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

[Jason Brougham <jaseb@amnh.org>]

Tim Williams wrote:

>The Inaccessible Island Rail was also reported to use its wings as
>brakes when jumping down sloped terrain (Hagen, 1952).

Sure, but I was referring to the new downy chicks which have no primary or 
secondary feathers.

(about rail chicks climbing using enlarged thumb claw):
>They are climbing/clambering using all four limbs.  But they are a
>long way from being arboreal quadrupeds (sensu stricto).

Right, because they aren't foraging mainly in trees. OK, so just theropods 
climbing quadrupedally then. Would you say murrelets are arboreal for their 
first weeks of life? They live in tree canopies for several weeks after 
hatching and have been reported as catching insects. This is before they can 
fly and they have no halluces. I wouldn't really. But if murrelets were 
extinct, and we found their fossils, and someone suggested trees were important 
in their biology, I think a strict minded adaptationist would say that was 100% 
impossible.

>In juvenile hoatzins (_Opisthocomus_), the feet do most of the work
>during climbing.  I don't think it's a great idea to compare
>scansoriopterygids too closely with hoatzins (juvenile or adult).
>There's a great deal of morphospace between the two.

Agreed. Of course, Opisthocomus is specialized to climb quadrupedally before it 
can fly. But manual claws, and delayed distal primary development, used in some 
cases for climbing, are distributed much more broadly in living birds. You 
suggested that Epidendrosaurus' manual claws were too weakly curved to be used 
quadrupedally but this seems to be common in the claws used by young birds for 
climbing: they look weakly curved.

Again, I am not saying Epidendrosaurus was arboreal. I am saying we must be 
cautious in ruling out climbing behaviors in animals that are not specialized 
in the ways that crown - group birds are for climbing.