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Re: Peering at review (Triassic dino-birds) (long)



You wrote:
>
> * I know drepanosaurids have also been re-interpreted as aquatic swimmers;
> but I think the evidence favors arboreal (chameleon-like?) habits for
these
> critters.
>
Somewhile ago while corresponding with HP David Peters (don't you just love
the sound of that? ;)), we discussed possible lifestyles for the
Drepanosaurs and based on the skeletal reconstructions that were on his
website, I came up with these possible lifestyles for Drepanosaurus as well
as Megalancosaurus:

The high pectorals probably took over the function that the scapula served
previously: muscle attachement. There seems to be some sort of red-line in
the evolution of the Drepanosaurs/ Megalancosaurus and that is to decrease
the size and with of the scapula and an increase in size of the pectorals.
In this case, it would make Megalancosaurus more advanced than
Drepanosaurus, the genera I use for comparison. As is seen in muscle
restorations, there is a big set of muscles attached from the humerus to the
scapula for which the distal is often enlarged. This in contrary in the
Drepanosaurs in which the scapula seems to decrease in size and reaches it's
climax in Megalancosaurus, were the scapula is just a thin strip of bone and
the enlarged pectorals took over this function for unknown reasons. Second,
the coracoid is in Drepanosaurus much smaller that it's relative relatively,
as is the size of the pectorals, so the purpose of the scapula was not  only
taken over by pectorals, but also by the coracoid to create a possibly more
effective and stronger way for muscle attachement and therefore to create a
possible stronger arm used for propulsion over the branches and through the
trees.

As for a possible lifestyle: as I said above, an arboreal lifestyle: as I
said above, an arboreal life-style is proven, something that was also said
by Feduccia in his book "The Origin and Evolution of Birds (second
edition)"(1996). This also explains the structure of the hands and feet of
Megalancosaurus, especially the hands which show a remarkable similarity to
the structure seen in koala-bears and chameleons, as well as owl feet to
some degree. The divergent toe on the foot is also evidence for an arboreal
lifestyle in the way that the divergent toe lay on the branche and the other
four surrounded the branche to give grip, a purpose all toes served in
Drepanosaurus (in HP David Peters all toes curve the same and therefore
could have served the function as a single working unit in clinging on to
the tree). From the reconstructions made for Megalancosaurus, a pretty
much horizontal animal comes to light, in contrary to the condition seen in
Drepanosaurus, which appears to be more vertical. The elongated and skinny
neck with the small pointed skull are therefore lowered and placed more
closely to the surface of the branche or the tree. With it's slender neck,
it would have had no problem at all reaching the insects inside the tree in
the crack.

Drepanosaurus on the other hand appears more interesting in it's lifestyle,
with it's food  being reached by clawing through and splitting the bark with
it's claws in search of insects. This would be the most logical food source
available in the trees and that can be coped with the tiny teeth that are
found in at least Megalancosaurus. The claw could also have been used for a
sort of woodpecker-style, with Drepanosaurus tapping on the bark of the
tree, which should result in that insects within the tree are coming out and
be an easy lunch for Drepanosaurus. But it's way of getting to it's meal is
not the interesting part, it seems that Drepanosaurs had a variety of ways
to get through the trees with Drepanosaurus being an alternative to the
quadrepal way as exemplified by Megalancosaurus. Drepanosaurus on the other
hand, seems to have adopted a bipedal way; the first piece of evidence is
the natural curve of the caudals and this is accompanied with the claw-like
structure on the end of the tail. The curvature seen in the caudals are
causing the tail to point upwards which would not make much sense in
deliberately doing so without a use. When placing the tail horizontal, for,
say, on a tree branche or trunk, it would place the arms free and create a
bipedal stance for this genus (also observed in Tanystropheus, following HP
David Peters off-list sended reconstruction...). The posture is further
enhanced because of the line that the remainder of the (dorsal) vertebrae
follow. This is the reason that, as earlier stated in this message,
Drepanosaurus was a more vertical animal, in contrary to Megalancosaurus.
The tail "claw" would than have served the purpose to give grip and not to
let the animal fall flat on his face. =) With this stance the forelimbs were
raised of the ground and would avoid that the BIG claw grew blunt and would
be no longer usefull in either ripping the bark from trees or tapping the
bark to get to the insects. All toes on the feet are used for creating grip
on the branche, like said previously. It would not have won any speed medals
with this way of life, but at least it got to the insects...
If things are not clear about this part of the text, please drop me a
message and I will send you a quick sketch of the posture that I mean.

Opinions please...

Cheers,

Rutger Jansma