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Re: Birds and mosasaurs [Rather long and theoretical]



At 10:14 PM 2/23/98 -0600, Toby White wrote:
>Unfortunately, one of the problems
>with articulating a disprovable hypothesis is that it can get disproven real
>fast.  

Actually, I'd change that first word to "Fortunately".  The problem with a
lot of scenario building is that the schemes are NOT disprovable...

>>By sedimentological fate, we are blessed with very good Late K deposits in
>>western North America and Mongolia, so we know a lot about these dinosaurs.
>>Deposits from earlier times are less extensive and/or less well exposed, and
>>we know correspondingly less about the dinosaurs from these time.  Not
>>coincidentally, we also know less about the mammals, lizards, amphibians,
>>unionid molluscs, etc. from these units.  This isn't surprising, and says
>>little about the organisms.
>
>The sampling error works both ways.  Your logic is that there may have been
>Dromeosaurids by the megaton in Early Jurassic days, or even earlier.

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.

>Based
>on the fact that much later Dromeosaurs are relatively birdy, you'd suggest
>(not unreasonably) that this improves the chances of some early Dromeosaur
>being the ancestor of birds.

No.  No.  Uh uh.  Nope.

To clarify the issue: most phylogenetic analyses indicate that
dromaeosaurids and birds are each others closest relatives (aka sister
taxa).  Neither is ancestral to the other, but both came from a common ancestor.

The Feduccia crowd have misled many people into thinking that Gauthier and
company suggest an ancestral position for dromaeosaurids.  They (read we) do
not.

>But look at it upside down, if you will.
>Dromeosaurs and other Coelosaurs were all the rage in the late Cretaceous,
>were the beneficiaries of some of the best-preserved geology in the
>Mesozoic, and also benefitted from large size,

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.

>further slanting the
>preservational bias.  In other words, the observed birdiness of the
>Dromeosaurs may well be an artifact of the fact that this group was very
>succesful, large and diverse, as well as unusually well-preserved (like my
>mother's side of the family).  We get to see much more of the genetic
>variation possible in the surviving Coelosaur clans than in earlier models.

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.

>No one (well, almost no one) seriously doubts that the first bird is in that
>lineage somewhere, so birdiness in some late Coelosaur clan is not a
>surprise, given the sampling bias.  The question is whether that similarity
>implicates a fairly direct genetic relationship.  This may well be a case,
>to put the matter in terms used in modern studies, of long chain attraction
>magnified by really impressive sources of sampling error.

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.

>The point being that
>regulatory mechanisms are no respecters of cladistics.

(Tell that to the evo-devo crowd!  They make a big case, and rightly so, of
demonstrating how patterns of regulating mechanism among the various groups
of metazoans can be used to resolve interrelationships among animal
lineages, as well as to predict various common ancestors of the different
subgroups).

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