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174
ated with this specimen, see Miller and Carranza-Castaneda
(1984).
Right carpometacarpus: Fragmentary proximal end, including
only trochlea carpalis and area immediately distal to it,
Florida Museum of Natural History, UF 123874, Leisey Shell
Pit 3, Hillsborough County, Florida, Bermont Formation, Irvingtonian
(between 1.66 and 1.4 Ma (MacFadden, 1995)).
This specimen was originally described and figured by
Emslie (1995:316-317), who referred it to Teratornis sp. cf. T.
incredibilis on the basis of its similarity to Teratornis merriami
and its size (-40% larger than T. merriami). We see no reason
to doubt that this specimen is a teratorn, but little else can be
said about it.
Discussion
Howard (1952) described Teratornis incredibilis on the basis
of the complete os carpi ulnare (cuneiform) noted above, allocating
the species to Teratornis on the basis of its general similarity
to T. merriami. The much greater size of the specimen
(43% larger than the same bone of T. merriami) and other characters
clearly established its distinction as a separate species
(Howard, 1952). Howard (1963, 1972) also referred to Teratornis
incredibilis the distal end of a radius and the anterior portion
of a beak discussed above on the basis of their general similarity
to T. merriami and the fact that both of these specimens
were about 40% larger than comparable bones of the latter species.
Although Howard (1972:343) considered the possibility
of generic separation of the larger species from Teratornis, she
considered all three specimens she had assigned to T. incredibilis
too undiagnostic or too poorly preserved to justify establishing
a new genus.
After restudying the holotype and three specimens previously
referred to Teratornis incredibilis we concluded that the differences
seen between T. incredibilis and T. merriami are such
that the erection of a new genus for the former was warranted.
In reexamining the holotypical os carpi ulnare we found that
the characters noted above for Aiolornis differ from those of
Teratornis at a level comparable to the differences seen among
the os carpi ulnare of the living genera of the family Vulturidae,
which are easily identified to genus, and the differences
hold for all specimens of the element from Rancho La Brea examined
(n=24). The unique characters of the humerus, ulna, radius,
and carpometacarpus assigned to Aiolornis incredibilis,
however, suggest that that species may have had different flight
adaptations from Teratornis merriami. Because the os carpi
ulnare is a bone integral to the flight of any bird, we would
have expected greater character differences to be reflected in
the os carpi ulnare of Aiolornis, which would serve to separate
it more readily from Teratornis. For this reason we would not
be surprised to find in the future that the older (Blancan and
Irvingtonian) specimens herein referred to Aiolornis incredibilis
actually are referrable to another genus and species.
SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
The other genus and species of teratorn, Cathartomis gracilis,
is known only from tarsometatarsi, one complete and one
distal end, from the late Pleistocene asphalt deposits at Rancho
La Brea, California. Although the length of the holotypical tarsometatarsus
of C. gracilis falls within the size range of tarsometatarsi
of Teratornis merriami, it is much more slender
and has several features that distinguish it from its more heavily
built contemporary. Miller and Howard (1938:169) reevaluated
the status of Cathartomis and concluded "that Cathartornis
is markedly similar to Teratornis merriami, though it is
undoubtedly a distinct species. We consider it also to be generically
distinct." Some of the characters listed as separating the
two genera are not particularly convincing, however, and with
more specimens of teratorns available now than at the time of
Miller and Howard's study, the case for maintaining Cathartomis
as a separate genus is weak. Resolution of the status of
Cathartomis is deferred pending completion of the studies of
the teratorns of Rancho La Brea by KEC. Given the marked
similarity of Cathartomis to Teratornis, we considered assignment
to Cathartomis of the specimens now referred to Aiolornis
inappropriate.
The size of the Incredible Teratorn, Aiolornis incredibilis, remains
its most remarkable known feature, even though we
know there was at least one larger species of teratorn. Howard
(1952) estimated the size of A. incredibilis to be 43% larger
than Teratornis merriami based on the holotypical os carpi
ulnare, which would give a wingspan of about 5 m. She estimated
(Howard, 1972:343) the radius she referred to A. incredibilis
to be "approximately 40% broader than a large radius of
T. merriami." The beak she referred to A. incredibilis was estimated
to be "43% larger than the largest of four measurable
specimens of T. merriami,'" based on what Howard (1972:343)
considered the best available measurement. The ulna that Jefferson
(1995:94) referred to A. incredibilis was said to be
"about 57% larger than the average (39.6) of five measured
specimens of Teratornis merriami from Rancho La Brea." Unfortunately,
it is not possible to draw a direct size comparison
between the partial humerus of A. incredibilis and humeri of T.
merriami. The only accurate measurements that can be taken
from the holotype of A. incredibilis are the width and depth
near midshaft, and the width to depth proportions of the shaft
are so different among the known genera of teratorns (Figure 3)
as to make such a size comparison meaningless. Neither is it
possible to determine accurately the size of the bird from which
came the carpometacarpi referred to A. incredibilis because of
the poor state of preservation of those specimens. A reasonable
"eyeball" estimate, however, suggests that both of these specimens
came from a bird with a wingspan intermediate between
that of T merriami (3.5-4 m) and Argentavis magnificens (6-8
m). This gives a wingspan estimate of 5.0-5.5 m for A. incredibilis,
which conforms with that estimated from the other specimens
referred to this species.