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Re: Arboreal Theropods: The prize at the bottom of the cracker jack box
Don Ohmes <email@example.com> wrote:
> A horizontal branch that 2x the length of my foot in diameter presents little
> challenge, in my experience.
> The above is special pleading of the most brazen sort. It boils down to a
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
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
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
Agreed. Nevertheless, if there are no arboreal adaptations... why
propose a "trees-down" path to flight at all?