Re: Anatomy Q & A

[David Marjanovic <david.marjanovic@gmx.at>]

Let's see if I can be of any help...

> My best guess so far is that, as a flexor of the
> digits, it is important in bracing the phalanges against the wieght of
> the body in digitigrade animals, but I have not been able to discover if
> the pattern of its development in other animals is consistent with that
> idea.  If it is as unimportant in apes as in humans then its is possible
> that, with the evolution of the brachiating hand, it decreased in
> importance compared with more digitigrade ancestral primates.  Anyway,
> that's the limit of my speculations.

We don't have any digitigrade ancestors. All primates, including the
quadrupedally running ones like baboons, are plantigrade.

> 4. Dorsal ribs in tetrapods.....are homologous with what structure in
> fish?  Sarcopterygians don't have ossified ribs (or, if they do, they
> are very short).

Although short, they are present.

> Actinoptergians can have dorsal and ventral ribs in the trunk

This has recently turned out to be a misinterpretation. Ribs are ribs, and
"dorsal ribs" simply don't exist. I forgot the ref, though.

> 6. Why does digit I of the ancestral pentadactyl limb have two
> phalanges, whilst all the others have three or more?

I don't think there's a better answer than "why not". The ancestral formula
for amniotes is 2-3-4-5-3/4, in any case, which makes some sense; the
anthracosaur ( = _some sort_ of basal tetrapod) *Silvanerpeton* has
2-3-4-5-6, which looks even more logical for a sprawling animal.

> 7.... and lastly for now, a question that is either surgically incisive,
> or just plain stupid (!).  Why is it that elbows bend forwards, and
> knees bend backwards?   Usually no attempt is made to explain it - the
> only offering I found was that 'pushing is more effecient that pulling'
> (explaning the knee, at least), but I don't see mechanically why this
> should be true.

Perhaps the answer can be found somewhere among basal tetrapodomorphs. The
elbows of *Eusthenopteron* already bend forwards.

> The only exception to this rule that I can think of is,
> of course, turtles, where the elbow bends back in at least some species

It just looks this way, because their humeri are twisted in quite weird
ways. But their elbows do bend forwards. Try rotating your shoulders
forwards to get an idea of what it is like.

> Which just proves that turtles really are from Mars......

They're certainly capable of morphological long-branch attraction.