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Ancestors and descendants (story for Nick)




Nick,
Well, I assume you are talking about a derived modern bird. Although there would be some shared features between the deinonychus and the bird, I think I would be even more struck by the large number of differences between the bird and all the saurischians.
Look at this thing, it's arms have turned into wings, it's lost its front claws and teeth. For Heaven's sake, there's hardly any tail left---just a stubby little pygostyle. It has a laterally facing shoulder joint and the feathers are asymmetric, I bet this beastie can fly. And it's a good thing, or those two theropods would make a quick meal of this defenseless little thing. And look it's got a fully opposable hallux so it can perch itself in a tree once it gets away. You ever seen anything with a palate like that?
Oh yes, and look at all these flight adaptations, deep thorax with a strut-shaped coracoid, a triosseal canal with the tendon of the supracoracoideus muscle (great for wing rotation). See the elastic furcula, deep sternal keel, and look at those pectoral muscles----this is a very strong flier (those pterodactyls will be so jealous). Not only strong flight, but controlled as well---see the alular feathers and the rectriceal fan. No crash landings for this baby.
Let's take it back to the lab and do a full autopsy. With all these derived features, we can hardly call this a dinosaur even if it does share some features with them. This bird beastie has been doing some serious evolving. I can hardly wait to dissect the internal organs too. This thing is almost as weird as that hairy mammal thing we found last week---you know, the one with the three shrunken jaw bones shoved up into it ears, and you decided to call it a rodent.
I thought turtles and tuataras were pretty weird, but this bird beastie clearly belongs in a class by itself (just like that mammal).
*********************************************************
From: NJPharris@aol.com
Reply-To: NJPharris@aol.com
To: dinosaur@usc.edu
Subject: Ancestors and descendants
Date: Thu, 22 Jun 2000 15:57:54 EDT

Ken Kinman asked, a while ago, whether one were more closely related to one's
parents and siblings, or to one's descendants a thousand generations removed.


That is an interesting question, and I feel it is best resolved by turning it
around: are my descendants a thousand generations removed more closely
related to me, or to my parents and siblings? The answer, as I think is
fairly obvious, is that they are more closely related to me, and that at some
level, those descendants possess--or their ancestors possessed--certain
features that link them to me, to the exclusion of the rest of my family.
Thus, if I were creating a classification from my family tree, I would be
justified in erecting a group containing myself and all of my descendants, to
the exclusion of my parents and siblings.


Likewise, I might examine a collection of living and fossil organisms, decide
they forma a natural group, and call them "birds". I might then happen upon
some specimens of _Deinonychus_ and _Ceratosaurus_. I would fairly quickly
notice that _Deinonychus_ and "birds" share a number of features not present
in _Ceratosaurus_, and I would therefore erect a group that included
_Deinonychus_ and birds, but not _Ceratosaurus_. But _Ceratosaurus_ shares
many features with the first two taxa that are not found in _Plateosaurus_,
so I group the first three together to the exclusion of the fourth. And so
on, and so on, until (ideally) I have classified every individual organism
accessible to scientific study.


My question to you, Dr. Kinman, is this: what is it, in your eyes, that
would make this "bottom-up" classificatory system less useful than one which
groups _Deinonychus_, _Ceratosaurus_, and _Plateosaurus_ together to the
exclusion of the birds?


Nick Pharris
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