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254 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
plagiaulax hunteri subzone) at the late part of chron 26 Reversed
(Sloan, 1987), with an age of about 61 million years.
ACKNOWLEDGMENTS.—I thank B.R. Erickson, Science Museum
of Minnesota (SMM), St. Paul, for the loan of the Paleocene
avian specimens reported in this paper and for discussions
concerning Wannagan Creek Quarry; P.G.P. Ericson and
S.L. Olson for sharing their knowledge of Presbyornithidae and
Paleocene birds; and Julie Martinez, Science Museum of Minnesota,
for drawing the figures. Comparative material used in
this study was that of the avian skeletal collection of the Bell
Museum of Natural History, University of Minnesota, and the
National Museum of Natural History (USNM, which houses
the collections of the former United States National Museum),
Smithsonian Institution. This study was partially supported by
funds (for illustrations) from the Department of Paleontology
of the Science Museum of Minnesota.
Systematic Paleontology
Class AVES
Order ANSERIFORMES
Family PRESBYORNITHIDAE
Presbyornis isoni Olson, 1994
FIGURE 1
REFERRED MATERIAL.—A badly crushed but mostly complete
right humems, lacking the external tuberosity, the proximal
half of the pectoral crest, and the central portion of the bicipital
area; SMM P96.9.2; collected by Michael P. Ryan, 28
June 1989. For measurements, see Table 1.
LOCALITY.—North Dakota, Billings County, -10 mi (16 km)
NW of Medora; Wannagan Creek Quarry, field map quadrant
P-6.
HORIZON AND AGE.—Late Paleocene, early Tiffanian 4,
Bullion Creek Formation, Wannagan Creek Quarry, Bed 2 (lignitic
shale); absolute age, -60 Ma.
COMPARISONS.—Specimen SMM P96.9.2 agrees in all characters
with the holotype of Presbyornis isoni (USNM 294116),
the distal two-thirds of a humems from the Aquia Formation of
Maryland, and confirms that P. isoni is an equally good Presbyornis
at both ends of its humerus by comparison with the
TABLE 1.—Measurements (in mm) of the two known humeri of Presbyornis
isoni
Measurement
Length from head to internal
condyle
Distal width
Depth through external (radial)
condyle
Greatest diameter of brachial
depression
SMM P96.9.2
194.9
-27
13.5
10.0
USNM 294116 (from
Olson, 1994)
23.3
12.9
8.8
type species Presbyornis pervetus Wetmore of the early
Eocene. The proximal end of the humems of P. isoni agrees
with that of the smaller-bodied P. pervetus in the following
characters: the head is undercut by a deep, arc-shaped extension
of the capital groove; the pectoral crest is gently curved
and long, about twice the length of the widely curved bicipital
crest; the attachment of M. scapulohumeralis posterior is a
wide, but elongated, kidney-shaped pit in the distal rim of the
bicipital crest; the median crest is continuous with the extremely
prominent central ridge of the shaft; and the attachment of
M. latissimus dorsi posterior is a prominent oval structure very
close to the central ridge. The proximal end of the humems of
P. isoni differs from that of P. pervetus in lacking a prominent
muscle-scar line distal to the attachment of M. latissimus dorsi
posterior. In the South American presbyornithid Telmabates
antiquus Howard, the humeral head and internal tuberosity are
both undercut by an extension of the capital groove (P.G.P.
Ericson, pers. comm., 1996), whereas in the North Dakota
specimen of Presbyornis isoni, only the head is undercut, as is
the case in Presbyornis pervetus; this is further indication that
P. isoni was correctly assigned to genus.
As in P. pervetus, the humems of P. isoni lacks the stoutness
typical of most of the modem anseriforms, although without
being very slender. The length/distal-width ratio of the humerus
of P. isoni is about 7.2, in contrast to 7.6 for the slightly
more gracile P. pervetus, 6.5 for the stout-winged Snow Goose
{Anser caerulescens), and 9.4 for the slender-winged Northern
Gannet {Morus bassanus). The ratios for Presbyornis spp. are
in general agreement with those of charadriiforms: American
Oystercatcher {Haematopus palliatus), 6.7; Double-striped
Thick-knee {Burhinus bistriatus), 6.8; American Avocet {Recurvirostra
americana), 7.0; Marbled Godwit {Limosa fedoa),
7.1; Black Skimmer {Rynchops niger), 7.2; South Polar Skua
{Catharacta maccormicki), 7.3; Franklin's Gull {Larus pipixcan),
7.5; and Common Murre {Uria aalge), 7.7. Similar values
obtain for primitive ducks, such as the Fulvous Whistling-duck
{Dendocygna bicolor), 7.2.
DISCUSSION.—The Presbyornithidae are primitive anseriforms
that share numerous character states with charadriiform
birds. For example, the proximal end of the humems of Presbyornis
is shallow, as in most charadriiforms and unlike modem
anseriforms. Postcranially, presbyomithids resemble the "transitional
charadriiform" Graculavidae, a group known from the
Late Cretaceous and the Paleocene of North America, from the
Paleocene of Sweden ("Scaniornithidae"; Olson and Parris,
1987) and France (Mourer-Chauvire, 1994), and apparently
from the earliest Eocene of Australia (Boles et al., 1994). Recently,
one genus formerly classified as graculavid, Anatalavis,
has been shown to be anseriform (Olson, this volume). Similarly,
Wetmore (1926), in describing Presbyornis pervetus, placed
the new family Presbyornithidae in the same suborder as the
Recurvirostridae. Feduccia and McGrew (1974) repeatedly
called P. pervetus "the Green River flamingo," although they
noted its duck-like appearance. The Recurvirostridae (avocets),