Newsgroups: rec.birds
From: maione@cs.toronto.edu (Ian Christopher Maione)
Subject: Bird Evolution
Organization: Department of Computer Science, University of Toronto
Distribution: na
Date: 13 Apr 92 03:58:43 GMT

   In article 5793 Joe Morlan writes ......

>The limestone imprints of the feathers of Arechaeopteryx indicate
>that it could fly.  The outer primaries are asymmetric, a feature
>found only on modern flying birds.  Modern flightless birds have
>symmetric wing feathers.  However, Archaeopteryx lacked the
>keeled sternum which anchors the large pectoral muscles of modern
>flying birds.  So it is not clear if Archaeopteryx was capable of
>flapping flight.  My view is that it probably was an excellent
>glider.

   Yes, Archeopteryx could certainly at least glide, which is in line
with the notion that birds evolved from "the trees down", as climbing
animals which eventually developed flight as a natural consequence of
moving around their tree environments in more effective ways.  After
all, if you're constantly moving from tree to tree, anything that can
help you break a fall is beneficial, and an ability to help break a
fall could have evolved into an ability to glide, and later to full
flapping flight.  Archeopteryx had a hallux (inner toe) which was
more backward pointing than any other theropod, and its limb claws are
very like those of modern climbing animals.  This suggests Archeopteryx
might indeed have been a tree climber.
    An alternate view of bird evolution is as "from the ground up", the
idea being that protobirds were fast ground running creatures which
developed flight to help catch flying insects.  However, this idea has
some serious problems, not the least of which is that such a creature
would have to eat a tremendous amounts of insects to keep itself going,
and besides, the early versions wouldn't have been able to catch
highly evolved flying insects very effectively anyway.

ICM>     Recently (I'm not sure whether this has been published
ICM> yet) there was also a discovery of another possible bird
ICM> ancestor from the late Triassic period (quite a bit earlier
ICM> than Archaeopteryx) dubbed "Protoavis", which has thrown a
ICM> bit of a twist into the issue.  However, Protoavis has some
ICM> characters (i.e a four-toed foot) which suggest that it is
ICM> not as close it birds, and it is certainly possible that
ICM> flight might have evolved more than once, with only one line
ICM> surviving to become modern birds.

>I've heard of this, but don't know the details.  However, most
>modern birds have four toes, so I don't understand why this
>character would mean it was not close to birds.  Modern birds
>with fewer than four toes are believed to have lost the extra
>toes in their evolutionary development.  How many toes did
>Archaeopteryx have?

    Actually, in terms of discussions of their evolution, birds are
counted to have only three toes, although they have the remnant of a
fourth.  If you look at any bird, you will see this (although in my
experience a goose is about the best example I've seen, becuase of its
large webbed feet).  A goose's foot has three large forward pointing
toes, and the much smaller one which points straight backwards.  Theropod
dinosaurs have exactly the same arrangement, but the last one isn't
counted with the others, so theropods are said to have three toes.  This
is in constrast to some other types of predatory dinosaurs
(paleodinosaurs, herreravians) which had four fully developed toes.
Perhaps ornithologists and paleontologists count their toes differently?
Actually, if you look at the foot of goose, you're going to get as close
as you're going to seeing the foot of T.Rex.

   Archeopteryx was a theropod, and thus had three toes if you count
the way I described above.  In fact, one specimen of Archeopteryx which
had no feather impressions preserved with it was mistakenly identified
as the theropod Compsognathus for over 30 years!  I suppose some birds
(i.e perching birds especially) would have kept that fourth toe to a
much greater extent than other birds (i.e waterfowl) which had no use
for them.

Regards,
Ian

"Birds are living dinosaurs, we can substitute the term 'dinosaur' for
'bird'; A dinosaur in the hand is worth two in the bush"
Padian K & Gautier J.A  "The Origin of Birds and the Evolution of Flight"

Newsgroups: rec.birds