[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Fwd: Aw: RE: Arboreal Theropods: The prize at the bottom of the cracker jack box

Don Ohmes <d_ohmes@yahoo.com> wrote:

> The feet of birds and their ancestors are shaped by daily foraging/escape 
> locomotor activities, not sleep
> habits

Yes, I see what you're getting at here.  Extant birds that forage on
the ground tend to have an elevated hallux, or none at all.  Some of
these birds spend time in trees (sitting / roosting / sleeping),
despite having a reversed hallux that is small and/or elevated, or
lacking a hallux altogether (like some tinamous).

Nevertheless, modern ground-foraging birds that sleep in trees at
night typically retain a hallux suitable for perching - such as
turkeys, seriemas and secretary birds.  I'm positing that a grasping
foot would be especially important for long-legged and highly
cursorial birds when roosting in trees.

So for this and other reasons I'm very cautious about extrapolating
roosting ability to bird ancestors (and their kin) that lacked a large
and reversed hallux.

Further, we do not yet know what shaped the grasping foot of avian
ancestors.  A grasping, anisodactyl foot entails more than just a
reversed hallux (= hind toe).  The hind toe also has to be large
enough, and positioned low enough on the foot, to be able to oppose
the other three digits.

It appears (based on fossil paravians, including basal avialans) that
the hallux was lowered and enlarged in theropod evolution *before* it
became fully reversed.  It is possible that a lower hallux was driven
by an entirely non-arboreal function.  (Note: The first metatarsal
lost contact from the tarsus early in theropod evolution, and
thereafter progressively migrated down the metatarsus; so the fully
descended hallux of perching birds represents the epitome of this
theropodan trend.)  It is also possible that the initial step in the
reversal of the hallux was unrelated to arboreality; although it seems
probable that full reversal was linked to perching.
Confuciusornithids and sapeornithids appear to have had an
incompletely reversed hallux - it seems likely that this intermediate
morphology was connected to arboreality, and optimized by full
reversal in later birds.

>-- and the implied argument that the perching foot is a convenience conveyed 
>to it's owner by sleeping on a
> branch needs to be put to rest (pun intended).

I think we have to agree to disagree here Don.   :-)   IMHO, he
ancestral terrestrial/cursorial bauplan of theropods means that modern
avians might not always be the best analogs for what microraptorines,
archaeopterygids etc were capable of.  Just because a petrel or a
tinamou can sit in a tree w/o the convenience of a perching hallux,
doesn't necessarily mean _Microraptor_ or _Archaeopteryx_ could.