Re: Carniadactylus paper and pterosaur ontogeny

[Mike Habib <habib@jhmi.edu>]

On Aug 1, 2009, at 6:08 PM, David Peters wrote:

> > I've done one better: measuring the original specimens, there are  
> > notable deviations from isometry.  For example, the small  
> > Santanadactylus at the AMNH is not geometrically similar to larger  
> > ones, though it's close in some dimensions.  Same for Quetz.  Also  
> > true of Anurognathus.
>
> Then they are without a doubt distinct taxa.

Er... how do you know this?  It would appear to be a circular  
argument: if we assume isometric growth, then allometric series  
suggest taxon separations.  However, that means starting with an  
assumption of isometry.  You need to test the isometry question,  
first.  Most living animals grow allometrically, and those that fly  
almost have to, in order to comply with some basic physics (the  
*strength* of the non-launching limbs can scale with isometry, though  
this still leaves the linear dimensions scaling allometrically.  The  
dimensions of the primary launching limbs are almost guaranteed to  
scale with substantial positive allometry in any given flying lineage,  
at least in the size range that includes all vertebrates).

> Send your reconstructions. I should also ask, which hatchlings? Send  
> species numbers. We may thinking of different hatchlings. The only  
> ones I am aware of are Pterodaustro hatchlings.

Can do.  For Pterodaustro, I had to work with literature values, so  
the calculations are a bit tentative, but still more than precise  
enough to demonstrate an allometric coefficient.  I'll send along some  
of the numbers a bit later (have to finish a grant proposal), but just  
as some perspective: the smallest Pterodaustro have a wingspan of  
about 0.3 meters, while the adult wingspans fall around 2.5 meters.   
That is an 8.3 fold difference in linear dimension.  Under isometry,  
length-corrected bone strength increases by the square, while mass  
follows a cubic function, so an isometric adult would have only 1/8th  
of the mass-specific bone strength of the baby.  That means the  
juvenile needs 8 times the relative strength of the adult for them to  
follow isometric patterns.


> > In both cases, the trend is not at all unexpected, because the  
> > small species and/or juveniles would need to be exceptionally  
> > "overbuilt" for isometric growth to result in a viable adult. As it  
> > is, they appear to have flown at a higher safety factor than the  
> > adults, but not by the margin that would exist under isometric  
> > growth.
>
> Let's check this out. Species numbers for a start.

I presume you mean specimen numbers?


Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280-0181
habib@jhmi.edu