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Re: Carniadactylus paper and pterosaur ontogeny




On Aug 1, 2009, at 3:35 PM, Mike Habib wrote:

David Peters wrote:

None of the embryos discovered are geometrically similar to the adults.

Not true. Trace and reconstruct the bones. Then get back to me. We'll compare notes.

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. All big pterosaurs derive from small ones. And many small ones derive from large ones. Bennett and Jouve thought Ctenochasma added teeth via ontogeny forgetting that every other known pterosaur, including the ancestral lineage of Ctenochasma, has fewer teeth.


It's also worth noting that for pterosaurs to be flighted at small size and retain flight ability through growth, they cannot grow isometrically.

1. Only Unwin said they were flighted at hatching.

2.Your statement is not backed up by the literature. Please share.

It just comes from basic biomechanics (which I have checked against bone strength calculations, using my own dimensions data and those from the literature) - if you scale the hatchlings up isometrically you end up with a wing spar that cannot handle flight loads.

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.

Same pattern appears with making comparisons among species - as body size increases, there is an allometric change in wing spar dimensions.

That may be so. But the differences are phylogenetic as I can show you.

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.

David



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