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Re: Carniadactylus paper and pterosaur ontogeny
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
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