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




On Tue, Jul 23, 2013 11:23 AM EDT Jason Brougham wrote:

>Alright. To recap as succinctly as I can.
>
>There seems to be consensus here that basal paravians could have engaged
>in 'occasional' climbing behaviors.
>
>But, to go further...
>
>There are living animals that lack climbing adaptations, or that are even
>members of clades that show 'trends' away from climbing, for which trees
>are still indispensable parts of their biology. These include murrelets
>and petrels, which have secondarily lost halluces. They include chicks and
>other birds with limited or absent flight ability (Inaccessible Rail,
>Kagu, hatchling tinamous, hatchling chukar).

And M gallopavo. I note that the hallux leaves no impression when turkeys walk 
on firm sand, and the claw tip does not extend to the rearward edge of the 
weight-bearing pad. Indeed, if memory serves, it has the gestalt of a dewclaw...

That said, it may indeed have function when hens are crouching on branches in 
brooding position.

>Basal paravians, living in habitats such as the Tiaojishan Formation which
>had diverse gymnosperm trees, arborescent bennettites, and tree ferns,
>could have utilized these habitats in some similar manner. Say, scrambling
>a meter or two up them to launch on imperfect wings. Or roosting on fallen
>tree limbs. Such behaviors do not require specialized adaptations but,
>rather, they could be the precursor of those adaptations. Animals that
>routinely entered trees without specializations would be ideal candidates
>to adapt to those habitats.
>
>The reasons we could suspect this in basal paravians, as opposed to other
>dinosaurs that show no climbing adaptations (such as, say, hadrosaurs),
>are 1) they are small 2) they are comparatively agile, with long, sharply
>clawed, hands and feet 3) they actually ARE the direct ancestors of the
>animals that specialized toward arboreality: the Avialae.
>
>For any given taxon the hypothesis can be tested by biomechanical analyses.

True. And I add that the iconic perching foot is most excellent gear for 
landing on a branch, or even grasping prey in hawk-like fashion, but what 
advantage would it grant to a flightless quadrupedal climber? 

Especially one that forages on the ground...

It would follow that the perching foot is a lagging indicator of flight, not a 
precursor.