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DISCUSSION.—Considering that the distal end of this tarsometatarsus
seems a strange mosaic, had the proximal end
been preserved, it might well have resembled that of some other
bird altogether, not to mention what the rest of the skeleton
may have resembled. Because the preserved part of this bone
is, however, a mosaic of exclusively charadriiform characters,
the Graculavidae is the best present assignment for this specimen.
This fragment would seem, at least in the absence of other
Paleocene fossils of more modem aspect, to represent a bird
older than the divergence time of any of the extant charadriiform
families, as does Dakotornis. On body size, the present
specimen could be referable to either of the monotypic genera
Dakotornis or Graculavus. The specimens of Dakotornis cooperi
are about one million years younger than this specimen,
and those of Graculavus velox Marsh are about four million
years older (see Olson, 1994, for the probable early Paleocene
age of G. velox and the other New Jersey graculavids).
GRACULAVIDAE, gen. et sp. indescript.
FIGURE 4
MATERIAL.—Distal 6 mm of a right tarsometatarsus, with
the outer trochlea broken away, SMM P96.9.3; collected by
Bmce R. Erickson and field crew, summer 1976.
LOCALITY.—North Dakota, Billings County, -10 mi (16 km)
NW of Medora; Wannagan Creek Quarry, field map quadrant
0-19.
HORIZON AND AGE.—Late Paleocene, early Tiffanian 4,
Bullion Creek Formation, Wannagan Creek Quarry, Bed 2 (lignitic
shale); absolute age, -60 Ma.
DESCRIPTION.—This tarsometatarsal fragment is from a bird
about the size of a Wilson's Plover {Charadrius wilsonia) and
has the following characters: (1) the distal foramen is moderately
large and oval and occupies the distal end of a shallow anterior
tendinal groove; (2) in posterior view, two tiny foramina
occur just proximal to the distal foramen; (3) the metatarsal
facet is shallow and oval; (4) the trochlear arch in distal view is
relatively low, with the outer trochlea not posteriorly retracted,
but with the inner trochlea retracted so that its central point is
posterior to the posteriormost extent of the middle trochlea; the
trochlear arch in posterior view (as is common in Charadriiformes)
forms a subsymmetrical basin between the three trochleae;
(5) the inner trochlea is bulbous (as in most Charadriiformes)
and is oriented distomedially; (6) the inner trochlea in
distal view bears a very slight, posteriorly oriented wing on its
medial side (a smaller wing than in modem Charadriiformes);
(7) the inner trochlea is elevated so that its distal extent is about
level with the proximal extent of the middle trochlea's digital
groove; (8) the middle trochlea extends considerably farther
distally than the other two; its lateral rim is slightly greater in
distal extent and in anterior extent than is the medial rim; and
(9) the outer trochlea would seem to have a slightly greater distal
extent than the inner trochlea (the broken-off outer trochlea
is preserved, but its contact with the adjacent part of the bone at
the breakage is lost).
SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
FIGURE 4.—Distal end of right tarsometatarsus of Graculavidae gen. et sp.
indescript., SMM P96.9.3 (x8): a, anterior view; b, medial view; c, posterior
view; d, distal view.
COMPARISONS.—Fossil tarsometatarsi of small, spurless galliforms
can be easily mistaken for those of charadriiforms (Olson
and Farrand, 1974). Indeed, none of the nine characters
listed in the previous paragraph would necessarily be inconsistent
with an assignment of the specimen to Galliformes. Olson
and Farrand (1974), however, list 10 other tarsometatarsal
characters, all relating to the trochleae, that distinguish galliforms
from charadriiforms. The present specimen differs from
the Galliformes in all 10 of these trochlear characters.
Specimen SMM P96.9.3 shares seven of the nine characters
listed above (1-5, 8, 9) with the plover family, Charadriidae, a
family not known in the fossil record earlier than the early Miocene
(Unwin, 1993). The specimen differs from Charadriidae
in two characters of the inner trochlea (6, 7), in which it more
closely resembles the Paleocene Telmatomis and modem avocets
(Recurvirostridae), respectively. It also resembles the Recurvirostridae
in characters 3 and 9.
DISCUSSION.—There is no evidence that the families
Charadriidae and Recurvirostridae had diverged prior to the
date of this fossil (Unwin, 1993), which may represent part of
the "graculavid" stock prior to the divergence of these families.
Of course, the presence of primitive charadriiform characters in
this specimen (and in the Brisbane tarsometatarsus described
above), perhaps by retention from much earlier forms, can tell
us nothing certain about the divergence times of the extant
charadriiform families.
The very small body size of the bird represented by specimen
P96.9.3, perhaps the smallest currently known Paleocene bird,
need not exclude it from the Graculavidae (which are otherwise
much larger birds), as this is only a form-family in any case