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Re: _Archaeopteryx_ and sterna
If Archaeopteryx had a sternum (and the new specimen confirms it did), the
sternum was small and could not have been the origin of large flight
muscles. But this does not mean it could not `fly'.
As others have said, look at bats, where there is no large, keeled sternum.
The pectoral muscles which drive the downstroke are still substantial in size.
The situation is not directly comparable, because the architecture of the
entire pectoral girdle in the two groups is very different. The absence of
a sternum in Arch. is however most informative.
In modern birds, two muscles originate from the sternum: the pectoralis
(aka pectoralis major), which imposes a down force on the wing (though see
below), and the supracoracoideus (aka pectoralis minor, which is a highly
derived system which imposes an up force on the wing). In fact, the
pectoralis is divided into at least two portions (work by Dial and
co-workers): simplifying greatly, the portion originating from the
furcula and from soft tissues directly below the shoulder joint depresses
the wing, while the portion posterior to and below the shoulder joint
originating on the sternum pronates the wing, by attaching to the
ventral side of the delto-pectoral crest of the humerus. It is reasonable
to regard these as providing separate functions.
In modern birds the proportions of these components and the geometry of
their origins are quite variable. A chicken, for instance, has a large
sternum because the elevating supracoracoideus and the wing-rotating
portion of the pectoralis are large. This comes - as in many galliform
birds - from their climbing flight.
Without a sternum, Archaeopteryx could easily have had a large pectoralis
to depress the wing (there are few bony structures which would be evidence
of its origin), but did not have a supracoracoideus, or the wing-rotating
portion of the pectoralis. The absence of the supracoracoideus was shown
in the 1970s by John Ostrom, on the basis of the morphology of the humeral
joint and coracoids.
This does not mean Arch. could not `fly'. For steady, level flight it
needs the ability to depress the wing, _but that is all_. Aerodynamic
forces will be sufficient to raise the wing in the upstroke. The sternum,
plus the muscles associated with raising the wing passively and with
controlling the pronation (and therefore aerodynamic pitch) of the wing
are adaptations associated with slow flight and a climbing take-off from
the ground. It is not surprising that these evolved as a responbse to
selection for improved flight soon after Archaeopteryx, and appear to be
present in the Spanish and Chinese early cretaceous birds.
This leaves Archaeopteryx as able to fly steadily at relatively high
speeds (what we would identify as cruising flight in a modern bird), but
probably unable to take-off unaided from the ground (Kevin Padian wrote a
few weeks ago about these problems), and with rather poor control in
flight. But the absence of the sternum does not rule out the
possibility that it could flap its wings to generate sufficient thrust for
level flight. By my definition, this is flight, though admittedly rather
poor by comparison with typical modern birds.
This may also explain the low asymmetry in some of Archaeopteryx's
primaries. The dichotomy between asymmetric = flight and symmetric =
flightless is too simple. Feathers are asymmetric so that they maintain
aerodynamic integrity as they deform under aerodynamic loads in flight.
The greatest degrees of non-emarginated asymmetry come in birds for which
pitching loads on the wing and the feathers are greatest, and these are
species for which manoeuvrability, climbing flight are most important.
Asymmetry is a derived adaptation for derived modes of flight which
Archaeopteryx did not have, but which were to evolve in birds
subsequently. It is not surprising that Arch has quite low asymmetry,
Dr Jeremy M. V. Rayner
School of Biological Sciences
University of Bristol
Bristol BS8 1UG
tel. 0117 928 8111, messages 0117 928 7476, fax 0117 925 7374