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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
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
Let's check this out. Species numbers for a start.
I presume you mean specimen numbers?
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205