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NUMBER 89 173
nis but not highly vaulted as in vulturids), with prominent central
ridge, or septum (which is only slightly developed anteriorly
in Teratornis), but lacking distinct median groove posterior
to ridge (as present in Teratornis); crista tomialis wider in
transverse section and more deeply grooved and more symmetrical
anteriorly than in Teratornis but forming comparable enclosure
of deep, narrow channel in anteriormost portion of
beak, in region of sharp curvature.
In teratorns, the crista tomialis forms a sharp ridge external
to the region of the anterior narial border, but farther anterior,
about halfway to tip of beak, this ridge declines in prominence,
and the median portion of the grooved crista tomialis forms a
sharper ridge. The point of this transition represents the posteriormost
portion of this specimen of beak that is preserved,
which suggests that the specimen came from a bird with a proportionately
much deeper beak than seen in Teratornis.
Left ulna: Proximal end and partial proximal shaft (Figure
4), Anza-Borrego Desert State Park Paleontological Collection,
ABDSP (IVCM) 519/5660, from Vallecito member of
the Palm Spring Formation (Woodward, 1963), June Wash
area of the Vallecito-Fish Creek Basin. Specimen from Vallecito
Creek local fauna (Jefferson, 1995), dating to about
1.8-0.9 Ma.
This ulna is severely crushed, but enough of the original
structure remains to determine that it differs from that of Teratornis
by having (1) olecranon long and very broad; (2) cotyla
ventralis lying at a much steeper angle to the long axis of the
shaft; and (3) cotyla dorsalis wider and relatively shallow. Although
the specimen is quite crushed, most of the bone of the
proximal end is present so it is possible to approximate closely
its original shape. The cotyla ventralis may be slightly rotated
as a result of crushing, but the original shape, in ventral view,
is clearly preserved. The amount of bone present suggests that
the proximal end was less deep anterioposteriorly immediately
distal to the cotyla ventralis, and the tuberculum Lig. collat.
ventralis was much less bulbous, than in Teratornis. In addition,
it appears that the olecranon was much more compressed
anteroposteriorly, although some post-mortem crushing has
taken place.
A total of about 26 cm of the ulna is preserved, in four pieces,
although only two of the four pieces actually fit together.
Two of the shaft fragments have feather papillae, but the smallest
does not. The small diameter of the smallest fragment,
which has no recognizable features, suggests that it may not
even be part of the ulna, an idea supported by the presence of a
fifth fragment that is unidentifiable to element, but which is
definitely not part of an ulna.
Right carpometacarpus: Fragmentary distal end, Universidad
Nacional Autonoma de Mexico, Instituto de Geologia,
Museo de Paleontologia, IGCU-6133, from locality GTO.31,
Blancan, Rancho Viejo area, State of Gaunajuato, Mexico,
about 240 km NNW of Mexico, Distrito Federal.
This specimen is too fragmentary to provide much information,
but it is clearly a teratorn. It differs from carpometacarpi
of Teratornis by having (1) the facies articularis digitalis major
with the medial rise more elongated and elevated and more distinctly
set off from the anterior portion; and (2) the os metacarpale
minoris with the distal area of fusion with the os metacarpale
majus proportionately shorter than in Teratornis and the
distal end more massive, projecting distad more distinctly and
at a greater angle from the os metacarpale majus. The single
known partial carpometacarpus of Argentavis is too poorly preserved
for comparison. For further details on the fauna associ-
FIGURE 4.—A,B, proximal end of the ulna referred to Aiolornis incredibilis
(ABDSP(IVCM) 519/5660): A, anterior view; B, ventral view. C,D, ulna of Teratornis
merriami (George C. Page Museum, B124): c, anterior view; D, ventral
view. Note the differences between the olecranons and the orientation of the
cotylae ventralis. Scale bar=3 cm.
B