RE: Archaeopteryx with bird book, was Re: Archaeopteryx flight

[Tim Williams <twilliams_alpha@hotmail.com>]

Jerzy Dyczkowski wrote:

> All birds and bird chicks of A. size are fully covered
> with feathers or down. Really few have naked head and
> neck for display purposes. In contrast A on many
> reconstruction is partialy bare, eg. on belly.

A naked head is also useful for scavenging - prevents fouling of the plumage 
while the scavenger is immersing itself in a rotting carcass.  I'm not 
saying that _Archaeopteryx_ was a scavenger - but the head (and neck) may be 
naked for purposes other than display.

> In living birds, femur and upper half of "tibia" are
> not visible under feathers. In contrast, most
> restorations portrait A. with visible femurs, more
> like human legs.

Yes, you're right - this doesn't make sense, given that _A_ apparently had 
femoral feathers.

> I fact, bird femur is kept parallel to backbone and
> "tibia" is permanently bent at angle, but I don't know
> if this was true of A. - one would need to examine leg
> joints in detail.

As I understand it, this subhorizontal (~ parallel to backbone) orientation 
of the femur in birds is associated with the position of the center of 
mass/weight (CM) in modern birds.  Modern birds tend to be much more 
'front-heavy' compared to non-avian theropods, due to a number of allometric 
changes (larger head, expanded pectoral musculature, short tail etc).  This 
bring the CM near the wings, much further forward than in their theropod 
ancestors, where it was situated near the pelvis.  Therefore, in order to 
bring the hindlimbs under the CM, the femur of birds is usually kept in a 
near-horizontal orientation, and stride generation occurs at the "knee", not 
at the hip socket.  Thus, modern cursorial birds tend to have very long 
hindlimbs, given that the tibiotarsus and tarsometatarsus serve as the 
effective hindlimb.  This isn't the full story; ratites do swing the femur 
when running, for example.  Ratites also show a heavy, posteriorly elongated 
pelvis, to help push the CM a little further back, as an aid to 
terrestrial/cursorial locomotion.

Jones et al. (2000) argued that _Caudipteryx_ had an anterior CM like modern 
birds, based on the high hindlimb/trunk ratio.  Christiansen and Bonde 
(2002) argued against this interpretation, noting (among other things) that 
the femur of _Caudipteryx_ is not robust enough to resist the torsion and 
bending associated with a role in stride generation.  In fact, no non-avian 
theropod appears to show this character.  For _Archaeopteryx_ to have its 
femur held near-horizontally implies a forwardly-located CM and stride 
generation comparable to modern birds, and I don't think there is any 
evidence for this.

Thanks Jerzy for these interesting posts.

Cheers

Tim