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re: altricial nestling bone fossilization



D. Marjanovics wrote:

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.

>>>> That can be true. Illusions do exist. So the bar must be raised and some 
>>>> things that one sees cannot be accepted because they don't meet the higher 
>>>> criteria. There are still a number of examples in which the baby matches 
>>>> the mother in every detail except a short list of proportional differences 
>>>> that are essentially constant across the Pterosauria. Some give insight 
>>>> into the mother that cannot be obtained directly, but matches sister taxa. 
>>>> More importantly, in my desperation I should be seeing babies everywhere. 
>>>> I don't. And believe me I look. Ratio of mothers to non-mothers across 
>>>> 100+ taxa? About 30+%. Some taxa are entirely free of the matrix, 
>>>> unfortunately.


> 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. 

>>>>>> How can you compare one type of bone growth which is real, to one that 
>>>>>> you claim is unreal? Besides, it can't be tested. At this point, it's 
>>>>>> imaginary. Hypothetical.

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.

>>>> Certainly redeposition occurs nearing maturity and when mass is at its 
>>>> greatest. But histologically pterosaur bones are unique in their 
>>>> structure.  Therefore direct comparisons regarding bone growth cannot be 
>>>> made. Only analogies, which may or may not be valid. Hey, I'm guessing too.


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, 


>>>>actually the aftermath of that scenario can be seen histologically, in 
>>>>fossilized adult pterosaurs.


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.

>>>> Think about it. In this scenario bending is okay. And maybe, considering 
>>>> the stresses it would be under, selected for. The baby is only a 
>>>> hitchhiker, not a walker. It seems to me, hypothetically speaking, the 
>>>> major stresses on the bones -- not compensated for my the springy joints 
>>>> -- would be compression and tension, as g-forces rule -- something coaxial 
>>>> cable and Chinese handcuffs handle quite well without changing the volume 
>>>> of their interiors drastically.

> 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.

>>>>> Prior to drepanosaurids this method seems to have arisen. And 
>>>>> drepanosaurids are in the club.

dp