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

Re: Sinocalliopteryx (Theropoda: Compsognathidae) ate confuciusornithids and dromaeosaurids



Jason Brougham <jaseb@amnh.org> wrote:

> Thank you for conceding that, if basal paravians could elevate the humerus 
> just a bit above the dorsum, they could have engaged in flapping flight.


Yes - if the forelimb was capable of a limited upstroke, I'm prepared
to accept that these basal avialans (such as _Archaeopteryx_) were
capable of some form of sustained flight ability.  This flight
wouldn't have been pretty, and it would have been very low-amplitude,
but it would have been powered flight nonetheless.  A ground-level
takeoff seems highly unlikely though, irrespective of whether the
wingbeat was low-amplitude (limited humeral elevation) or non-existent
(no humeral elevation).


> However, as I mentioned before, pinning down the exact position, geometry, 
> and range of motion in the shoulders of extinct animals known only from 
> fossils - to a certainty good
> enough to settle this matter - may be impossible.


Yes, I fear you may well be right.  In today's world, it's fairly easy
to sort birds into those that can fly and those that can't.  A few
extant species are a bit tricky (such as the kagu and kakapo), in the
sense that their flight capabilities are extremely weak.  However, in
these extant forms the wing is capable of full humeral excursion, so
these birds may not be appropriate analogs for incipient flight
behavior by avian ancestors.  For the first aerial maniraptorans,
early powered flight was probably nothing like that of any modern
bird.  So the transition from non-flight to flight would have produced
animals whose aerial behavior was not easy to define as "flighted" or
not.  This transitional state may be extremely difficult to discern
from anatomy alone.


> I agree with your observations about the advanced features of the flight 
> apparatus in modern birds, but I noticed you did not mention my reference on 
> bats. Bats may be useful analogs
> for early stages of flight evolution, as animals that achieved powered flight 
> with fewer radical modifications of the basic tetrapod pectoral anatomy.


If the initial flapping flight of non-ornithothoracean avialans
involved a deltoideus-driven upstroke then, yes, the early stages of
flight might have been quite bat-like in this respect. The caveat is
that bats evolved from mammals that were highly adapted for
quadrupedal arboreal locomotion.  So the pectoral girdle was already
suited to a wide range of forelimb movement.


> I am afraid I cannot agree with your certainty in this statement:
>
> "But it's safe to say that an inability to raise
> the humerus above the dorsum precludes powered flight, and therefore
> indicates flightlessness."
>
> as nature has a way of providing counter examples to almost all a priori 
> rules we may try to impose on it.
>
> Rotation in the humerus, for example, could produce elevation of the wingtip 
> that could be sufficient for weak flapping.


If humeral elevation was not possible (= no upstroke), I think the
most parsimonious interpretation is that these animals couldn't flap.
To me, this is preferable to invoking a novel kind of wingbeat.  The
idea that _Xiaotingia_, _Anchiornis_, _Microraptor_, _Archaeopteryx_
and _Jeholornis_ were non-powered gliders who used their feathered
forelimbs during descents to the ground seems entirely reasonable to
me.







Cheers

Tim