[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Carniadactylus paper and pterosaur ontogeny




On Aug 1, 2009, at 6:57 PM, Mike Habib wrote:


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?

Start with embryos. We have those. We never knew for sure before. Now we know.

Quote from Chiappe on his Pterodaustro egg: "This specimen is indistinguishable from both juvenile and mature specimens of Pterodaustro — with its long, slender snout and evidence of filament-like mandibular dentition."


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.

Measuring several pelvic openings and stuffing a baby inside an egg that can pass through requires an 8-fold increase in size to adulthood, as you say.




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