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NUMBER 89 215
caput humeri perpendicular to the long axis of the bone; the
middle of the caput humeri without a distal protrusion on the
caudal side; distal part of humerus without a dorsal crest; humerus
with tuberculum M. pronator superficialis detached from
tuberculum supracondylare ventrale; distal end of humerus
with tuberculum supracondylare ventrale adjacent to the
condylus ventralis; proc. flexorius of humerus projecting weakly
distad; ulna with relatively small tuberculum lig. collateralis
ventralis; and double insertion of M. biceps brachii on both the
radius and ulna.
The Jungornithidae resemble the Apodidae in having a high,
robust, tapering, and proximally placed deltopectoral crest. In
the Apodidae the structure and position of the deltopectoral
crest correlates with reinforcement of M. coracobrachialis cranialis
and M. pectoralis, pars cranialis. All these features are
among the characters that provide a highly developed ability
for gliding flight in the true swifts (Karhu, 1992a). The similarity
of structure and placement of the deltopectoral crest in the
true swifts and jungornithids suggests that the latter could be
well adapted for gliding flight, too.
At the same time, Jungornis and the Trochilidae share some
essential characters that distinguish them from the Apodidae.
The structure of the humeral head in Jungornis clearly demonstrates
a trochilid-like specialization: presence of the distal enlargement
on the caudal surface. In the Trochilidae this modification
of the humeral head is associated with high
specialization of the shoulder joint, which allows extreme supination
of the adducted humerus during hovering flight (Cohn,
1968; Karhu, 1992a). In the Hemiprocnidae, Apodidae, and all
known fossil genera, with the exception of Jungornis, the humeral
head lacks any distal enlargement on the caudal surface.
Although Argornis possesses a double insertion of M. biceps
brachii both on the ulna and on the radius, there is no sign of
the radial insertion in Jungornis. This fact implies the presence
of a single insertion on the ulna, although it cannot be determined
directly because of poor preservation in the holotype of
Jungornis. If so, it would represent another trochilid-like specialization
within the Jungornithidae.
Taking into account that the trochilid-like characters under
discussion are absent in the relatively more generalized jungornithid
genus Argornis, their occurrence in Jungornis should be
considered a result of intrafamilial evolution parallel to that in
the Trochilidae. Because two jungornithid genera, Argornis
and Jungornis, show obvious similarities to the Apodidae, the
origin of trochilid-like features in the Jungornithidae demonstrates
the feasibility of developing trochilid-like specializations
from apodid-like adaptations. It suggests that the Trochilidae
could have arisen from an apodid-like ancestor as well.
This inference is supported by the analysis of evolutionary
transformation of the forelimb muscles in the Apodiformes,
which shows the widespread progression of certain specializations
in the Trochilidae relative to the Apodidae. Agreement in
the conclusions based on paleontological and myological data
clearly contradicts the opinion of Cohn (1968) that similarities
between the Apodidae and Trochilidae are convergent.
Both Argornis and Jungornis are similar to the Trochilidae
and differ from the Apodidae in having the cotyla ventralis ulnae
with a weakly pronounced ventroproximal edge. Owing to
this peculiarity, the condylus ventralis of the humerus can slide
ventroproximally relative to the ulna during the supination of
the forearm. In the Apodidae the ventroproximal edge of the
cotyla ventralis is prominent and strongly restricts the possible
rotational movements of the elbow joint in the spread wing
(Karhu, 1992a).
The position of the proc. supracondylaris dorsalis of the humerus
is among the number of especially important characters
in Apodiformes. The tendency for proximal displacement of
this process is obviously conditioned by the proximal enlargement
of the places of origin of M. extensor digitorum communis
and the ventral head of M. extensor metacarpi radialis on
the craniodorsal side of the distal part of the humerus. Very interesting
indirect evidence of such a correlation is provided by
Jungornis. Its humerus resembles the true swifts and hummingbirds
in overall configuration, but it has the proc. supracondylaris
dorsalis placed approximately on the same level as in Aegialomis,
the most generalized genus of Apodiformes known
(Karhu, 1992b). The relatively distal position of the proc. supracondylaris
dorsalis in Jungornis may be explained by the
presence of the high crest distally adjacent to the process. This
crest allows enlargement of the place of origin of both extensors
without proximal displacement of the supracondylar process.
Hummingbirds, which have the best developed M. extensor
digitorum communis and ventral head of M. extensor
metacarpi radialis, demonstrate both conditions: proximal displacement
of the proc. supracondylaris dorsalis, and muscle origin
from the crest. In Argornis the supracondylar process is located
distally, and the dorsal crest is absent, suggesting
relatively weak development of both these muscles.
The well-developed distal process of the caudal margin of
the proximal phalanx of the major digit indicates the presence
of long first primaries in Argornis (Stegmann, 1965). Evidence
for long first primaries in other Eocene apodiforms (e.g., Lydekker,
1891; Peters, 1985) suggests that the elongation of the
distal part of the wing occurred in the early stages of apodiform
evolution. In particular, this elongation might precede the divergence
of humeral structure that distinguishes the hemiprocnid
and apodid-trochilid directions of specialization of the
flight apparatus within Apodiformes (Karhu, 1992a, 1992b).