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Re: Birds and mosasaurs [Rather long and theoretical]



>Although the pattern is fairly consistent, as dromaeosaurids clearly share
>the vast majority of these with basal birds, and other taxa varying degrees
>fewer.

If the derived taxa simply have successively more synapomorphies with the
test group (here, birds), while other branches from the ancestral species
don't, then my scenario doesn't apply.  Unfortunately, one of the problems
with articulating a disprovable hypothesis is that it can get disproven real
fast.  

>>right into the latest Cretaceous,
>
>As stated plenty of times, Late K members of the various theropod groups are
>NOT more "birdy" than their Early Cretaceous members.  There isn't a trend
>towards more birdlike features *within* Oviraptorosauria or
>Ornithomimosauria or Troodontidae or Dromaeosauridae over time (with the
>exception of a very few features previously mentioned, such as the loss of
>teeth within ornithomimids).  What there is is a difference in the amount of
>birdlike features between these groups.

So perhaps there's hope after all.

>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.  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.  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, 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.
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.

>>all without requiring
>>very much in the way of missing links or introducing biostratigraphic
>anomalies.
>
>Although it does require an entirely hypothetical genetic scheme.

Not at all.  The model may not apply here, but its not a hypothetical scheme
and hasn't been since the earliest days of experimental embryology.  A lens
has no physical similarity to a leg, but transplant embryonic axolotl lens
tissue to the abdomen, and you'll grow a limb.  The point being that
regulatory mechanisms are no respecters of cladistics.  Mixing and matching
regulatory and structural genes can result in startling novelties literally
overnight.  Most novelties are, of course delterious and are eliminated.
All I've added is the thought that the permissible, viable novelties are not
only few, but may tend to be the *same* few over the life of a linaeage.
Thus, it may be possible to speak of a lineage having a genetic propensity
to develop bird-like traits, although the actual LCA with birds had no
bird-like features to speak of.

>... basing an evolutionary scheme primarily on the
>lack of recovery of particular terrestrial vertebrate taxa in a particular
>epoch is probably not a very secure methodology.

Agreed.

  --Toby White