PLIENSBACHIAN (LOWER JURASSIC) BIOSTRATIGRAPHY AND ...

D escription. Shell midvolute, whorl section ellipsoidal.
Umbilical wall low, shallow; umbilical shoulder
rounded. Flanks convex.
Ornamentation consists of strong, simple rectira-
diate and slightly sinuous ribs that arise at the
umbilical shoulder. The ribs bend sharply forward just
below the ventrolateral shoulder and continue onto the
venter to form chevrons.
Discussion. The Spatsizi specimens of U ptonial sp.
compare with specimens from Alaska assigned to
U ptonial sp. B by Imlay (1981). Although similar to
Uptonia distinct a Tutcher and Trueman (1925), the
small size and state of preservation of the material
preclude a definite species assignment. Problematical
age relationships are discussed below.
Occurrence. In Spatsizi, U ptonial sp. first appears
near the base of the range of Dubariceras freboldi,
above the ranges of the genera Tropidoceras and
Acanthopleuroceras. U ptonial sp. B and D. freboldi
were not found at the same locality in Alaska, so the
relation between their ranges is uncertain (Imlay,
1981). In the Queen Charlotte Islands, a single
specimen similar to U pto n ia l sp. was found in
association with D ubariceras fre b o ld i and, again,
above the ranges of A n c a n th o p le u ro c e ra s and
Tropidoceras (H.W. Tipper, pers. comm., 1985).
A position within the Freboldi Zone (approximately
equivalent to the upper Ibex Zone and much of the
Davoei Zone) is at variance with the age of Uptonia in
Europe where it never ranges above the Masseanum
Subzone of the Jamesoni Zone. It is possible that the
Spatsizi U ptonial sp. is more closely related to the
coeval South American genus E oam aitheus. The
absence of tubercles and a distinct keel would seem to
make this unlikely, but Hillebrandt (pers. comm.,
1987) has such forms in his collected material that are
probably derived from E oam aitheus (Hillebrandt,
1984, and mostly unpublished; but see, for example,
G a la tic e ra sl in H illebrandt, 1987). The exact
taxonomic position of the rather rare North American
material must therefore await the details of the South
American succession.
U ptonial sp. was found at localities 3, 37, 82, and
98.
Age. Early Pliensbachian (Freboldi Zone).
Genus Dayiceras Spath, 1920
Type species. Dayiceras polym orphoides Spath, 1920
(p. 541, PI. 15, figs. 1-4), by original designation.
Rem arks. Shell midvolute, whorl section ellipsoidal to
ogival. Ornament consists of fine, dense, sinuous ribs
that are rursiradiate on the umbilical wall, sinuous on
the flanks, and terminate on the ventrolateral shoulder
in radially elongate tubercles. The venter bears a
median row of tubercles that commonly coalesce to
form a crenulate keel.
A ge and distribution. Rare specimens of Dayiceras
have been found in the Ibex Zone of southern England
(Spath, 1920) and possibly northwest Germany
(Hoffmann, 1982), but the genus has not been reported
from any other part of the northwest European
Province. It has been reported from the Ibex Zone of
Portugal (Mouterde, 1951; Mouterde and Ruget,
1970), North Africa (Rakus, 1972), and northern Italy
(Wiedenmayer, 1977, 1980). This suggests that
Dayiceras was most common in the Tethyan region
rather than being primarily boreal in its distribution, as
suggested by Dommergues et al. (1984).
Dayiceras sp.
Plate 4, figures 3-5
Material. Four fragments preserved as internal and
external moulds in siltstone.
M ea su re m e n ts. Not available because of poor
preservation.
Description. Shell evolute to midvolute; whorl section
is compressed-rectangular. Umbilical wall is low, steep;
umbilical shoulder rounded. Flanks are convex;
ventrolateral shoulder abruptly rounded. The venter is
convex, narrow.
Ornament consists of dense, fine sinuous ribs that
arise on the umbilical wall where they trend
rursiradiately, then become flexuous on the flanks and
bend gently forward near the ventrolateral shoulder
where they are marked by radially elongate tubercles.
The ribs continue weakly past the tubercles onto the
venter, but fade rapidly. A low, weakly beaded keel is
present on the venter.
Discussion. The Spatsizi specimens of Dayiceras sp. are
similar to Dubariceras freb o ld i in their form of ribbing
and volution, but they differ from D ubariceras
freboldi by their low median keel, narrower whorl
section, and slightly coarser ribbing. These specimens
of Dayiceras can be distinguished from Polym orphites
by virtue of their more compressed whorl section,
denser and more sinuous ribbing, irregular keel, and
large size.