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Re: tiny-armed theropods



Le 22/10/2011 18:49, Robert Schenck a écrit :
  @ Keesey.

Wow, interesting that PhyloCode specifically addressed the issue AND
the example!

On Sat, Oct 22, 2011 at 11:55 AM, Jocelyn Falconnet
<j.falconnet@gmail.com>  wrote:
So far as I know, few paleontologists would dispute the fact that they
are closer to each other than to, let's say, *Diplodocus longus* or
*Iguanodon bernissartensis*.

I might be a bit behind the times on this, but don't some cladograms
posit meaning for the lengths of branches between points, with the
length of a path between two species being a quantitative measure of
relatedness?
So is passer or iguanodon closer to megalosaur? Isn't it technically iguanodon?
Er... I think that most paleontologists agree to the fact that *Megalosaurus* is much closer to *Passer* than to *Iguanodon*. Branch length has nothing to do with relationships in cladistics - but it would if you were a phenetician (in which case I would ask you to stop immediately). The issue is: who is close to who (rather than to whoever else) ? (Oops, I am not quite sure of my English syntax, here - please forgive me - but it works fine in French)

As you cannot infer relationships from primitive (=plesiomorphic) characters, you have to rely instead on share derived (=apomorphic) characters to decipher the relationships between these three dinosaurs. For instance, the retention of 'arms' instead of the bird wings in *Tyrannosaurus* (*Megalosaurus* is currently restricted to a partial dentary, so...) and *Iguanodon* does not support their relationship - as a quick glance to other tetrapods shows that this is an inherited feature. In contrast, the presence of, let's say, a furcula (= wishbone) in *Tyrannosaurus* suggests a closer relationship with *Passer* than with *Iguanodon* because this structure is absent in more distant relatives (=outgroup) such as pterosaurs or crocodilians.