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possess well-developed Mm. extensor longus alulae and ulnometacarpalis
dorsalis. Both of these muscles usually sustain
a reduction when the automatic conjunction of movements in
the elbow and carpal joints becomes more efficient (Stegmann,
1970). The Hemiprocnidae also are characterized by relatively
simple inner differentiation of such important flight muscles as
M. pectoralis, M. flexor carpi ulnaris, and M. extensor metacarpi
radialis. In the crested swifts, comparatively weak development
of M. extensor digitorum communis, M. flexor digitorum
profundus, and M. ulnometacarpalis ventralis is obviously associated
with their limited role of resisting aerodynamic forces,
whereas in the true swifts and hummingbirds these muscles
also participate in active rotation of the manus and its major
digit.
Certain flight muscles are developed in constant proportion
to body size in all representatives of the families compared: M.
rhomboideus superficialis, the group of Mm. serrati (with exception
of M. serratus superficialis pars metapatagialis, it being
absent in the hummingbirds), M. coracobrachialis caudalis, M.
tensor propatagialis pars brevis, M. scapulotriceps, M. brachialis,
M. expansor secundariorum, M. ectepicondylo-ulnaris, M.
abductor alulae, and M. flexor digiti minoris. All three families
are characterized by the weakness of both M. deltoideus minor
and M. scapulohumeralis cranialis.
Relative development of the following flight muscles increases
from the Hemiprocnidae to the Apodidae to the Trochilidae:
M. subcoracoideus caput ventrale, M. pectoralis, M.
supracoracoideus, M. humerotriceps, M. flexor digitorum profundus
caput humerale, M. flexor carpi ulnaris, M. extensor
metacarpi radialis, M. extensor digitorum communis, M. extensor
longus digiti majoris, M. supinator, M. ulnometacarpalis
ventralis, and M. abductor digiti majoris. Thus, in addition to
an obvious and quite understandable hypertrophy of M. pectoralis
and M. supracoracoideus, the reinforcement of the flight
muscles in the true swifts and hummingbirds involves those
that supinate the humerus and forearm, extend the elbow, extend
and flex the wrist, rotate the manus, supinate the major
digit of the manus, and flex the major digit and pronate its distal
phalanx.
In the same sequence, the following muscles become relatively
less developed: M. scapulohumeralis caudalis, M. rhomboideus
profundus, M. deltoideus major, M. latissimus dorsi
pars cranialis, M. biceps brachii, M. extensor longus alulae, M.
ulnometacarpalis dorsalis, and M. flexor alulae. In the true
swifts and hummingbirds, a relative weakness of the muscles
that elevate and retract the humerus without causing rotation
(M. latissimus dorsi pars caudalis, M. scapulohumeralis caudalis,
M. deltoideus major) obviously results from a hypertrophy
of both M. pectoralis and M. supracoracoideus. These two
muscles provide mainly rotational mobility of the humerus relative
to its long axis in the true swifts and hummingbirds,
which correlates with the caudal orientation of the caput humeri
and the shortening of the humeral shaft in these families
(Karhu, 1992b). The retracting action of M. pectoralis grows as
SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
its sternal attachment widens caudally, increasing the amount
of muscular fibers oriented in a craniodorsolateral direction.
In contrast to the large number of muscles with similar tendencies
of specialization in both the Apodidae and the Trochilidae,
there are few muscles in which specific reinforcement or
reduction is unique either to the Apodidae or to the Trochilidae.
The Apodidae exceed the Trochilidae in relative development
of M. coracobrachilais cranialis, M. subscapularis caput
laterale, M. flexor digitorum superficialis, M. extensor metacarpi
ulnaris, and M. interosseus ventralis. In the Apodidae, enlargement
of the muscles listed above provides more efficient
maintenance of the spread wing and prevents passive extension
of the wrist and passive dorsal flexure of the major digit. These
peculiar transformations of the flight muscles in the Apodidae
correspond to their greater ability in gliding and fast, forwardflapping
flight in comparison with the Trochilidae.
In comparison with the Apodidae, the Trochilidae have
much better developed Mm. pronator superficialis and pronator
profundus but less developed M. subscapularis, M. latissimus
dorsi pars caudalis, M. tensor propatagialis pars longa, and M.
interosseus dorsalis. In addition, the hummingbirds lack both
propatagial parts of M. pectoralis, and M. flexor digitorum superficialis
remains only as a short, stout tendon, attaching on
the proximal part of the lig. humeroulnare. They have only two
of the four alular muscles, namely, M. abductor alulae and M.
adductor alulae, the latter being greatly reduced.
Reinforcement of Mm. pronator superficialis and pronator
profundus in hummingbirds indicates an extensive rotational
mobility of the forearm relative to the humerus. This conclusion
conforms with the structure of the elbow joint in the hummingbirds,
which allows significant rotational movements, unlike
the more restricted mobility in the true swifts.
A conspicuous example of divergence between the Apodidae
and Trochilidae is provided by M. biceps brachii. In living
Apodiformes, only the crested swifts have the M. biceps
brachii ending on both the proximal end of the ulna and the
proximal end of the radius. In hummingbirds there is a single
insertion on the ulna, whereas in the true swifts the insertion is
on the radius. Taking into account that the double insertion of
M. biceps brachii is typical for most birds, it is obviously more
generalized, and a single insertion, either on the ulna or on the
radius, represents a morphological specialization.
Discussion
The following features show the general level of specialization
to be lower in Argornis than in Jungornis: coracoid with
facies articularis sternalis saddle-shaped; sternal facet of coracoid
relatively narrow dorsoventrally with only the medial part
ventrally protruded; sternal facet of coracoid with the angulus
lateralis projecting beyond the level of the angulus medialis;
both proximal and distal ends of humerus relatively narrow;
humeral shaft more slender; only the smaller ventral part of the