Re: Bats and Archaeopteryx

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>I too was wondering about their
>evolutionary development with regards to flight. I was also wondering if
>there has been any evidence in their fossil record  (scant as it is) to
>show that they evolved from a gliding form (through the stages that you
>list, ie. fingers becoming longer and incorporating into the wing structure)?
>Also, have there been any studies done comparing the flight energy
>needs/consumption between bats and birds? Don't bats require/expend more
>energy than birds for flight?

No good protobats are known from the fossil record (which isn't surprising,
since the record of bats themselves is remarkably spotty).  Many recent
phylogenetic analyses show that bats are part of a complex of mammals, the
Archonta, which also includes Dermoptera (the "flying lemurs"), Scandentia
(the "tree shrews"), and Primates (us).  The body of evidence show that
this is an ancient clade of arboreal small mammals which radiated into a
variety of life styles.

>>RE. Archaeopteryx: Hmmm, let me try to rephrase my questions on this. My
>question is simply one of whether the evolutionary pressure from an
>arborial gliding existence would tend toward a bird-like wing development
>or a flying squirrel-like skin-flap development. Extant arboreal gliders
>all use the skin flap method. We have no modern analog to show that a
>bird-like wing can be derived from this (or that a bird-like wing will
>even evolve in an arboreal species). None of the modern gliding
>animals is even in an interim state of development <towards> the wing
>structure that birds have.

One thing that hasn't been mentioned much during this year's go 'round on
bird origins is that morphological condition which first led Ostrom to
propose the "cursorial hypothesis".  Unlike other flyers and gliders, birds
from Archaeopteryx on up have forelimb and hindlimb complexes which are
independant.  Birds ancestrally have an upright hind limb posture which
works very well for terrestrial locomotion (as well as perching), while the
forelimb is highly transformed.  All other modern terrestrial vertebrate
flyers (and arguably the pterosaurs) have hindlimbs which are incorporated
into the wing structure, with the femur held out from the body.

Ostrom (and others) have suggested that, given the hindlimb structure of
Archaeopteryx and modern birds, there is no evidence of an evolutionary
passage through a "gliding squirrel"-type stage.

>Another question is in regard of the tucked wing position. Extant
>tree-dwelling gliders sprawl themselves out to bring their center of
>gravity closer to the tree. They don't tuck their front limbs close to
>the body. Tucking isn't likely to occur as long as you are climbing the
>trunk of the tree. However, once you have started to develop a tucked
>limb position, you can still hop up into the lower branches. Now, correct
>me if I am wrong (or my facts out-of-date), but I seem to recall that
>Archaeopteryx limbs were pretty-much tucked in (like a birds) but with a
>greater range of motion. This would lead me to suspect that they had at
>least evolved out of the trees, and (if Archies were tree dwelling) moved
>there subsequent to the development of the wing-limb.

The forelimbs of Archaeopteryx are anatomically scaled down (in absolute
size) versions of the forelimbs of other maniraptoran dinosaurs, such as
dromaeosaurids, oviraptorids, troodontids, etc. (as beautifully shown in
the type of Sinornithoides).  It appears birds inherited this "tucking"
ability, which may have been used to keep the forelimbs out of the way
during normal locomotion.

>Now, IF the structures were already present prior to tree dwelling, they
>most likely were used to the greatest advantage (which would include
>stabilizers for sharp turns, added lift on jumps/hops, etc. All of which
>are perfectly good flight precursers) while on the ground. Once in the
>trees, the evolutionary advantage may have been ENHANCED and thus develop
>into what we see today, but the beginnings, I believe, started on the ground.

Also, the structure of the pelvis and tail of Archaeopteryx is similar
(although in some ways less derived) than those of dromaeosaurid dinosaurs.
These transformations appear to have been associated with the development
of the tail as a "dynamic stabilizer".  Such a structure is useful in the
trees (or on the highwire, as in the balancing pole of an acrobat) or on
the ground (felids use their tails in such a manner to make rapid, sharp
turns while pursuing prey).

Also, the vocabulary word of the day:

Scansorial:  of or pertaining to animals which habitually climb trees or
other vegetation, but spend a majority of their time on the ground.

(This, I think, will become more and more important as Archaeopteryx
studies continue...)

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.