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Re: More reptile stuff

At 10:28 AM 12/17/98 -0600, Chris Campbell wrote:

>> You're wrong, at least under modern vertebrate systematics.  To be terribly
>> unfashionable and quote myself:
>> "The category Reptilia is now considered a node-based taxon: the most recent
>> common ancestor of turtles, lepidosaurs (lizards [including snakes] and the
>> tuatara), and archosaurs (crocodiles and birds and their extinct relatives).
>I guess my question here is this: isn't this rather arbitrary?

In one sense, yes.  Choosing a particular explicit definition is arbitrary.
There might be various justifications for it (taxonomic history, species
diversity, important transformations associated with it), but as with ALL
taxonomic assignment, the name chosen and the definition chosen is
ultimately arbitary.  (The group "turtles + (lepidosaurs + archosaurs)"
could just have easily been called 'Fred', for instance, but for historical
reasons has been labeled 'Reptilia').

What is important, however, is *sticking* to the definition.  If an explicit
definition is given, then whether a particular species or genus or other
group belongs to the defined taxon becomes a testable hypothesis rather than
a matter of taste (the pre-phylogenetic taxonomy method).

>I don't
>think you're saying that the amniotic condition arose separately in the
>lines which let to mammals and reptiles, and as such mammals and
>reptiles had a common amniotic ancestor. 

No, I'm not.  In fact, abundant data would support that this condition arose
in their common ancestor.

> Does that ancestor have less
>in common with, say, testudines than testudines have with archosaurs? 

What do you mean by "in common"?  It would be the same ancestor, the common
ancestral population of ALL amniotes.

If you mean "do turtles more closely resemble the most recent common
ancestor of mammals and reptiles than do archosaurs", the answer might be:
in some features, probably; in others, no way!!

(For example, turtles have some HIGHLY transformed anatomical features, for
which mammals, lepidosaurs, and archosaurs retain the primitive condition).

As Gauthier has said, there are no animals alive which closely resemble a
Paleozoic amniote.  All living amniotes (mammals, turtles, tuataras,
squamates, crocodilians, and birds) have highly specialized anatomies.  The
idea that Permian or earlier amniotes were somehow "lizard-like" or
"turtle-like" comes from peoples unfamiliarity with how sophisticated the
anatomies of lizards or turtles actually are.

>What is it about these early amniotes that warrants placing them in a
>taxonomic no man's land instead of placing them with reptilia (which
>would then necessitate either ditching the term reptilia -- which gets
>my vote -- 

(A parenthetical aside -- Is it REALLY so difficult for people to try and
adopt a new view of Reptilia?  Didn't everyone once go through the phase
when you learned that pterosaurs and plesiosaurs and _Dimetrodon_ WEREN'T
dinosaurs, but you moved on and began to appreciate Dinosauria more?   The
same goes for learning that spiders aren't insects, or that whales are
mammals. I *know* that Bakker calls for abandoning "Reptilia" in _Dinosaur
Heresies_, but is that good enough reason?)

>or calling pretty much all of amniota reptilia as well, which
>is somewhat silly)?

These early amniotes wouldn't be in a taxonomic no man's land: they would
either be synapsids, sauropsids, or possibly their own unique branch coming
off from that common ancestor.  Reasons they might be excluded from
Synapsida or one of its component groups (such as Mammalia) or Sauropsida or
one of its component groups (such as Reptilia) would be lack of positive
common ancestry with one particular line or another (shared derived
characters, for example).
I will turn the question around on you: in what positive way would basal
amniotes be more like turtles + (lepidosaurs + archosaurs) than they would
like mammals?  Shared primitive traits doesn't cut it: but that reasoning,
humans are more like turles (in having a full set of fingers and toes) than
like horses.

Recognition of the derived features linking turtles, lepidosaurs, crocs, and
birds is nothing new to cladistics: it goes back to Haeckel (coiner of the
term "phylogeny") and Huxley in the 19th Century.  However, the idea of a
Scala Natura has traditionally kept non-mammalian, non-avian amniotes to a
"lower vertebrate" "class".

Time to break free from the Ladder, and start appreciating the Tree!
>> Greg Paul, in his review of Dougal Dixon's _The New Dinosaurs_,

>Is this review on-line anywhere?  I'd like to read it.

I doubt it.  Ask Greg.

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