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re: Endothermic Crocs in Nature

David Peters (davidrpeters@earthlink.net) wrote:

<Considering that early crocs are hard to distinguish from early dinos (witness
Macelognathus, Scleromochlus, etc.), especially when just their bits and pieces
are known, I could easily buy into endothermy devolving into something less in
a sit-and-wait predator. As Greg Paul has noted earlier, apparently the easiest
time for a taxon to revert is when it is only a few steps into the next level.
Perhaps the same has occurred here as well.>

  Such is the problem that plagues taxonomists, i.e., homoplasy. Your ancestors
will look like your sister-group no matter what you think they are different
for. That difference in truth can be as little as a 1k nucleotide variation, or
the expression of a process or divot where your lineage has none, or blonde
versus brown hair. However, basing groups on those homoplasies ignores both the
minor role initial speciation may have on a lineage in developing unique
physical characteristics, and the nature of perfect hindsight. During
speciation it's likely the species cared little, or had no idea, about the
event in question, and go about breeding and producing young (and NOT breeding
with others). Recent work posits that the "accepted" dinosaur
*Procompsognathus* may also be crocodylomorphan, as may the animal to which the
teeth named *Revueltosaurus* be. This is based both on variation in a group,
convergence, and basic innate similarities that are not lost as lineages
separate, all forming homoplasious similarities. I cnanot stress more how much
such basic features cannot be taken so plainly as to indicate this or that, and
as such the language is so that the people who do propose relationships based
on "a little more similarity here than there" should be doing so on the edge of
their seat, and in such language as to leave no doubt this is based on
tentative evidence:

  "Based on this data, the animal in question appears to share more features in
common with Sphenosuchidae than with basal Dinosauria, and so we place this
taxon in that clade (where it also seems to clar up some phylogenetic
questions) and further research may support this conclusion or place is

  [I'm not quoting anyone; this is used to illustrate a point.]

  It's happened, and continues to, for much of the same reasons people,
including me, object to such conclusions being more definite than not. Note
that basal dinosaurs were neither theropod, ornithischian nor sauropodomorphan,
but may have had characteristics said to be common to all three, such as the
apparent saurischian-esque "theropod" *Eoraptor* or the ornithischian-esque
"non-dinosaur dinosaur" *Silesaurus*. The more species we find at these
"critical" nodes the more this area will be confusing, until we get so many,
perhaps, it begins to clear up.


Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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