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Re: birds and/or/with dino's
On Tue, Sep 15, 2009 at 8:35 AM, B tH <email@example.com> wrote:
> --- On Tue, 9/15/09, David Marjanovic <firstname.lastname@example.org> wrote:
>> Ancestry cannot be cut off.
> OF COURSE! ÂBefore I had really ever begun to read much about dinosaurs and
> ancient life in earnest as a child, I recognized that bird feet and theropod
> feet were no coincidence. BUT to the common masses (of which ah'm) when one
> says "dinosaur" they are speaking about the classic perception of them -
> instead of the "non-avian dinosaur" terminology of today.
Which "classic" perception? Brontosaurs in swamps? Featherless
dryptosaurs jumping like kangaroos? Lumbering stegosaurs?
Seaweed-eating trachodonts using their crests as snorkels? Dimetrodons
and plesiosaurs and pterosaurs and trilobites?
Dinosauria has always been a relatively heterogenous group. Birds just
turn out to be a small part of that. (Although they do make up the
majority of the *known* part of that, seeing as they're still around.)
Furthermore, the "classic perception" is wrong in many ways. Why
should the scientific community bend over backwards to accomodate it?
Incidentally, "non-avian dinosaur" is a bit of a mouthful, but there
alternate ways to say the same thing, or nearly the same thing,
depending on what, exactly, you're driving at:
- "stem-group dinosaur"
- "stem-avian" (includes "lagosuchids", silesaurids, Scleromochlus,
and probably pterosauromorphs as well)
- "Mesozoic dinosaur" (includes some avians)
- "flightless dinosaur" (includes some avians; excludes some non-avians)
- "classic dinosaur" (includes some non-avians, using a strict
definition for "avian")
- "fossil dinosaur" (includes some avians)
- "extinct dinosaur" (includes some avians)
- "non-avialan dinosaur" (includes all primarily flightless dinosaurs)
> ÂSometimes, it makes it difficult to ask questions due to getting answers
> involving modern birds. ÂI believe we can all keep in our heads the ancestry
> of birds etc., etc., when speaking about the classic idea of dinosaurs.
Depends on the context. I think keeping in mind that avians are
dinosaurs is enlightening, and causes people to question what exactly
they're trying to determine. For example, why would you be interested
in what the smallest non-avian dinosaur was? Is it because most
non-avians are flightless? Should you be asking what the smallest
flightless dinosaur is? Would you be similarly interested in the
smallest non-chiropteran mammal?
>Â After all, when we talk of mammals the issue of the mammal-like reptiles
>(archaic) rarely crops up into the conversation.
"Stem-mammals" is a better term for that paraphyletic group, seeing as
they aren't generally considered reptiles any more. And this is a
false analogy, since "mammals", unlike "dinosaurs", has always been
used as a clade. Better analogy: when people speak of "Synapsida",
confusion does crop up sometimes, since the name was originally used
for a paraphyletic group (=stem-mammals) and is now generally used for
a total clade (=pan-mammals).
> As for the length of the tail being used as the cut-off line, while noting
> the transitional forms, I'd say just use what is normal for today's birds -
> short 'n' stubby.
If what you're interested in is "today's birds", then the cut-off line
should be their final common ancestor. Tail length is interesting when
it comes to flight dynamics (and convergences with chiropterans and
pterosaurs), but it's hardly the only synapomorphy of avians with
regard to other extant amniotes. There's also tridactyl manus,
toothlessness, feathers, hard eggshells, endothermy, beaks, etc.
There's no reason to highlight any of these as being so special that
it deserves a cut-off point while the others have none.
> Ok - I await the paleo-stoning/caning deserving of the amateur . . .
An old tradition on this list. I got mine back in the mid '90s. ;)
T. Michael Keesey
Technical Consultant and Developer, Internet Technologies