PDF (Lo-Res) - Smithsonian Institution Libraries

264 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
minate on a prominent, raised linear scar 7 mm long, extending
from within the tricipital fossa near the base of the ventral tubercle
distally in the axis of the shaft. The scar may be the central
vane of a partly pinnate M. humerotriceps. Similar, although
shallower, striations occur in the tricipital fossa of the
Western Gull, Larus occidentalis. A similar long scar is present
in a slightly different position in several modem larids examined
and in the Burhinidae. A short, lower crest or ridge extends
from close to the proximal end of the long scar proximad
and deeper into the triceps fossa. Dorsal to the longer scar there
is a small, rough, irregular surface that may be the impression
for M. scapulohumeralis cranialis.
The caudal margin of the humems is very robust and distinct.
It is far dorsal to the ventral tubercle. At the level of the base of
the ventral tubercle the caudal margin bends abmptly dorsad to
terminate at the proximal articular surface of the humerus
about halfway between the apex and the dorsal tubercle. The
area between the ventral tubercle and the caudal margin of the
humems is broader than that in Graculavus velox.
Discussion
Cretaceous and Paleogene neornithines with the proximal
end of the humems similar to that of Graculavus are Telmatornis,
Presbyornis, Telmabates, and Zhylgaia. Olson and Parris
(1987) compared Graculavus most closely to the Burhinidae in
the Charadriiformes. Telmatomis Marsh, 1870, has been referred
to the Charadriiformes (Shufeldt, 1915; Cracraft, 1972;
Olson and Parris, 1987) and is most similar to the Scolopacidae.
Presbyornis Wetmore, 1926, is referable to the Anseriformes
based on associated cranial material (Olson and Feduccia,
1980). Telmabates Howard, 1955, evidently is referable to
the Presbyornithidae (Feduccia and McGrew, 1974). Zhylgaia
Nesov, 1988, was referred originally to the Presbyornithidae in
Charadriiformes but later (Nesov, 1992) was referred, only tentatively,
to the Presbyornithidae. Zhylgaia has a very steeply
angled ventral tubercle (almost 90° to the axis of the shaft), a
distinct although shallow impression for M. coracobrachialis
cranialis, the head not much undercut, the capital incisure
broad and shallow, and the caudal margin of the humems well
defined but not dorsal to the ventral tubercle. These conditions
suggest that it does not belong with either the Anseriformes or
the Charadriiformes. Thus, the humems in all of these early
waterbirds is very similar, but the birds are not all referable to
the same modem higher-level taxon.
In general, the proximal end of the humems in these birds
differs from that of most modem Charadriiformes as follows:
1. Among the articular head and associated structures, the
head is smaller, and the dorsal tubercle is farther from the head
and is smaller but more protmdent.
2. Impressions for ligaments and muscles on the cranial surface
are much less pronounced, including a shallow impression
for M. coracobrachialis cranialis and a short sulcus for the
transverse ligament.
3. Attachments of the muscles in and around the tricipital
fossa are more robust, especially in Graculavus itself, but there
is no dorsal (second) tricipital fossa.
These differences suggest a distinctive flight mechanism.
Graculavus, Telmatomis, Presbyornis, Telmabates, and
Zhylgaia from the Late Cretaceous and the Paleogene are more
similar to each other than to any modern bird. Many of the
characters of Graculavus are not unique to the Charadriiformes
but occur also among extant waterbirds in a mosaic pattern.
The double-basined tricipital fossa occurs in virtually identical
form in the Charadriiformes and the Oceanitidae (Procellariiformes).
Extreme elongation of the ventral tubercle, very deeply
undercut articular head, and very prominent caudal margin
of the humerus are present in many Charadriiformes and in
most Procellariiformes. A small articular head occurs in many
Procellariiformes and in the Pelecaniformes. A small articular
head, wide distance between the dorsal and ventral tubercles,
and strong protmsion of the dorsal tubercle occur in some petrels,
especially Calonectris (Procellariiformes), in Phaethon,
and in some other pelecaniforms (see comparisons in Olson,
1977, figs. 18-20). Judged on varied morphological and behavioral
grounds, Phaethon is highly plesiomorphic among Pelecaniformes
(Cracraft, 1985; Elzanowski, 1995).
This mosaic pattern of sharing characters with Graculavus
suggests that the various extant groups of waterbirds have retained
different suites of characters that were present in a common
ancestor. The occurrence of many characters of Graculavus
in the overall very plesiomorphic Phaethon supports this
suggestion.
Conclusions
The new, very large species extends the Cretaceous range of
Graculavus from eastern to western North America. Presbyornithids
are now reported from the Late Cretaceous through the
early Eocene in North and South America, Antarctica, and
Mongolia (Howard, 1955; Feduccia and McGrew, 1974; Olson,
1994; Noriega and Tambussi, 1995). Zhylgaia comes from
an estuarine habitat in the late Paleocene of Kazakhstan (Nesov,
1988). Graculavus, Telmatomis, Zhylgaia, and the Presbyornithidae
show that similar graculavid-like birds were widespread
in the Late Cretaceous through the early Tertiary.
Reports of graculavid-like birds represented only by other parts
of the skeleton are harder to evaluate in this context (e.g., Olson
and Parris, 1987; Kurochkin, 1988; Nesov and Jarkov,
1989; Elzanowski and Brett-Surman, 1995).
Graculavus augustus was a very large bird and a strong flyer.
The bones were delicately sculpted and were not highly
pneumatic, resembling contours in flying swimmers and
divers; they lacked the extreme inflation of soaring birds. Impressions
for tendons and muscles differ sufficiently from
those of modem Charadriiformes to suggest distinctive flight
mechanics. Graculavus is most similar to the Charadriiformes,
but it shows a high proportion of characters that may be plesio-