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

[Tim Williams <tijawi@gmail.com>]

Don Ohmes <d_ohmes@yahoo.com> wrote:

> A horizontal branch that 2x the length of my foot in diameter presents little 
> challenge, in my experience.


While sleeping?


> The above is special pleading of the most brazen sort. It boils down to a 
> decree.


Not at all.  Among birds, the ability to roost in trees without a
grasping hallux is not universal.  For example, the only cranes
(Gruidae) that roost in trees are the crowned cranes (_ Balearica_
spp.), which have a hind toe capable of perching (this is inferred to
be a primitive trait for Gruidae).  Hence, although turkeys and other
gallinaceous birds might be able to roost without the benefit of a
grasping hallux, this does not hold for all birds - especially
long-legged birds like cranes.


> The turkey is presumably descended from an ancestor with the iconic "perching 
> pes", and roosts nightly --
> yet the hallux has *become* vestigial relative to grasping function -- as one 
> would expect, given it's foraging
> habits.


Ground-foraging habits favor a reduced and more elevated hallux in
birds.  Perching or roosting typically favors retention of the hallux.
 The hallux of many ground-foraging birds that roost in trees may not
be vestigial, if it is still capable of grasping (though obviously not
as well as a long, low hallux).  A specialized perching foot with a
large and fully descended hallux make it easier for a bird to grasp
narrower branches.


In some highly cursorial birds, the hallux is lost altogether (as it
was in ornithomimids).


> Reality and evolutionary logic stand contra the contention that sheltering in 
> trees inevitably alters the foot --


It depends on which theropods (birds or non-birds) we're talking
about.  In general, in light of the differences in hindlimb carriage,
center of mass, etc I'm reluctant to extrapolate modern avian
behaviors like roosting to non-avialan theropods.  I include close
bird relatives in this list, such as _Microraptor_.


> and rebuts the contention that a trees-down path to flight is necessarily 
> associated with "arboreal
> adaptations".


Agreed.  Nevertheless, if there are no arboreal adaptations... why
propose a "trees-down" path to flight at all?






Cheers

Tim