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Re: altricial nestling bone fossilization



> Responding to D. Marjanovics observation that the wings appear to be
capable of flying in my neonate reconstructions: That's true. They are full
size or sometimes larger than their adult counterparts, which seems
counterintuitive when you consider bat and bird altricial young and their
short nonvolant wings. [- which means, if I were making this stuff up I
would have been smarter to make the wing fingers shorter. Again,
demonstrating the importantance of reporting what is, rather than what
should be.]

Nope. It demonstrates nothing more than that you saw something and
interpreted it as a pterosaur -- influenced by the fact that you know what
pterosaurs normally look like. That the face on Mars looks like a face while
nobody expected one there isn't any evidence for its existence either.
        I've never meant to say you were making it up. I'm just saying that
you desperately try to see _anything_ in the empty matrix, not necessarily
anything _specific_. It's a Rorschach test.

> The bottom line to my questioning is still this: which grows faster? A
fully ossified bone eroding and redepositing itself? Or a softer sort of
bone, held together by pterosaurs' unique 'Chinese handcuff/coaxial cable'
woven -type ossification pattern, loose, poorly ossified and wonderfully
expandable at first - increasingly dense and inflexible as maturity
(puberty) arrives?

The former. Clearly the former. Why? Because a pneumatic bone is constantly
being eroding and redepositing itself -- bone is deposited by osteoblasts in
mechanically stressed places, while it is constantly being eroded
_everywhere_ by the osteoclasts in the lining of the air sacs. In birds,
pneumaticity increases greatly during ontogeny. A pneumatic bone is not
"poorly ossified". It's _completely_ ossified, lacking cartilage altogether
except as a lining of the articular surfaces.

> What a great way to maintain extremely thin-walled bones - and increase
their length without epiphyses!

How do you mean "without epiphyses"? In mammals, the epiphyses ossify
separately from the diaphyses, and when they fuse to the diaphyses growth
ceases. That's the mammalian way of achieving determinate growth. Other
tetrapods don't have these separate ossifications. They simply retain thick
cartilage caps that slowly ossify from the diaphysis outwards, and these are
called epiphyses.

How do you imagine an expandable bone? Twenty separate ossifications
embedded in rapidly growing and slowly ossifying cartilage? Not only is such
a thing entirely unknown, it also wouldn't have _any_ mechanical stability
compared to the size of the animal, which couldn't stand on its legs, or
cling to its mother without all its long non-bones bending rachitically.

> What predators were climbing through the trees looking for babies in the
Triassic? I'm not sure I can think of many, other than small lizards and
that sort, some with gliding ribs.

Drepanosaurids.