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Re: tiny-armed theropods



On Sat, Oct 22, 2011 at 9:49 AM, Robert Schenck <schenck.rob@gmail.com> wrote:
>  @ Keesey.
>
> Wow, interesting that PhyloCode specifically addressed the issue AND
> the example!

:)

> 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?

Branch lengths in cladograms can mean various things, including, but
not limited to:
- number of supporting characters
- molecular change (several ways of measuring this)
- number of generations
- time of divergence

> So is passer or iguanodon closer to megalosaur? Isn't it technically 
> iguanodon?

The trouble here is that there are several ways to define "closely
related". Jocelyn was probably referring to propinquity of descent,
i.e., Megalosaurus and Passser share more common ancestry with each
other than with Diplodocus.

If you want to use another metric:
- number of supporting characters: this is pretty arbitrary, and I
don't think there are any studies that cover it specifically for these
species anyway
- molecular change: unavailable for 2 out of the 3 species
- generations: could be estimated, but probably not that reliably
- time of divergence: probably the only other one we can use, for what
it's worth

Let's assume that Dinosauria originated in the MTr, say, 240 Mya.
We'll assume the Megalosauroidea-Neotetanurae clade originated around
the EJ/MJ boundary, say, 175 Mya. Times for the species are--Iguanodon
bernissartenesis: ~125Mya, Megalosaurus bucklandii: ~165 Mya, Passer
domesticus ~0ya.

Total time divergence between I. bernissartensis and M. bucklandii:
(240 - 125) + (240 - 165) = 190Ma.
Total time divergence between P. domesticus and M. bucklandii: (175 -
0) + (175 - 165) = 185Ma.

Pretty close to even! Considering how nebulous some of my estimates
are, I'd call it a tie.

-- 
T. Michael Keesey
http://tmkeesey.net/