PDF (Lo-Res) - Smithsonian Institution Libraries

278
ment of the radius that are continued from the main slab (Figures
2, 3). This slab also bears several impressions of the wing
feathers. The counterslab and associated slab form a broken
contact with each other (Figure 3).
VERTEBRAE.—The preserved 15 or 16 cervical and thoracic
vertebrae (3-17(18)) are ventrally exposed on the main slab
(Figures 1, 3, 5). The first (atlas) and second (axis) vertebrae
are absent, although a probable fragment of the latter is preserved
in front of the first preserved vertebra, which I consider
to be the third cervical. The first two preserved vertebrae each
have a short, wide, flat ventral body surface; thus, I consider
them to be the third and fourth cervicals. Most of the portions
of the probable fifth and sixth cervicals are disarticulated from
the other vertebrae. The seventh and eighth vertebrae each
show a long, flattened body ventrally. They have well-developed
cranial zygapophyses with costal processes and caudal
zygapophyses (Figures 1, 3, 5). The carotid processes form a
shallow open carotid canal. The strong, paired, caudal, transverse
processes are developed on the ventral side lateral to the
caudal articular surface. Such processes are present in Lithornis
and in Rhea; they are different from the postlateral processes in
grebes (Zusi and Storer, 1969).
The ninth cervical was extracted in order to investigate the
articular surfaces of the vertebral bodies because in earlier papers
the vertebrae of Ambiortus were regarded as probably
amphicoelous. Although this was widely used to establish the
relationships of this bird, it appears to have been a misinterpretation.
As a result of this new preparation of the vertebrae,
it has become evident that the caudal articular surface of the
eighth cervical and the cranial surface of the tenth cervical
were certainly heterocoelous. The tenth and eleventh vertebrae
have a longer centrum with a narrower ventral side than do
those of the third and fourth vertebrae and those of the thirteenth
through sixteenth vertebrae, but they have shorter centra
than in the seventh and eighth vertebrae. The twelfth, thirteenth,
and fourteenth cervical vertebrae show shortened
bodies and enlarged costal processes represented only by the
sturdy basal portions. The basal portions of two well-expressed
ribs are preserved near the right side of the fifteenth
vertebra. These ribs are small and represent the floating ribs.
The sixteenth vertebra is very compressed. The seventeenth
vertebra has a wide and nearly flat ventral body surface and a
very narrow caudal articular surface. On the left side of this
vertebra the dorsal portion of a large rib is present; it has two
articular facets, although the dorsal one is not preserved in this
sample. A probable portion of the eighteenth vertebra is preserved
caudal to the seventeenth vertebra in the angle between
the coracoid and sternum. The thirteenth through seventeenth
vertebrae have shortened bodies. The ventral crests (hypapophyses)
are not present in either the cranial cervical or the thoracic
vertebrae. I think that this specimen preserves at least 15
cervical vertebrae, which mainly have wide and short centra.
Thus, Ambiortus was a short-necked bird.
SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
FIGURE 2.—Associated slab with Ambiortus dementjevi, PIN 3790-272; Khurilt
Ulan Bulak locality, central Mongolia, Neocomian. (Scale bar= 1 cm.)
SHOULDER GIRDLE.—The cranial portion and left costal
margin of the sternum preserves the base of a thick pillar of the
sternal keel and a damaged sternal costal margin (Figures 1, 3).
The cranial surface of the pillar is directed down and somewhat
caudally. The furcula is represented by the dorsal portions of
both clavicles, which terminate at slightly thinned ends that are
neither enlarged nor flattened. The small articular facet for the
coracoid is directed caudomedially. The cross section of the
clavicle in its middle portion is nearly circular. The medial side
of the clavicle is slightly flattened. Originally, the holotype
preserved the mold of the interclavicular symphysis, which was
placed superficially on the level of the fifteenth vertebra. This
was destroyed during preparation. The mold of the ventral furcula
showed that the ventral portion of both clavicles have the
same diameter as dorsal ones. In the interclavicular area, a thin,
slightly projecting eminence around the symphysis was
present, without any vestige of the hypocleideum.
In the left scapula, the caudal end of the scapular body is absent.
The humeral articular facet is flat and wide and faces latero-cranially
but is not nearly perpendicular to the scapular
blade as noted by Elzanowski (1995). In its cranial area, the
facet converges into a slightly convex, ellipsoidal coracoidal
tubercle. A strongly projecting and sturdy acromion is very distinctive
for this scapula (Figure 4). The acromion is dorsoventrally
compressed, and its cranial ending is blunt and attenuated
cranially. The dorsal surface of the acromion possesses a conspicuous
dorsal tubercle (not crest) that is very similar to that in
Lithornis celetius Houde, 1988. The acromion is not turned
mediad, contrary to Elzanowski (1995). The scapular body is
blade-like, strongly flattened lateromedially, and slightly