RE: Why Thulborn's ideas on dinosaur polyphyly are wrong

[Tim Williams <twilliams_alpha@hotmail.com>]

Mickey Mortimer wrote:

> Based on the above, Thulborn finds the chance of sauropods and 
> ornithischians being secondarily > quadrupedal to be implausible, due to 
> the number of changes that would have to occur (e.g. enlarged pedal digit I 
> in sauropodomorphs, redevelopment of pedal digit V in sauropods, 
> semiplantigrade posture).

Although associated with quadrupedality, these characters are more broadly 
correlated with a shift in the distribution of body weight.  
Therizinosauroids show similar changes in pedal morphology, even though they 
remained strict bipeds.

Many birds also show an "enlarged pedal digit I", but in this case this 
character is associated with arboreality (perching).

Cheers

Tim

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