 175 Ma).  You can get from mesonychids to whales in a 10th that time!!

>If this is the
>case, we might expect to see all of the main Maniraptoriform lineages with
>essentially world-wide distribution since there were no sea barriers during
>most of this time.

That is true, and a bit puzzling.  Of course, maniraptoriform distributions
are somewhat greater then some of us (including me) thought in the ancient
days of 1994...  European ornithimimosaurs in the early Early Cretaceous
were a bit of a surprise to me, but the bones are there.

Also, say that the phylogeny of Maniraptoriformes is something like:
((Birds + Dromaeosaurs) + (Oviraptorosaurs + Therizinosaurs)) +
(Tyrannosaurs + (Ornithomimosaurs + Troodontids))

Presence of a bird or a dromaeosaurid in the Middle Jurassic would require a
mininum of four lineages to be present at that time:  Birds, dromaeosaurids,
the stem of the oviraptorosaur-therizinosaur group, and the stem of
Arctometatarsalia.  The presence of Middle J birds or dromaeosaurids would
NOT, however, require that the oviraptorosaurs and therizinosaurs had split,
nor the arctometatarsalians split into the various subgroups.  Also, it
would not require that any of these latter forms had acquired all (or any!)
of their derived characters at this time.

>I don't know, so I'll have to ask, is this the case?  If
>not, I admit its no disproof, but its a point against your thesis.

Known coelurosaurs from the Middle Jurassic include Proceratosaurus, the
fragmentary ?dromaeosaurid teeth, possible ?troodontid teeth, and the
Morocco foot.  The Middle Jurassic record for small dinosaurs sucks.

But, as stated in previous postings, lack of recovery is not evidence.
Phylogenetic reconstruction should be based on what is observed, not what
isn't.  Marginocephalians are not known from Middle Jurassic rocks (yet),
but I predict that they are there in some form (whether hypsilophodont-like,
fabrosaur-like, or heterodontosaur-like remains to be seen...).

>>The Feduccia crowd have misled many people into thinking that Gauthier and
>>company suggest an ancestral position for dromaeosaurids.  They (read we) do
>>not.
>
>Its probably not the Feduccia crowd so much as the fact that some of us (who
>are already beginning to fossilize) completed our formal educations before
>cladistics.  We tend to speak in terms of descent when there is no real
>reason to do so.

That may be your case, but the Feduccia crowd explictly make the case that
the phylogenetic schemes of Gauthier or Norell or so on requires a
dromaeosaurid *ancestry* of birds.  Whether this is their misunderstanding
or their reworking of the hypothesis, I don't know.

>>Well, larger than typical birds, but not particularly large sized.  My pet
>>dog as a kid was bigger than any Velociraptor or Saurornitholestes, and she
>>was only a golden lab-collie cross.
>
>Jeez!  Picky, picky...  

Yes, picky, picky!!  I have to put up with reporters and so on who "know"
(because Feduccia told them so) that theropods were too large, and that they
were all evolving shorter arms, and so forth.  These misconceptions need to
be confronted, and starting with a well-informed population with a keen
interest in dinosaur paleontology seemed like a good idea to me.

>>But there isn't that much variation there, within each coelurosaur lineage.
>>Most dromaeosaurids look extremely similar to each other.  Most troodontids
>>look extremely similar to each other.  Most tyrannosaurids look extrememly
>>similar to each other.  And so on.
>
>I'll take your word for it, although the male and female dromeosaurs would
>probably have disagreed with you.

:-)  (Also, I suspect their plumage was very different between sexes,
between ontogenetic stages, and between species, but that will be very
difficult to test without time machines or some really spectacular, as yet
undiscoved lagoonal or lacustrine deposits...).

>There is still a lot of variation between
>the Maniraptoriform groups, a lot more than we see in Carnosaurs for
>example.

Yes.

>In any case, doesn't this make matters worse for you? You must
>posit that essentially nothing happened in the way of evolutionary novelty
>in this group for 100-150 My.  That's a huge amount of stability for a group
>that had an average species lifetime of (if I remember the estimate) about 3My.

Actually, this rather unexpected pattern turns out to be commonplace in
vertebrate evolution.  It is one of the main themes in Robert Carroll's new
and thought-provoking volume Patterns and Processes of Vertebrate Evolution
(1997, Cambridge Univ. Press).

"Within many groups, major episodes of cladogenesis are limited to
relatively short lengths of time, after which the individual clades persist
for long stretches with relatively little morphological or trophic change.
This pattern certainly requires explanations beyond those possible by
extrapolation from the pattern of evolution common to most living species
and genera"  - p. 166.

"The pattern of large-scale evolution shows that all major taxa retain a
basically similar mophological pattern for much longer periods of time than
had been assumed by Darwin.  Among vertebrates, many families, defined on
both morphological and phylogenetic criteria, retain a similar structure for
100 million years of longer.  This constancy of form may be attributed to
evolutionary constraints: aspects of their biochemistry, genetics,
development, and/or physical aspects of the environment." - p. 179.

>>Long branch attraction may result in molecular convergence, but would be
>>extraordinarily unlikely to result in a consistent pattern of distribution
>>of "birdlike" features between each of the coelurosaur lineages, while
>>simultaneously NOT producing variation of the birdlike features within each
>>lineage.
>
>Can you spell this out a bit more?  Molecular-type long branch effects are
>statistical anomalies resulting from random events.

And the fact that an "A" can only change to a "T", "C", or "G": other
options being unavailable in Terran life.

>Phenotypes, on the
>other hand, are not random because only some are viable.

But many more options are open (compare the morphology of metacarpal I of
Ichthyostega to Caseia to an opossum to a bat to whale to a hominid to a
lizard to a croc to a pterosaur to Allosaurus to...).

>Thus, we'd not
>expect to *literally* see long branch attraction in paleontological
>cladograms.

I agree.

>Instead, we'd see independent, mosaic recurrence of permissible
>(i.e. viable) novelties.  This is the phenotypic analog (and direct result)
>of genotypic LBA.

In other words, we could see convergences.  Agreed.

>Actually, I'm trying to apply some of the evo-devo stuff to this problem.

(Definitely pick up Carroll!  He's got quite a discussion of hox genes and
development and evolution of vertebrates.  I think you'll find it interesting.)

>The point is that highly conserved genetic regulatory structures (e.g. hox
>boxes) and highly conserved structural genes can result in radically
>different phenotypes essentially overnight by changing which regulatory
>domain governs which structural gene.  This is exactly why it isn't
>necessary for the LCA of  (to use neutral terms) Z and Z-like A's to have
>much in the way of Z-like features.

But, if a form IS known to possess features in common with Z, and indeed
that various other forms also possess some (but not all) of these, and if
these taxa can be arranged in a nested hierarchy with regards to the
distribution of Z-like forms, what is the more parsimonious conclusion:

A) these forms independantly derived Z-likeness.
B) these forms represent a series of progressive divergences from a common,
non Z-like ancestor, and that the forms with the most Z-like characteristics
represent taxa which derived most recently from the Z-branch.

Or, to put this another way, say you have a few taxa (like 9600 species)
that have a particular character state (say an ascending process of the
astragalus).  Is it more parsimonious to hypothesize that their most
immediate common ancestor did not have this structure, but only the genetic
capacity for it; or that their most immediate common ancestor had this trait?

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661