[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Ken Kinman wrote:
Aves is a little trickier, being mired in all
this controversy, but warm-bloodedness certainly is not a candidate as a
Nevertheless, it is why you separate Aves from the archosaurs, and the
Mammalia from the other synapsids.
The crocs and turtles and rest of the "reptiles" didn't diversify
anywhere near the extent that birds and mammals did, so they have never
seen as candidates for elevation to class status.
So we're talking Linnaean hierarchies, yes? One nice thing about cladistics
is that you can dispense with these ranks. "Mammals, the grades are just
in, and you achieved an A+ in Morphological Diversity, so you get to have
your own Class. Congrats! Crocs - oh, bad news I'm afraid. Unfortunately
that spurt of evolution in the Triassic wasn't quite enough. You get a D
for Diversity - I'm afraid you'll have to stay in the Reptilia. Better luck
Apologies for the facetiousness, but do you get my point?
If birds had died out at
the K/T extinction, I would classify them as another order of reptiles, but
that didn't happen.
Here's something else that didn't happen: Birds didn't switch ancestors the
moment they became successful. They began as theropod dinosaurs, and
they'll continue to be theropod dinosaurs until Armageddon.
And like it or not, these early thecodonts are going to problematic
a long, long time, so I'm not abandoning Thecodontiformes any time soon.
And as for Sphenosuchidae, I show it as sister group to Order
Is the phenetic gap between Sphenosuchidae and
Poposauridae all that much bigger than the phenetic gap between
Sphenosuchidae and Protosuchidae? If so, I'll consider making
Sphenosuchidae the basal clade of Crocodyliformes.
"Phenetic gaps"? What be "phenetic gaps"? You see the hole you're digging
yourself into Ken? You have to attach some kind of quantitative value to
Say (for example) the Troodontidae, Dromaeosauridae, and Archaeopterygidae
represent three consecutive outgroups to the Aves (i.e. a paraphyletic group
of three families relative to the Aves (birds)). You want to find out
whether archeopterygids belong in the Saurischia or the Aves. So, you have
to determine if the morphological differences between _Archaeopteryx_ and
birds is greater or lesser than that between dromaesaurids and
_Archaeopteryx_? How do you *measure* this? Do you have a formula? Write
a list? What if you do determine that _Archaeopteryx_ belongs in the Aves.
If you happen to discover a fossil intermediate between dromaeosaurids and
_Archaeopteryx_, you have to go thru the whole damn process again.
With cladistics, _Archaeopteryx_ is a member of the Aves - as well as a
member of the Saurischia, AND a member of the Dinosauria. There is no
reason for these categories to be mutually exclusive.
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com