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Re: altricial nestling bone fossilization
I have a few more thoughts that are probably relevant and/or useful to this
1) There is a long continuum from fully altricial to superprecocial.
Passerines are very stubby and underdeveloped, certainly, but so-called
'semialtricial' herons and raptors are also mostly nest-bound and helpless,
though their eyes are open almost immediately and they have some limited
mobility. It is quite likely that some fossil forms were essentially
altricial, but were not as underdeveloped at birth as passeriforms (because
super-altriciality seems to be a derived character of passerines).
2) Ducks are certainly precocial. Black ducks, for example, are
superprecocial. However, precociality is defined by the independence of
neonates, not by their flight capabilities. Many (most?) precocial birds
actually take a long time to become volant, but they run or swim at an early
age. In this case, the hindlimbs will be of near-adult proportions and
strength (relative to body size). The
forelimbs presumably will not be as strong as the hindlimbs until loading
begins (and it sounds like, form David's example, that the sternum does not
ossify early on). It has been shown in gulls that the forelimbs strengthen
rapidly as they are loaded in pre-flight exercise (but overall this
hindlimb/forelimb comparison work is mostly missing for birds...see the end of
3) There are really two issues at stake here: length of the limbs and strength
of the limbs. In some birds, the length of the limb reaches adult proportions
early, but is not capable of supporting heavy loads until later in development.
In humans, the length of the femur and the strength of the femur are largely
set separately. Length is largely genetically determined, whereas femur
strength increases when loading begins (ie. when we start walking). Chris Ruff
has done some excellent work on this.
4) The growth in hindlimbs and forelimbs need not follow the same pattens.
They do not in humans,
for example. Of course, this might be very different in those birds that use
the forelimbs for propulsion as well.
So really, a comparison of limb strength and growth across taxa for hindlimbs
and forelimbs would be quite useful. And there I will have to stop, because I
can't say more at this time. I should have a lot more to say on the subject
come this spring.
> They look like they could even fly. They don't look like newly hatched
> passeriforms that can barely lift their huge heads off the nest floor.
> I don't know if ducks qualify as precocial here... in any
> case, I've
> got an impressive digital photo of a duckling's sterna plus one
> coracoid (I
> don't know what precise age). The sterna are two flat plates,
> there's no
> trace of an ossified keel, and the two bones aren't even sutured
> yet -- even
> though the coracoid is completely ossified, and the facets for the
bs and the coracoids are fully formed in bone, too.
> I'll risk a guess: they start out short and stubby and then grow
> mainly in
> length. Though not quite as impressive, our own extremities are a
> case of
> just this happening.