RE: Campbell's even crazier than a MANIAC? (archeopteryx climbing)

[Tim Williams <twilliams_alpha@hotmail.com>]

Jaime Headden wrote:


> The foot of pelicans is pretty characteristic of a non-back-toe-pointing pes 
> in 
> an animal that can stand on branches and "perch". That toe cannot reverse, 
> as in medially oriented..


As I recall, this is to do with the fact that the back toe (hallux) is 
incorporated into the webbed foot.  Nevetheless, the orientation of the hallux 
in pelicans is somewhat reversed, with an angle of 125 degrees observed for the 
brown pelican (Middleton, 2001).  (In Middleton's study, the orientation of the 
halluxias defined as the angle between the long axis of the third toe and the 
long axis of the hallux - so the more reversed the hallux, the greater the 
angle.)  So pelicans would have a posteromedially directed hallux.


Some waterbirds do have a medially directed hallux, like gannets, which tend to 
roost and nest in cliffs or on rocks.


>From Middleton (2001):

"The range of hallucal orientations among the birds quantified was 100°, from a 
low of 80 degrees in the northern gannet (_Morus bassanus_) to 180 degrees in 
both the rhinoceros hornbill (_Buceros rhinoceros_) and the maleo 
(_Macrocephalon maleo_) ... With a hallux oriented at less than 90 degrees, the 
northern gannet is the only species of bird studied that does not have a 
reversed hallux.  Increasing hallucal orientation angles are found in such 
species as the white-tailed tropicbird (_Phaethon lepturus_) and the Peruvian 
booby (_Sula variegata_), which have hallucal orientation angles of 110 degrees 
and 120 degrees, respectively ... Many of the birds with low hallucal 
orientation angles are thought to be closely related (e.g., boobies, pelicans, 
tropicbirds, gannets, and cormorants); all were formerly classified in the 
order Pelecaniformes and share the characteristic of interdigital webbing."


Reference:


Middleton, K.M. (2001).  The morphological basis of hallucal orientation in 
extant birds.  J. Morphol. 250: 51–60.


Incidentally, the secretarybird (_Sagittarius serpentarius_) has one of the 
highest hallucal angles (175 degrees), even though it hunts on the ground.


Middleton's study is worth reading because it spells out just what achieving a 
reversed hallux entails.  Anatomically speaking, retroversion of the hallux is 
no simple matter.  The presence of individual  traits associated with perching 
(especially related to the morphology of metatarsal I) allows this ability to 
be inferred for fossil taxa.  Simply having the hallux of a fossil bird 
preserved in an apparently posterior/reversed orientation does not 
automatically mean it was reversed in real life - which is a common mistake 
when inferring perching ability for many ancient birds (especially 
_Archaeopteryx_, in which the metatarsal I shows no evidence of reversal).



Cheers

Tim
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