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



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